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website MapLesson planGlossary TimelineQuestions & AnswersCentral home Page (9b) The Planets Index7.Precession8. The ring Earth8a. The Horizon8b. Parallax8c. Moon dist. (1)8d. Moon dist. (2) 9a. Earth orbits Sun? 9b. The Planets 9c. Copernicus to Galileo10. Kepler"s LawsKepler"s laws (For teachers)10a. Scale of Solar Sys.11. Graphs & Ellipses11a. Ellipses and very first Law 12. 2nd Law many stars we observe type fixed constellations in the sky, undergoing daily motion (e.g. Rising and setting) but maintaining resolved positions loved one to every other--like the stars of Orion, or the big Dipper. The ancients however noted that 5 stars constantly moved--all adhering to close come the routes of the Sun and Moon across the heavens, i.e., close come the ecliptic. The Greeks referred to as them planets, i.e. Wanderes, a name still used.

Venus and also Mercury

The five planets recognized to the ancients were named after major Greek gods, later on replaced through their roman inn equivalents: Mercury, Venus, Mars, Jupiter and also Saturn. Castle were reasonably bright--Venus and also Jupiter deserve to be brighter than any fixed star--though their brightness seemed to vary. Venus and Mercury never show up far indigenous the Sun and (outside the polar regions, at least) are just visible just after sunset or before sunrise, arguing that those planets to be confined close to the Sun. The Greeks called Venus "Hesperus" when it showed up as the night star and "Phosphorus" once as morning star it rose prior to sunrise, despite they establish both were the same object. Mercury, i m sorry is fainter and closer to the Sun, is an especially hard come detect by eye, and also only once its visible position is much from the Sun"s. Every planets appeared to move past background the stars in the very same direction together the Moon (and of the Sun)--with one strange variation: sometimes their evident motion is temporarily reversed ("retrograde motion"). That is most obvious with Mercury and also Venus, i m sorry shuttle back and forth across the position of the Sun. Together the sunlight moves among the stars--along the constellations of the zodiac--these planets sometimes move the same method and include their movement to that of the Sun, yet sometimes their apparent motion opposes the one of the Sun, leading to them to seem to relocate backwards or "retrograde."

Mars, Jupiter and also Saturn

The other three planets clearly shows to the eye deserve to be seen almost everywhere along the ecliptic--even in ~ midnight, directly opposite the Sun, i beg your pardon was when they show up brightest. Mars appears to move the fastest, Jupiter next, and Saturn the slowest. But all exhibit that puzzling quirk--near the allude of their celestial path precisely opposite the sunlight ("opposition"), their motion amongst the stars temporarily transforms around.
Schematic drawing of the apparent reversal of motion (retrograde motion) observed through Mars. Positions 1...7 that the earth correspond to location 1...7 of Mars, i beg your pardon moves more slowly. As the earth overtakes Mars (positions 4 and also 5) the planet"s position in the sky moves backwards.
now we undestand all that very well (see photo above). Planets room spherical objects choose Earth--Venus, Mercury and Mars space smaller, Jupiter and Saturn lot bigger. Earth is a world too and also others exist too (too pass out to be viewed without a telescopes), all orbiting the sun on or close to the aircraft of the ecliptic. Their speed but varies--the closer to the Sun, the faster (see section 10a and also in specific Kepler"s 3rd law). Therefore, once the three external planets are close to opposition, the earth orbiting closer come the sunlight overtakes them, and also they seem to relocate backwards. The retrograde activity of the two inner planets has actually a similar cause. Gift closer come the Sun, lock overtake the planet in your motion.

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Summary of what is now known about Planets

A collection of 14 sections included to "Stargazers" in 2008 describes planets of the solar system, but for completeness, below is a quick an overview of the contents of the solar system. A full of 8 major planets orbit the Sun, consisting of Uranus and also Neptune which space too dim because that the unaided eye, and likewise Earth (Pluto is no much longer counted here). Four distinct classes that objects room usually recognized: major planets, in stimulate of distance from the Sun--Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and also Neptune. All however the inner two have satellites, and also all 4 outer ones have rings as well, composed of little orbiting chunks the matter. Asteroids or young planets, most however not all in between Mars and Jupiter. Ranging in diameter up to about 500 km. The "Kuiper Belt" the icy objects exterior the orbit of Neptune, of i beg your pardon the best known (though as of currently only the second largest) is Pluto, uncovered in 1930 and about the size of our Moon. The belt is called after the Belgian astronomer Gerard Kuiper, may extend to double the street of Neptune and is estimated to covers as many as 100,000 objects (about 1000 that them determined so far), many only 100 km throughout or smaller. Comets, traditionally divided into "non returning" (official name, "long period comets") and also "periodic" ones." Non-returning comets are thought to come indigenous the "Oort cