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Heliacal Rising

Heliacal rising

The heliacal rising of a star (or other body such as the moon or a planet) occurs when it first becomes visible above the eastern horizon at dawn, after a period when it was hidden below the horizon or when it was just above the horizon but hidden by the brightness of the sun. Each day after the heliacal rising, the star will appear to rise slightly earlier and remain in the sky longer before it is hidden by the sun (the sun appears to drift eastward relative to the stars along a path called the ecliptic). Eventually the star will no longer be visible in the sky at dawn because it has already set below the western horizon. This is called the heliacal setting. A star will reappear in the eastern sky at dawn approximately one year after its previous heliacal rising. Not all stars have heliacal risings: some may (depending on the latitude of observation on the earth) remain permanently above the horizon, making them always visible in the sky at dawn, before they are hidden by the brightness of the sun. Constellations containing stars that rise and set were incorporated into early calendars or zodiacs. The ancient Egyptians based their calendar on the heliacal rising of Sirius and devised a method of telling the time at night based on the heliacal risings of 36 stars called decan stars (one for each 10° segment of the 360° circle of the zodiac/calendar). The Sumerians, the Babylonians, and the ancient Greeks also used the heliacal risings of various stars for the timing of agricultural activities. To the Maori of New Zealand, the Pleiades are called Mataariki and their heliacal rising signifies the beginning of the new year (around June). The corresponding rising of a celestial body above the eastern horizon at nightfall, for example, that of the full moon, is called its acronychal rising. Category:Astronomy

Star

:This article is about celestial bodies. A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Small (dwarf) stars such as the Sun generally have essentially featureless disks with only small starspots. Larger (giant) stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb-darkening (the brightness decreases towards the edge of the stellar disk). Stellar astronomy is the study of stars.

Star formation and evolution

Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or collision of two galaxies (as in a starburst galaxy). High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula. Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe (13.6 billion years), no black dwarfs exist yet. As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume both Mercury and Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process. Since iron nuclei are more tightly bound than any heavier nuclei, they cannot be fused to release energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. In old, very massive stars, a large core of inert iron will accumulate in the center of the star. An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into black dwarfs over very long stretches of time. white dwarf In larger stars, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before. Eventually, most of the matter in a star is blown away by the explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars, a black hole. The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

Appearance and distribution of stars

All stars except the Sun appear to the human eye as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars. The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the known universe [http://news.bbc.co.uk/2/hi/science/nature/3085885.stm]. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way. The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.

Age and size of stars

galactic halo Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometers. However, these have a much lower density than the Sun. One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive star is long extinct, however, and currently only theoretical. With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.

Star classification

There are different classifications of stars ranging from type W, which are very large and bright, to M, which is often just large enough to start ignition of the hydrogen. Some of the more common classifications are O, B, A, F, G, K, M, and can perhaps be more easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon (1863-1941). There are many other mnemonics for star classification; the most frequent addition tacks "Right Now, Sweetheart" for the red dwarf sub-types R, N and S. The new types L and T have also been recently appended to the end of the OBAFGKM sequence to classify the coldest low-mass stars and brown dwarfs, prompting such additions as "Lovingly Tonight" to the mnemonic. Each letter has 10 subclassifications. Our Sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units.
For example, the mass of the Sun is :M_Sun = 1.9891×10³⁰ kg The masses of other stars can be given in terms of M_Sun.

Naming of stars

Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named. See star designations for more information on how stars are named. For a list of traditional names, see the list of stars by constellation.

Energy production

The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measured by its apparent magnitude.

Nuclear fusion reaction pathways

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis). Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. In the Sun, with a 10⁷ K core, hydrogen fuses to form helium in the proton-proton chain: :4¹H → 2²H + 2e⁺ + 2ν_e (4.0 MeV + 1.0 MeV) :2¹H + 2²H → 2³He + 2γ (5.5 MeV) :2³He → ⁴He + 2¹H (12.9 MeV) These reactions result in the overall reaction: :4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV) In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle. In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process: :⁴He + ⁴He + 92 keV → ^{8
-}Be :⁴He + ^{8
-}Be + 67 keV → ^{12
-}C :^{12
-}C → ¹²C + γ + 7.4 MeV For an overall reaction of: :3⁴He → ¹²C + γ + 7.2 MeV

Star mythology

As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead, or gods/goddesses.

References

- Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press
- John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.

External links

- [http://www.mrao.cam.ac.uk/telescopes/coast/betel.html Images of starspots on the surface of Betelgeuse]
- [http://simbad.u-strasbg.fr/sim-fid.pl Find out what is known about any given star by entering its name or position]
- [http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml Star Classification] Category:Astronomical objects ko:항성 ms:Bintang ja:恒星 simple:Star th:ดาวฤกษ์

Planet

A planet is generally considered to be a relatively large mass of accreted matter in orbit around a star that is not a star itself. The name comes from the Greek term πλανήτης, planētēs, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. Based on historical consensus, the International Astronomical Union (IAU) lists nine planets in our solar system. Since the term "planet" has no precise scientific definition, however, many astronomers contest that figure. Some say it should be lowered to eight by removing Pluto from the list, whilst others claim it should be raised to fifteen, twenty, or even higher.

Planetary formation

It is not known with certainty how planets are formed. The prevailing theory is that they are formed from those remnants of a nebula that don't condense under gravity to form a protostar. Instead, these remnants become a thin disc of dust and gas revolving around the protostar and begin to condense about local concentrations of mass within the disc. These concentrations become ever more dense until they collapse inward under gravity to form protoplanets. When the protostar has grown such that it ignites to form a star, its solar wind blows away most of the disc's remaining material. Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb. Meanwhile, protoplanets that have avoided collisions may become moons of larger planets. With the discovery and observation of planetary systems around stars other than our own, it is becoming possible to elaborate, revise or even replace this account.

Within our solar system

Main article: Solar system The process of naming planets and their features is known as planetary nomenclature. All the currently accepted planets in the solar system are named after Roman gods, except for Uranus (named after a Greek god) and the Earth, which was not seen as a planet by the ancients but rather the centre of the universe. The designated planetary names are near-universal in the Western world, but some non-European languages, such as Chinese, use their own. Moons are also named after gods and characters from classical mythology, or, in the case of Uranus, after Shakespearean characters. Asteroids can be named after anybody or anything at the discretion of their discoverers, subject to approval by the IAU's nomenclature panel.

Accepted planets

Asteroid According to the authority of the IAU, there are nine planets in our solar system. In increasing distance from the Sun they are: #Mercury (astronomical symbol ) #Venus () #Earth () with one confirmed natural satellite, Luna (the Moon) #Mars () with two confirmed natural satellites, Deimos and Phobos #Jupiter () with sixty-three confirmed natural satellites #Saturn () with forty-six confirmed natural satellites #Uranus (Uranus) with twenty-seven confirmed natural satellites #Neptune () with thirteen confirmed natural satellites #Pluto () with three confirmed natural satellites (Charon, S/2005 P 1, S/2005 P 2) However, there is some pressure for Pluto to be reclassified as a Kuiper Belt object, especially in light of the discovery of . This object, however, has not yet received a definitive classification from the IAU.

Other candidates

When Ceres was found orbiting between Mars and Jupiter in 1801, it was initially touted as a planet, but after many smaller objects were found with a similar orbit, it was classified as an asteroid. However, due to its large size (relative to the other asteroids), and its roughly spherical shape, Ceres would be considered a planet by some astronomers' definitions. Similarly, since 1992 many objects have been found in the predicted Kuiper Belt that exists beyond Neptune. Several of the largest of these have challenged the planetary status quo, as they are both spherical and larger than the bodies in the Mars-Jupiter asteroid belt, and are similar in size, orbit and composition to Pluto. However, as yet none have been accepted as planets by the IAU. The most significant of these are (in order of increasing distance from the Sun) 90482 Orcus, , 50000 Quaoar, , , 28978 Ixion, 20000 Varuna, 19521 Chaos, and 90377 Sedna. (However, it should be noted that Sedna is often considered to be beyond the Kuiper Belt; being either a member of the scattered disc or the inner Oort Cloud). Like Ceres before it, Sedna was widely touted as a planet when it was discovered in 2003, as it was the largest object found since Pluto. However, mainly due to its size still being smaller than Pluto's, it did not achieve planetary status from the IAU. However, the discovery in 2005 of (nicknamed Xena), with a size and mass larger than Pluto seems to have forced the issue. As of September 2005 it has not yet been accepted as a planet, but the IAU is expected to announce a definition of a planet by the end of the year, which will either see become a planet, or have Pluto stripped of its status.

Extrasolar planets

:Main article: Extrasolar planet. Of the 173 extrasolar planets (those outside our solar system) discovered to date (October 2005) most have masses which are about the same or larger than Jupiter's. Exceptions include a number of planets discovered orbiting burned-out star remnants called pulsars, such as PSR B1257+12, the planets orbiting the stars Mu Arae, 55 Cancri and GJ 436 which are approximately Neptune-sized [http://www.eso.org/outreach/press-rel/pr-2004/pr-22-04_pf.html], and a planet orbiting Gliese 876 that is estimated to be about 6 to 8 times as massive as the Earth and is probably rocky in origin. It is far from clear if the newly discovered large planets would resemble the gas giants in our solar system or if they are of an entirely different type as yet unknown, like ammonia giants or carbon planets. In particular, some of the newly discovered planets, known as hot Jupiters, orbit extremely close to their parent stars, in nearly circular orbits. They therefore receive much more stellar radiation than the gas giants in our solar system, which makes it questionable whether they are the same type of planet at all. There is also a class of hot Jupiters that orbit so close to their star that their atmospheres are slowly blown away in a comet-like tail: the Chthonian planets. The National Aeronautics and Space Administration of the United States has a program underway to develop a Terrestrial Planet Finder artificial satellite, which would be capable of detecting the planets with masses comparable to terrestrial planets. The frequency of occurrence of these planets is one of the variables in the Drake equation which estimates the number of intelligent, communicating civilizations that exist in our galaxy. Astronomers have recently [http://www.nature.com/news/2005/050711/full/050711-6.html] [http://www.jpl.nasa.gov/news/news.cfm?release=2005-115] detected a planet in a triple star system, a finding that challenges current theories of planetary formation. The planet, a gas giant slightly larger than Jupiter, orbits the main star of the HD 188753 system, in the constellation Cygnus, and is hence known as HD 188753 Ab. The stellar trio (yellow, orange, and red) is about 149 light-years from Earth. The planet, which is at least 14% larger than Jupiter, orbits the main star (HD 188753 A) once every 80 hours or so (3.3 days), at a distance of about 8 Gm, a twentieth of the distance between Earth and the Sun. The other two stars whirl tightly around each other in 156 days, and circle the main star every 25.7 years at a distance from the main star that would put them between Saturn and Uranus in our own Solar System. The latter stars invalidate the leading hot Jupiter formation theory, which holds these planets form at "normal" distances and then migrate inward through some debatable mechanism. This could not have occurred here, the outer star pair disrupting outer planet formation.

Brown dwarf "planets"

The discovery of a planet-sized satellite of a brown dwarf has blurred the distinction between "planet" and "moon." A brown dwarf, though a star in theory, in practice is often described as in between a planet and a star. It is formally defined by the IAU by its official statement that "Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located." To the IAU, the question of whether an object in orbit around a brown dwarf is a "planet" or a "moon" was simply not relevant, as it does not use the term "moon," only "satellite" and as yet has no official definition for "planet."

Interstellar planets

Interstellar planets are rogues in interstellar space, not gravitationally linked to any given solar system. No interstellar planet is known to date, but their existence is considered a likely hypothesis based on computer simulations of the origin and evolution of planetary systems, which often include the ejection of bodies of significant mass. Such objects are not formally called planets, however, since the IAU has not defined the term "planet".

Definition and classification of planets

Much like "continent", "planet" is a word without a precise definition, with history and culture playing as much of a role as geology and astrophysics. Recent definitions have been vague and imprecise; The American Heritage Dictionary, for instance, formerly defined a planet as: :A nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star, such as the sun, around which it revolves. In the solar system there are nine known planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.' However, for some time that definition has been viewed by many as inadequate. The eight largest planets (which are also the eight nearest to the Sun) are universally recognised as such, and for this reason are often universally referred to as "major planets", but there is controversy over Pluto and other smaller objects.
Suggested wide definitions
Since the discoveries of many of the objects in the Kuiper belt and around other stars, there has been a concerted push amongst scientists to come up with a precise definition of what constitutes a planet. In 1999, the IAU set up a working group to develop a scientifically plausible recommendation, but as of August, 2005 they had not reached a conclusion. After the discovery of (informally called "Xena"), a member of the committee, Alan Stern, has said that the group wanted "to get something done, pronto". He also informed journalists that a "consensus" in the group was moving towards the following definition: :A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior. Note that this definition also covers disputes at the upper end of a planet's size, which provides the extra benefit of forming a barrier between planets and brown dwarfs. Many consider this definition the best option as it sets up divisions based on physical characteristics rather than an arbitrary size limit. It is also somewhat universal in its application where other definitions have been crafted mainly to sort our own solar system into simple categories (such as placing the size limit as just under Mars, Mercury or Pluto). Depending how it is interpreted, objects counted as planets under such a new system would include some or all of the objects listed above, with potentially many more yet to be found. Gibor Basri, head of astronomy at the University of Berkeley, has suggested a similar definition and has also proposed the terms "fusor" (any object that achieves fusion in its core) and "planemo" (an object that is round from self-gravity but not a fusor) to help improve the astronomical nomenclature. Under Basri's definition: :A planet is a planemo orbiting a fusor These definitions have the advantage of creating a group including larger moons (which share many characteristics with the smaller planets) and also covering large free-roaming objects, which some astronomers think should be included in the definition of a planet. Basri has also suggested 'liberal use of adjectives' such as "major", "beltway", "dwarf", "giant", "super" and "historical".[http://astron.berkeley.edu/%7Ebasri/defineplanet/Mercury.htm] Others have suggested categories of planet/planemo based on composition such as "rock" (composed mainly of silicate), "gas" (composed mainly of hydrogen and helium), and "ice" (composed mainly of oxygen and carbon).
Suggested narrow definitions
There are alternate suggestions which would instead reduce the number of planets in the system. Upon his discovery of Sedna, Mike Brown of Caltech suggested a definition which would exclude both Sedna and Pluto from being classified as planets, proposing the following: :A planet is any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit [http://www.gps.caltech.edu/~mbrown/sedna/#What%20is%20the%20definition%20of%20a%20planet?] This definition generally plays down the importance of size, but instead focuses on the formation of the proposed planet. Under this definition, no Kuiper Belt objects (including Pluto) would be considered planets. Brown's wish to "demote" Pluto prompted many to criticize him for setting out to create a purely scientific definition for a term which had an existing popular (albeit 'flawed') application. Upon his discovery of , Brown indicated he had become a convert to this way of thinking, and proposed that whatever definition of planet be adopted, it should include both Pluto and any Kuiper Belt object found to be larger than Pluto. [http://www.gps.caltech.edu/~mbrown/planetlila/index.html]
Further classification
Astronomers distinguish between minor planets, such as asteroids, comets, and trans-Neptunian objects; and major (or true) planets. Planets within Earth's solar system can be divided into categories according to composition.
- Terrestrial or rocky: Planets that are similar to Earth — with bodies largely composed of rock: Mercury, Venus, Earth, Mars
- Jovian or gas giant: Those with a composition largely made up of gaseous material: Jupiter, Saturn, Uranus, Neptune. Uranian planets, or ice giants, are a sub-class of gas giants, distinguished from true Jovians by their depletion in hydrogen and helium and a significant composition of rock and ice.
- Icy: Sometimes a third category is added to include bodies like Pluto, whose composition is primarily ice; this category of "icy" bodies also includes many non-planetary bodies such as the icy moons of the outer planets of our solar system (e.g. Triton). Many consider the Earth and its Moon to be a double planet, for several reasons:
- The Moon, as measured by its diameter, is 1.5 times larger than Pluto.
- The gravitational force of the Sun on the Moon is larger than the gravitational force of the Earth on the Moon by a factor of approx. 2.2. (This is not a unique situation in the solar system. The Sun's gravity is also stronger than the primary's on Jupiter's moon S/2003 J 2; Uranus' moon S/2001 U 2; Neptune's moons S/2002 N 4 and Psamathe; and several asteroid moons. However, Luna is the sole case of this phenomenon affecting an object of planetary mass.)
See also

- Definition of planet
- Planetary habitability
- Planetary science
- Planemo
- Planetoid
- Brown Dwarf
- Planets in science fiction
- Prograde and retrograde motion
- Skies of other planets
References

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External links

- [http://www.nineplanets.org/ NinePlanets.org] - tour of the solar system
- [http://www.iau.org International Astronomical Union]
- [http://www.fourmilab.ch/cgi-bin/uncgi/Solar/ Solar System Live] (an interactive orrery)
- [http://janus.astro.umd.edu/javadir/orbits/ssv.html Solar System Viewer] (animation)
- [http://www.sky-pics.net/ Pictures of the solar system]
- [http://gw.marketingden.com/planets/sun.html Renderings of the planets]
- [http://planetquest.jpl.nasa.gov/ NASA Planet Quest]
- [http://www.ciw.edu/IAU/div3/wgesp/definition.html Working definition of "planet"] from IAU WGESP — the lower bound remained a matter of consensus in February 2003
- Dan Green's page on [http://cfa-www.harvard.edu/cfa/ps/icq/ICQPluto.html planet classification]
- [http://www.spacedaily.com/news/outerplanets-04b.html Gravity Rules: The Nature and Meaning of Planethood]; S. Alan Stern; March 22, 2004
- [http://www.iau.org/IAU/FAQ/PlutoPR.html On the status of Pluto]; IAU, February 3, 1999
- als:Planet ko:행성 ms:Planet ja:惑星 simple:Planet th:ดาวเคราะห์ zh-min-nan:He̍k-chheⁿ

Horizon

:For other uses of horizon, see Horizon (disambiguation) Horizon (disambiguation) The horizon is the line that separates earth from sky. More precisely, it is the line that divides all of the directions you can possibly look into, into two categories: those which intersect the Earth, and those which do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc. The resulting intersection of earth and sky is instead known as the visible horizon. However, if you are on a ship at sea, the true horizon is strikingly apparent. Historically, the distance to the visible horizon has been extremely important as it represented the maximum range of communication and vision before the development of the radio and the telegraph. Even today, when flying an aircraft under Visual Flight Rules, a technique called attitude flying is used to control the aircraft, where the pilot uses the relationship between the aircraft's nose and the horizon to control the aircraft. He also retains his spatial orientation by referring to the horizon. spatial orientationIn astronomy the horizon is the horizontal plane through (the eyes of) the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points which have an altitude of zero degrees. The regular horizon is a little below that. The distance of the horizon on earth, in a plain (standing on the ground or on a tower, or from a plane) or on a hill or mountain surrounded by plains, is approximately

\sqrt

kilometers, where h is the height in meters of the eyes. Examples:
- standing on the ground with h = 1.70 m, the horizon is at a distance of 4.7 km
- standing on a hill or tower of 100 m height, the horizon is at a distance of 36 km These figures indicate theoretical visibility (what can be seen depends also on how clear the air is, of course) of objects at ground level. To compute to what distance the tip of a tower, the mast of a ship or a hill is above the horizon, add the horizon distance for that height. For example, standing on the ground with h = 1.70 m, one can see, weather permitting, the tip of a tower of 100 m height at a distance of 41 km. This formula is reasonable when h is much smaller than the radius of the Earth (6371 km). The exact formula for distance from the viewpoint to the horizon, applicable even for satellites, is :

\sqrt

where R is the radius of the Earth (note: both R and h in this equation are in kilometers). A different formula is given by :

\cos\frac=\frac.

This formula gives the arc length distance s along the curved surface of the Earth to to bottom of object, whereas the above formula is for the straight line of sight distance to the top of the object of view. Both formulas agree when the height of the object is negligible compared to the radius.

External links

- [http://newton.ex.ac.uk/people/sque/physics/horizon/ Derivation of the distance to the horizon]
- [http://www.robertbdance.com/PaintingAtmosphericEffects.html An artistic treatment of the horizon]

Acknowledgements

The first version of this article originates from Jason Harris' Astroinfo which comes along with KStars, a Desktop Planetarium for Linux/KDE. See http://edu.kde.org/kstars/index.phtml Category:Horizontal coordinate system Category:Spherical astronomy

Latitude

Latitude, sometimes denoted by the Greek letter φ, gives the location of a place on Earth north or south of the Equator. Latitude is an angular measurement ranging from 0° at the Equator to 90° at the poles (90º N or 90º S). Co-latitude is the complement of latitude. complement showing lines of latitude, which appear straight and horizontal in this projection, but are actually circular with different radii.]]

Lines of latitude

All locations of a given latitude are collectively referred to as a line of latitude or parallel, because they are coplanar, and all such planes are parallel to the Equator. Lines of latitude other than the Equator are approximately small circles on the surface of the Earth; they are not geodesics since the shortest route between two points at the same latitude involves moving farther away from the equator. A specific latitude may then be combined with a specific longitude to give a precise position on the Earth's surface.

Subdivisions

Each degree of latitude is further sub-divided into 60 minutes (one arcminute of latitude is exactly one nautical mile or 1852 metres), each of which divides into 60 seconds. A latitude is thus specified as 13° 19′ 43" N. For high accuracy, the seconds are specified with a decimal fraction. An alternative representation uses degrees and minutes, where parts of a minute are expressed as a decimal fraction, thus: 13° 19.717′ N. Degrees expressed as a decimal number (Decimal Degree notation) is more often used: 13.32861° N. Sometimes, the North/South suffix is replaced by a negative sign for South (-90º for the south pole).

Important latitudes

Latitudes of particular importance are the Tropic of Cancer (latitude 23°27′ north), the Tropic of Capricorn (latitude 23°27′ south), the Arctic Circle (latitude 66°33′ north), and the Antarctic Circle (latitude 66°33′ south). Only at latitudes between the Tropics is it possible for the sun to be at the zenith. Only north of the Arctic Circle or south of the Antarctic Circle is the midnight sun possible.

Effect of latitude

A region's latitude has a great effect on its climate and weather. Latitude more loosely determines tendencies in climate, polar auroras, prevailing winds, and other physical characteristics of geographic locations.

Types of latitude

Because the Earth is slightly flattened by its rotation, cartographers refer to a variety of auxiliary latitudes to precisely adapt spherical projections according to their purpose.

Common "latitude"

- In common usage, "latitude" refers to geodetic or geographic latitude φ and is the angle between the equatorial plane and a line that is normal to the reference spheroid, which approximates the shape of the Earth to account for flattening of the poles and bulging of the equator. The expressions following assume elliptical polar sections with eccentricity e, and that all sections parallel to the equatorial plane are circular. Geographic latitude (with longitude) then provides a Gauss map.

Reduced latitude

- Reduced or parametric latitude β is the latitude of the same radius on the sphere with the same equator. ::

\beta=\arctan\!\left\\,\!

Authalic latitude

- Authalic latitude ξ gives an area-preserving transform to the sphere. :::

\xi=\arcsin\!\left\\,\!

\mbox Q\!\left\=\left|\frac-\frac\ln\!\left\\right|

Rectifying latitude

- Rectifying latitude μ is the surface distance from the equator, scaled so the pole is 90°. Unfortunately, it is an incomplete elliptic integral: ::

\mu=k\int_^\phi\fracdx\,\!

Conformal latitude

- Conformal latitude χ gives an angle-preserving (conformal) transform to the sphere. ::

\chi=2\arctan\!\left\-\frac\,\!

Geocentric latitude

- The geocentric latitude φ_g is the angle between the equatorial plane and a line from the center of the Earth. ::

\phi_g=\arctan\left\\,\!

For other planets such as Mars, geographic and geocentric latitude are called "planetographic" and "planetocentric" latitude, respectively. Most maps of Mars since 2002 use planetocentric coordinates.

Approximate difference from geographic latitude
φ	reduced φ − β	authalic φ − ξ	rectifying φ − μ	conformal φ − χ	geocentric φ − φ_g
0°	0.00′	0.00′	0.00′	0.00′	0.00′
5°	1.01′	1.35′	1.52′	2.02′	2.02′
10°	1.99′	2.66′	2.99′	3.98′	3.98′
15°	2.91′	3.89′	4.37′	5.82′	5.82′
20°	3.75′	5.00′	5.62′	7.48′	7.48′
25°	4.47′	5.96′	6.70′	8.92′	8.92′
30°	5.05′	6.73′	7.57′	10.09′	10.09′
35°	5.48′	7.31′	8.22′	10.95′	10.96′
40°	5.75′	7.66′	8.62′	11.48′	11.49′
45°	5.84′	7.78′	8.76′	11.67′	11.67′
50°	5.75′	7.67′	8.63′	11.50′	11.50′
55°	5.49′	7.32′	8.23′	10.97′	10.98′
60°	5.06′	6.75′	7.59′	10.12′	10.13′
65°	4.48′	5.97′	6.72′	8.95′	8.96′
70°	3.76′	5.01′	5.64′	7.52′	7.52′
75°	2.92′	3.90′	4.39′	5.85′	5.85′
80°	2.00′	2.67′	3.00′	4.00′	4.01′
85°	1.02′	1.35′	1.52′	2.03′	2.03′
90°	0.00′	0.00′	0.00′	0.00′	0.00′

Astronomical latitude

- A more obscure measure of latitude is the astronomical latitude, which is the angle between the equatorial plane and the normal to the geoid (ie a plumb line). It originated as the angle between horizon and pole star.

Latitude and wealth

It is frequently observed that there is a distinct correlation between latitude and the wealth of nations. The continents along the equator, Africa and South America are the poorest. Even within Africa and South America this can be seen as the nations furthest from the equator are wealthier. In Africa the wealthiest nations are the three on the southern tip of the continent, South Africa, Botswana, and Namibia, and the countries of North Africa. Similarly in Latin America Argentina, Chile and Uruguay have long been the wealthiest. Within Asia, Indonesia, located on the equator, is among the poorest. The wealthiest nations of the world with the highest standard of living tend to be those at the northern extreme of areas open to human habitation, Canada, and the Nordic Countries. Within the wealthy continents, and even within large countries wealth increases with distance from the equator. Southern Europe has long been poorer as has the Southern United States. There have been a number of explanations for this phenomenon. The first to describe and try to assess it was the French philosophe Montesquieu who proposed that cold weather means less blood in the extremities, which makes the flesh less elastic; this gives northerners more strength and makes them less able to relax. This forcing of the blood inward, according to Montesquieu also means more flows through the heart, increasing vitality. These findings have been wholly discredited by modern science.

Evolutionary explanations

One explanation is grounded in evolutionary theory. Some have argued that as humans migrated into higher latitudes and encountered colder weather there, the cold weather forced the evolution of higher group intelligence by forcing inhabitants to perform more intellectually demanding tasks, such as building shelter, fires, and clothing, in order to survive (Lynn, 1991). One study that supports this notion was performed by Beals et al. (1984, p. 309), who found a correlation of 0.62 (p=0.00001) between latitude and cranial capacity in samples worldwide and reported that each degree of latitude was associated with an increase of 2.5 cm³ in cranial volume. Researchers such as J. Philippe Rushton have argued that the association of greater brain size with greater latitude is due to the fact that cold weather imposes on its inhabitants more cognitively demanding tasks such as the need to construct shelter, make clothing, and store food.

Non-evolutionary explanations

Another explanation that is still widely held is that modern technologies and institutions were designed primarily in a small area of north western Europe. Thus agricultural techniques, machines, and medicines were designed to suit a temperate climate. These technologies and models readily spread to areas of similar climate, such as North America and Australia. As these areas also became centres of innovation this bias was further enhanced. Vastly less effort has been put into improving tropical agriculture than temperate because of this. Technologies, from automobiles to power lines are designed for colder drier regions and tend to work far worse in the tropics. In simple words, the life in tropics doesn't create a strong natural urge for new technology development since life conditions in terms of basic body needs are comfortable enough. The colder the weather, the more life necessities are required for survival, which creates a strong motivation for ongoing innovative process. To make a comfortable life farther from tropics requires a more advanced technology. But once the life is comfortable enough, the 'innovation belt' is moving further from equator again. Thus, there exists a vicious cycle of technologies being designed for the wealthy, which makes them more wealthy and thus more able to fund technological development. One piece of evidence for this is that the far north has not always been the wealthiest latitude. Until only a few centuries ago, the wealthiest belt stretched from Southern Europe through the Middle East, northern India and southern China. A dramatic shift in technologies beginning with ocean going ships and culminating in the Industrial Revolution saw the most developed belt move north, in Europe, in China, and in the Americas. Northern Russia became a superpower while southern India became impoverished and colonized. Such dramatic changes argue that the current distribution of wealth is not due to immutable factors such as climate or race. Linked to this explanation is that of disease. The tropics are far more prone to devastating diseases due to their temperature that makes life easier on vectors such as insects and rodents. There has long been a malarial belt spanning the globe; this made human life more difficult. Most notably it was almost impossible for most forms of northern livestock to thrive. These problems have been compounded by the wealth of the north: vastly more research money goes into curing the ailments of northerners. Physiologist Jared Diamond, in his Pulitzer Prize-winning work Guns, Germs, and Steel, made the case that the Europe-Asia land mass is particularly favorable for the transition of societies from hunter-gatherer to farming communities. The continent stretches much further along the same lines of latitude than any of the other continents. As it is much easier to transfer a domesticated species along the same latitude than it is to move it to a warmer or colder climate, any species developed at a particular latitude will be transferred across the continent in a relatively short amount of time. Thus the inhabitants of this continent have a built-in advantage in terms of earlier development of farming, and a greater range of plants and animals from which to choose. He also linked this progression to the development of diseases that were later to threaten the inhabitants of other continents. The close association of people in Europe-Asia with their domesticated animals provided a vector for the rapid transmission of diseases. Inhabitants of lands with few domesticated species were never exposed to the same range of diseases, and so, at least on the American continents, succumbed to diseases introduced from Europe.

References

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-

External links

- [http://www.bcca.org/misc/qiblih/latlong.html Look-up Latitude and Longitude]
- [http://jan.ucc.nau.edu/~cvm/latlon_find_location.html Resources for determining your latitude and longitude]
- [http://geography.about.com/library/howto/htdegrees.htm Convert decimal degrees into degrees, minutes, seconds] - Info about decimal to sexagesimal conversion
- [http://sundials.org/links/local/pages/dd_dms.htm NASS - Convert degrees/minutes/seconds to decimal degrees and vice versa] - JavaScript version
- [http://www.marinewaypoints.com/learn/greatcircle.shtml Distance calculation based on latitude and longitude] - JavaScript version
- [http://www.cia.gov/cia/publications/factbook/reference_maps/pdf/political_world.pdf Zoomable version of the map] (pdf) Category:Navigation Category:Angle ja:緯度 th:ละติจูด

Zodiac

: This article is about the astrological concept. For other uses, see zodiac (disambiguation) zodiac (disambiguation) , medieval woodcuts]]The zodiac (from Greek zoon, "animal") is an imaginary belt in the heavens extending approximately 8 degrees on either side of the Sun's apparent path (the ecliptic), that includes the apparent paths of the Moon and the planets Mercury, Venus, Mars, Jupiter, Saturn. There is a zodiac in western astrology, a different one in Vedic astrology, and a very different one in Chinese astrology.

Historical origin

The origins of the zodiac lie in Sumer in Mesopotamia. See the History of astrology article for a detailed discussion.

Astronomy

In astronomy, the zodiac is the region of the sky close to the circle on which the orbital plane of our solar system intersects the celestial sphere. It includes the apparent path of the sun across the sky, known as the ecliptic, and the apparent paths of the naked eye planets which move in a zone just above and below this. It is a useful region of the sky to define, because it has practical implications for observations from the earth's surface. A bright object lying outside of the zodiacal region cannot be a planet. Polar observatories cannot easily observe the planets, because the zodiac is too close to the horizon. The zodiac is traditionally thought of as comprising a certain set of constellations. The constellations of both zodiacs are shown in the table below, including Ophiuchus, which was recognised as a zodiacal constellation at least as far back as Ptolemy's Almagest in the 2nd century. Most of Ophiuchus is north of the ecliptic - however, there are a few stars of Ophiuchus which are south of the ecliptic. Ptolemy recognised 4 of them, which are today known as 36 Oph, 42 θ Oph, 44 Oph and 51 Oph, and he recognised that they were south of the path of the sun through the sky. Thus, although the 1930 decision by the International Astronomical Union to adopt constellation boundaries is a common reason given by astrologers for the inconsistency, Ophiuchus as a recognised zodiacal constellation predates this IAU decision by at least 1,700 years. In modern astronomy, the zodiacal constellations, like all constellations, are recognized as chance visual groupings of stars, with no natural significance. In most cases they are not groupings of stars in three-dimensional space. In a few cases, parts of constellations are made up of stars that are close in space as well as in the sky. We see the sky without any perception of its depth; two stars that are neighbours in a constellation are usually three-dimensionally not close to each other. Star clusters and star systems are exceptions.

Astrology

In western astrology the zodiac is a band on the celestial sphere which contains the perceived paths of the Sun, Moon, and principal planets and is divided into twelve equal parts of 30° each, called "signs of the zodiac" or astrological signs, each named for a constellation. At the centre of this band is the plane of the ecliptic. The width of the zodiac allows for the fact that the orbits of the other bodies are inclined relative to plane of the ecliptic, and thus extend about 8º above and below the ecliptic. In western tropical astrology, the zodiacal year begins at the point where the plane of the ecliptic intersects with the earth's equatorial plane at the vernal equinox. This is when the Sun moves into the northern hemisphere of the earth's equatorial plane. Thus the Sun always enters the astrological zodiacal sign of Aries at the vernal equinox about March 20, but it will not cross into the astronomical constellation of the same name until nearly a month later. So in western tropical astrology, although the signs derive their names from the constellations, they are not the same thing. In sidereal astrology the zodiacal signs are aligned to their correct astronomical stellar constellations. The alignment still isn't precise, because the constellations all take up varying amounts of space along the ecliptic, whereas the zodiacal signs each take up exactly 30°.

Zodiacal Constellations

The zodiac includes twelve of the constellations that the ecliptic crosses. It actually crosses a thirteenth, Ophiuchus, but this constellation is not considered part of the zodiac (see above). Because the ecliptic lies in the general plane of the solar system, the Sun, Moon, and planets seem to move through the zodiacal constellations. A traditional mnemonic: : The Ram, the Bull, the Heavenly Twins, : And next' the Crab, the Lion shines, : The Virgin and the Scales. : The Scorpion, Archer, and the Goat, : The Man who holds the Watering Pot, : And Fish with glittering scales. The following table compares the dates that the sun enters a sign in tropical (Ptolemaic) and sidereal (Vedic) astrological systems, and the date when it enters the astronomical constellation by the same name. The dates can vary by as much as 2 days, from year to year, depending on the cycle of leap years, and the precession of the perihelion of the Earth's orbit over many centuries. The "tropical" zodiacal dates (those that divide the zodiac starting with the vernal equinox) are used primarily in the United States and western Europe. The "sidereal" dates (those that divide the zodiac starting with the alignment of the Sun with a distant star in Aries) are used primarily in India. Many people who don't know anything about constellations have heard about the signs of the Zodiac in an astrological context. Astrologers would use astronomical observations of the movements of the night sky for divinatory purposes. Some of these applications were founded on correspondences between practical knowledge and celestial observations (for example, the relationship between solar position and stellar positions depends on the season, which has practical implications for agriculture), whilst others have no natural source. The familiar "star sign", or more precisely the "sun sign", under which a person is born refers to the position of the sun in the signs on the tropical ecliptic at the time of his or her birth. Because of the precession of the equinoxes over the last three thousand years or so, the signs are out of phase with the astronomical constellations for which they are named by about a month. A few modern western astrologers, in common with those of India, cast horoscopes with reference to the actual constellations, rather than the signs. This is known as sidereal astrology. sidereal astrology The idea of an astrological birth sign is that the person would have some characteristics of the mythic symbolism that the ancients identified with that constellation; so, for example, a "Libra" (the scales) will be balanced and stable. Also, because the planets are all found in the zodiac, the position of the Moon or a planet in a particular sign would have an effect on the life of that person. For example: A person may be born on June 1st. This is near the center of the sign of Gemini, and so Gemini would be his sun sign. Any planets also observed near the center of Gemini, would be in "conjunction" with the sun, and said to have a particularly strong effect on the destiny and personality of the person. At the same time, other planets are in other signs of the zodiac, and their effects would be felt on the portions of a person's life "ruled" by that sign. Significance is also associated to the angular positions of planets and signs relative to each other at the moment of a birth or other significant event.

Angels of the zodiac

With the Christianization of Europe, angels were associated with the signs of the zodiac to replace the traditional gods. The traditional correlations are as follows:

The zodiac as a calendar

The concept of the zodiac was originated by the Babylonians certainly before 2000 BC as a method of visualizing the passage of time. The zodiac worked as a symbolic calendar. It was divided into twelve parts as suggested by the appearance of 12 moons in a year. The signs are geometric divisions of the celestial sphere, each corresponding to one twelfth of a year. The signs of the zodiac, as enumerated by Egyptian astronomer, Ptolemy, in the 2nd-century AD, are the ones we know today. The same names are used for both signs in astrology and for constellations in astronomy, but it's important to make a distinction between signs and constellations. Signs are geometric sections, each 30° wide, corresponding with particular periods of time of the year, but which don't necessarily physically correspond with the constellations of the same name. By the time of Ptolemy the zodiac was already at least two thousand years old. But together with its burgeoning astrological use the basic function and structure of the "calendar of the zodiac" remained. The sign of Aries marks the beginning of the year at the vernal equinox. The retreating crab in Cancer represents the retreat of the Sun from its farthest northern point at the time of the summer solstice. Leo, the symbol of fire, represents summer heat. The scales of Libra signify the balance between day and night at the autumnal equinox. The decline of the sun's power is represented in Scorpio by the scorpion, the symbol of darkness. The water-bearer, Aquarius, represents the rainy season which, in Egypt, meant the yearly flooding of the Nile. The fishes of Pisces, symbolize the return of life and the resumption of agriculture. The concept of the zodiac spread from Babylonia to Greece and, from there, to Egypt where the Egyptians substituted their own symbolism. Aries became the Fleece. Two Sprouting Plants replaced the twins of Gemini. Cancer was re-named Scarabaeus. Leo became the Knife and Libra the Mountain of the Sun. Sagittarius was reduced to just an arrow. Capricorn became the image of life, represented by a mirror. Scorpio became a serpent. Aquarius became simply water, while Taurus, Virgo and Pisces were not changed.

External links

- [http://www.char4u.com/chinese-zodiac-sign.php Chinese Zodiac Chart] Find your Chinese Zodiac sign based on your date of birth.
- [http://www.shadowdrake.com/celtic/celticzodiac.html Celtic Zodiac] — Folklore and history of the Celtic zodiac.
- [http://www.geocities.com/astrologyzodiacs/ Zodiacs] — A site detailing the various zodiacs in depth.
- [http://www.geocities.com/astrologyconstellations/ophiuchus.htm Ophiuchus] — The problem of Ophiuchus - the so-called 13th sign of the Zodiac.
- [http://free-horoscopes.typepad.com A guide to free horoscopes] with selected sites, lessons for beginners and horoscopes freeware.
- [http://www.griffithobs.org/SkyOphiuchus.html Born Under Ophiuchus and Ignored by the Horoscopes: A Modern Dilemma] — An article discussing the omission of Ophiuchus from the Zodiac by astrologers. Category:Constellations Category:Astrological factors category:pseudoscience ko:황도대 ja:十二宮 th:จักรราศี

Sirius

Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) is the brightest star in the nighttime sky, with a visual apparent magnitude of −1.46. It is located in the constellation Canis Major. Its name comes from the Latin sīrius, from Greek σείριος (seirios, "glowing" or "scorcher"). As the major star of the "Big Dog" constellation, it is often called the "Dog Star". Sirius can be seen from every inhabited region of the Earth's surface and, in the Northern Hemisphere, is known as a vertex of the Winter Triangle. At a distance of 2.6 pc or 8.57 light years (50.3 trillion miles), Sirius is also one of the nearest stars to Earth. The best time of year to view it is around January 1, when it reaches the meridian at midnight. It is a main sequence star of spectral type A0 or A1 and has a mass about 2.4 times that of the Sun. It is also known by the Latin name Canicula ("little dog") and as الشعرى aš-ši’rā in Arabic astronomy, from which the alternate name Aschere derives. Its closest large neighbour star is Procyon, 1.61 pc or 5.24 ly away. In 1909, Ejnar Hertzsprung suggested that Sirius was part of the Ursa Major Moving Group; however, more recent research by Jeremy King et al. at Clemson University in 2003 questions whether that is true, since the two components of Sirius appear to be too young.

Binary system

In 1844 Friedrich Wilhelm Bessel deduced that Sirius was actually a binary star. In 1862 Alvan Graham Clark discovered the companion, which is called Sirius B, or affectionately "the Pup". The visible star is now sometimes known as Sirius A. The two stars orbit each other with a separation of about 20 AU and a period of close to 50 years. In 1915 astronomers at the Mount Wilson Observatory determined that Sirius B was a white dwarf, the first to be discovered. Interestingly, this means that Sirius B must have originally been by far the more massive of the two, since it has already evolved off the main sequence. Robert Hanbury Brown measured the diameter of Sirius for the first time in 1956.

History

Historically, many cultures have attached special significance to Sirius. Sirius was worshipped as Sothis in the valley of the Nile long before Rome was founded, and many ancient Egyptian temples were oriented so that light from the star could penetrate to their inner altars. The Egyptians based their calendar on the heliacal rising of Sirius, which occurred just before the annual flooding of the Nile and the Summer solstice. In Greek mythology, Orion's dog became Sirius. The Greeks also associated Sirius with the heat of summer: they called it Σείριος Seirios, often translated "the scorcher." This also explains the phrase "dog days of summer". In the astrology of the Middle Ages, Sirius was a Behenian fixed star, associated with beryl and juniper. Its kabbalistic symbol Image:Agrippa1531_Canismaior.png was listed by Heinrich Cornelius Agrippa.

Mysteries

There are a few unsolved mysteries regarding Sirius.
- Firstly, apparent orbital irregularities in Sirius B have been observed since 1894, suggesting a third very small companion star, but this has never been definitely confirmed.
- Second, ancient observations of Sirius describe it as a red star, when today Sirius A is bluish white. The possibility that stellar evolution of either Sirius A or Sirius B could be responsible for this discrepancy is rejected by astronomers on the grounds that the timescale of thousands of years is too short and that there is no sign of the nebulosity in the system that would be expected had such a change taken place. Alternative explanations are either that the description as red is a poetic metaphor for ill fortune, or that the dramatic scintillations of the star when it was observed rising left the viewer with the impression that it was red.

The Dogons

It has been suggested that the Dogon tribe of Mali knew about unseen companion star(s) before they were discovered in the 19th century. This is a source of speculation for UFO enthusiasts and was the subject of the book The Sirius Mystery by Robert Temple. This work has been debunked by astronomer Carl Sagan, among others, as based on confabulation and selective evidence. Careful research reveals there was probably cultural contamination on the part of visiting astronomers who went to the region to observe a transit of Venus.

Sirius in Fiction

- In the Known Space series of stories by Larry Niven a high gravity world called "Jinx" is the moon of a gas giant in the Sirius system.
- In the fantasy novel "Dogsbody" by Diana Wynne Jones the star Sirius is an intelligent being falsely accused of murdering another star by his peers. As punishment he is sent to Earth in the body of a new born puppy to find the weapon he supposedly used.
- Sirius is also known as the planet/star/vortex/whatever Mental is supposedly on in the Serious Sam games.
- In the Harry Potter book series, the character Sirius Black, who can change shape into a large black dog, takes his name from the star.
- Sirius is the home star system of the "visitors" in the tv series "V"
- Sirius is part subject matter of the volume Cosmic Trigger I by Robert Anton Wilson
- Sirius is mentioned in Les Miserables by Victor Hugo, en passant
- The Sirius Cybernetics Corporation
- Tom Robbins' 1994 novel, Half Asleep in Frog Pajamas deals greatly with various Dogon/Sirius mysteries as well as much of the general mythology surrounding the star.
- There is also a metal & plastic yo-yo named after the Sirius star. It's manufactured by a company called [http://www.yoyojam.com/ YoYoJam] and is the signature model of Nathan Crissey

References and external links

- Benest, D., & Duvent, J. L. (1995, July). [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995A%26A...299..621B&db_key=AST&high=40dcacf76b29443 Is Sirius a triple star?] Astronomy and Astrophysics, 299, 621-628. (available at [http://adswww.harvard.edu/ The NASA Astrophysics Data System])
- Ridpath, Ian (1978) [http://www.csicop.org/si/7809/sirius.html Investigating the "Sirius Mystery"]. The Skeptical Inquirer. Retrieved July 11, 2005.
- [http://www.solstation.com/stars/sirius2.htm Detailed information on Sirius]
- [http://www.sankey.ws/siriustime.html Getting Sirius about time] Canis Majoris, Alpha Category:Binary stars Category:Canis Major constellation 48915 Category:Mythological dogs Category:White dwarfs Category:White main sequence stars ja:シリウス

Babylonians

Babylonia, named for the city of Babylon, was an ancient state in Mesopotamia (in modern Iraq), combining the territories of Sumer and Akkad. Its capital was Babylon. The earliest mention of Babylon can be found in a tablet of the reign of Sargon of Akkad, dating back to the 23rd century BC.

History

During the first centuries of the "Old Babylonian" period (that followed the Sumerian revival under Ur-III), kings and people in high position often had Amorite names, and supreme power rested at Isin. A constant intercourse was maintained between Babylonia and the West — with Babylonian officials and troops passing to Syria and Canaan, while "Amorite" colonists were established in Babylonia for the purposes of trade. One of these Amorites, Abi-ramu or Abram by name, is the father of a witness to a deed dated in the reign of Hammurabi's grandfather. The city of Babylon was given hegemony over Mesopotamia by their sixth ruler, Hammurabi (1780–1750 BC; dates highly uncertain). He was a very efficient ruler, giving the region stability after turbulent times, and transforming it into the central power of Mesopotamia. A great literary revival followed the recovery of Babylonian independence. One of the most important works of this "First Dynasty of Babylon", as it was called by the native historians, was the compilation of a code of laws. This was made by order of Hammurabi after the expulsion of the Elamites and the settlement of his kingdom. A copy of the Code of Hammurabi was found by J. de Morgan at Susa, where it had been taken as plunder, and is now in the Louvre. Ammiditana, the great-grandson of Hammurabi, still titled himself "king of the land of the Amorites", and his father and son bore the Canaanite names of Abieshuh and Ammisaduqa. The armies of Babylonia were well-disciplined, and they conquered the city-states of Isin, Elam, and Uruk, and the strong Kingdom of Mari. The rule of Babylon was even obeyed as far as the shores of the Mediterranean. But Mesopotamia had no clear boundaries, making it vulnerable to attack. Trade and culture thrived for 150 years, until the fall of Babylon in 1595 BC. The last king of the dynasty was Samsu-Ditana, son of Ammisaduqa. He was overthrown following the sack of Babylon in 1595 BC by the Hittite king Mursili I, and Babylonia was turned over to the Kassites (Kossaeans) from the mountains of Iran, with whom Samsu-Iluna had already come into conflict in his 6th year. The Kassite dynasty was founded by Kandis or Gandash of Mari. The Kassites renamed Babylon "Kar-Duniash", and their rule lasted for 576 years. With this foreign dominion — that offers a striking analogy to the contemporary rule of the Hyksos in Egypt — Babylonia lost its empire over western Asia. Syria and Canaan became independent, and the high-priests of Asshur made themselves kings of Assyria. Most divine attributes ascribed to the Semitic kings of Babylonia disappeared at this time; the title of "god" was never given to a Kassite sovereign. However, Babylon continued to be the capital of the kingdom and the 'holy' city of western Asia, where the priests were all-powerful, and the only place where the right to inheritance of the old Babylonian empire could be conferred.

Neo-Babylonian Empire

Through the centuries of Assyrian domination, Babylonia enjoyed a prominent status, or revolting at the slightest indication that it did not. However, the Assyrians always managed to restore Babylonian loyalty, whether through granting of increased privileges, or militarily. That finally changed in 627 BC with the death of the last strong Assyrian ruler, Ashurbanipal, and Babylonia rebelled under Nabopolassar the Chaldean the following year. With help from the Medes, Niniveh was sacked in 612, and the seat of empire was again transferred to Babylonia. Nabopolassar was followed by his son Nebuchadnezzar II, whose reign of 43 years made Babylon once more the mistress of the civilized world. Only a small fragment of his annals has been discovered, relating to his invasion of Egypt in 567 BC, and referring to "Phut of the Ionians". Of the reign of the last Babylonian king, Nabonidus (Nabu-na'id), and the conquest of Babylonia by Cyrus, there is a fair amount of information available. This is chiefly derived from a chronological tablet containing the annals of Nabonidus, supplemented by another inscription of Nabonidus where he recounts his restoration of the temple of the Moon-god at Harran; as well as by a proclamation of Cyrus issued shortly after his formal recognition as king of Babylonia. It was in the sixth year of Nabonidus (549 BC) that Cyrus, the Achaemenid Persian "king of Anshan" in Elam, revolted against his suzerain Astyages, "king of the Manda" or Medes, at Ecbatana. Astyages' army betrayed him to his enemy, and Cyrus established himself at Ecbatana, thus putting an end to the empire of the Medes. Three years later Cyrus had become king of all Persia, and was engaged in a campaign in the north of Mesopotamia. Meanwhile, Nabonidus had established a camp in the desert, near the southern frontier of his kingdom, leaving his son Belshazzar (Belsharutsur) in command of the army. In 538 BC Cyrus invaded Babylonia. A battle was fought at Opis in the month of June, where the Babylonians were defeated; and immediately afterwards Sippara surrendered to the invader. Nabonidus fled to Babylon, where he was pursued by Gobryas, the governor of Kurdistan, and on the 16th of Tammuz, two days after the capture of Sippara, "the soldiers of Cyrus entered Babylon without fighting." Nabonidus was dragged from his hiding-place, and Kurdish guards were placed at the gates of the great temple of Bel, where the services continued without interruption. Cyrus did not arrive until the 3rd of Marchesvan (October), Gobryas having acted for him in his absence. Gobryas was now made governor of the province of Babylon, and a few days afterwards the son of Nabonidus died. A public mourning followed, lasting six days, and Cambyses accompanied the corpse to the tomb. Cyrus now claimed to be the legitimate successor of the ancient Babylonian kings and the avenger of Bel-Marduk, who was assumed to be wrathful at the impiety of Nabonidus in removing the images of the local gods from their ancestral shrines, to his capital Babylon. Nabonidus, in fact, had excited a strong feeling against himself by attempting to centralize the religion of Babylonia in the temple of Merodach (Marduk) at Babylon, and while he had thus alienated the local priesthoods, the military party despised him on account of his antiquarian tastes. He seems to have left the defence of his kingdom to others, occupying himself with the more congenial work of excavating the foundation records of the temples and determining the dates of their builders. The invasion of Babylonia by Cyrus was doubtless facilitated by the existence of a disaffected party in the state, as well as by the presence of foreign exiles like the Jews, who had been planted in the midst of the country. One of the first acts of Cyrus accordingly was to allow these exiles to return to their own homes, carrying with them the images of their gods and their sacred vessels. The permission to do so was embodied in a proclamation, whereby the conqueror endeavoured to justify his claim to the Babylonian throne. The feeling was still strong that none had a right to rule over western Asia until he had been consecrated to the office by Bel and his priests; and accordingly, Cyrus henceforth assumed the imperial title of "king of Babylon." A year before Cyrus' death, in 529 BC, he elevated his son Cambyses II in the government, making him king of Babylon, while he reserved for himself the fuller title of "king of the (other) provinces" of the empire. It was only when Darius Hystaspis ("the Magian") acquired the Persian throne and ruled it as a representative of the Zoroastrian religion, that the old tradition was broken and the claim of Babylon to confer legitimacy on the rulers of western Asia ceased to be acknowledged. Darius, in fact, entered Babylon as a conqueror. After the murder of Darius, it briefly recovered its independence under Nidinta-Bel, who took the name of Nebuchadnezzar III, and reigned from October 521 BC to August 520 BC, when the Persians took it by storm. A few years later, probably 514 BC, Babylon again revolted under Arakha; on this occasion, after its capture by the Persians, the walls were partly destroyed. E-Saggila, the great temple of Bel, however, still continued to be kept in repair and to be a center of Babylonian patriotism, until at last the foundation of Seleucia diverted the population to the new capital of Babylonia and the ruins of the old city became a quarry for the builders of the new seat of government.

Science and mathematics

Among the sciences, astronomy and astrology occupied a conspicuous place in Babylonian society. Astronomy was of old standing in Babylonia, and the standard work on the subject, written from an astrological point of view, later translated into Greek by Berossus, was believed to date from the age of Sargon of Akkad. The zodiac was a Babylonian invention of great antiquity; and eclipses of the sun and moon could be foretold. Observatories were attached to the temples, and reports were regularly sent by astronomers to the king. The stars had been numbered and named at an early date, and we possess tables of lunar longitudes and observations of the phases of Venus. Great attention was naturally paid to the calendar, and we find a week of seven days and another of five days in use. In Seleucid and Parthian times, the astronomical reports were of a thoroughly scientific character; how much earlier their advanced knowledge and methods were developed is uncertain. The development of astronomy implies considerable progress in mathematics; it is not surprising that the Babylonians should have invented an extremely simple method of ciphering, or have discovered the convenience of the duodecimal system. The ner of 600 and the sar of 3600 were formed from the unit of 60, corresponding with a degree of the equator. Tablets of squares and cubes, calculated from 1 to 60, have been found at Senkera, and a people acquainted with the sun-dial, the clepsydra, the lever and the pulley, must have had no mean knowledge of mechanics. A crystal lens, turned on the lathe, was discovered by Austen Henry Layard at Nimrud along with glass vases bearing the name of Sargon; this could explain the excessive minuteness of some of the writing on the Assyrian tablets, and a lens may also have been used in the observation of the heavens. The Babylonian system of mathematics was sexagesimal, or a base 60 numeral system (see: Babylonian numerals). From this we derive the modern day usage of 60 seconds in a minute, 60 minutes in an hour, and 360 (60 x 6) degrees in a circle. The Babylonians were able to make great advances in mathematics for two reasons. First, the number 60 has many divisors (2, 3, 4, 5, 6, 10, 12, 15, 20, and 30), making calculations easier. Additionally, unlike the Egyptians and Romans, the Babylonians had a true place-value system, where digits written in the left column represented larger values (much as in our base ten system: 734 = 7×100 + 3×10 + 4×1). Among the Babylonians mathematical accomplishments were the determination of the square root of two correctly to seven places ([http://it.stlawu.edu/%7Edmelvill/mesomath/tablets/YBC7289.html YBC 7289 clay tablet]). They also demonstrated knowledge of the Pythagorean theorem well before Pythagoras, as evidenced by [http://www.tmeg.com/bab_mat/bab_mat.htm this tablet] translated by Dennis Ramsey and dating to c. 1900 BC: 4 is the length and 5 is the diagonal. What is the breadth? Its size is not known. 4 times 4 is 16. 5 times 5 is 25. You take 16 from 25 and there remains 9. What times what shall I take in order to get 9? 3 times 3 is 9. 3 is the breadth.

Literature

:Main article: Babylonian literature

Location

The city of Babylon, the main city of Babylonia, was found on the Euphrates River, about 110 kilometres south of modern Baghdad, just north of what is now the Iraqi town of al-Hillah.

External links

- [http://ancientneareast.tripod.com/Old_Kingdom_of_Babylonia.html The History of the Ancient Near East]
- [http://www.math.tamu.edu/~don.allen/history/babylon/babylon.html Babylonian Mathematics]
- [http://www-groups.dcs.st-andrews.ac.uk/~history/HistTopics/Babylonian_numerals.html Babylonian Numerals]
- [http://www.halloran.com/babylon1.htm Babylonian Astronomy/Astrology]
- [http://www.phys.uu.nl/~vgent/babylon/babybibl.htm Bibliography of Babylonian Astronomy/Astrology]
- [http://www.sacred-texts.com/ane/rbaa.htm The Religion of Babylonia and Assyria by Theophilus G. Pinches (Many deities' names are now read differently, but this detailed 1906 Work is a classic)]
- [http://fax.libs.uga.edu/BM530xK531l/ Legends of Babylon and Egypt in Relation to Hebrew Tradition], by Leonard W. King, 1918 (a searchable facsimile at the University of Georgia Libraries; DjVu & [http://fax.libs.uga.edu/BM530xK531l/1f/legends_of_babylon_and_egypt.pdf layered PDF] format)
- [http://fax.libs.uga.edu/BL1620xB7/ The Babylonian Legends of the Creation] and the Fight between Bel and the Dragon, as told by Assyrian Tablets from Nineveh, 1921 (a searchable facsimile at the University of Georgia Libraries; DjVu & [http://fax.libs.uga.edu/BL1620xB7/1f/babylonian_legends_of_creation.pdf layered PDF] format) Category:Former monarchies Category:Civilizations Category:Ancient Iranian provinces ja:バビロニア

Maori

Māori is the name of the indigenous people of New Zealand, and their language. It is also the name of the people and language of the Cook Islands, referred to as Cook Islands Māori. The word māori means "normal" or "ordinary" in the Māori language and denotes mortal beings as distinct from the gods. "Māori" has similarities in some other Polynesian languages such as Hawaiian in which the cognate word maoli means native, indigenous, real or actual. Māori often refer to themselves as tāngata whenua (literally "people of the land") to emphasise their indigenous status.

Māori arrival in New Zealand

It is not precisely known when Māori arrived. Polynesian voyagers are believed to have migrated to what is now New Zealand from eastern Polynesia in the latter part of the 1st millennium. As th

Heliacal rising

Star

Star formation and evolution

Appearance and distribution of stars

Age and size of stars

Star classification

Naming of stars

Energy production

Nuclear fusion reaction pathways

Star mythology

References

See also

Related lists

External links

Planet

Planetary formation

Within our solar system

Accepted planets

Other candidates

Extrasolar planets

Brown dwarf "planets"

Interstellar planets

Definition and classification of planets

Suggested wide definitions

Suggested narrow definitions

Further classification

See also

References

External links

Horizon

See also

External links

Acknowledgements

Latitude

Lines of latitude

Subdivisions

Important latitudes

Effect of latitude

Types of latitude

Common "latitude"

Reduced latitude

Authalic latitude

Rectifying latitude

Conformal latitude

Geocentric latitude

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Latitude and wealth

Evolutionary explanations

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Further reading

See also

References

External links

Zodiac

Historical origin

Astronomy

Astrology

Zodiacal Constellations

Angels of the zodiac

The zodiac as a calendar

See also

External links

Sirius

Binary system

History

Mysteries

The Dogons

Sirius in Fiction

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References and external links

Babylonians

History

Neo-Babylonian Empire

Science and mathematics

Literature

Location

See also

Further reading

External links

Maori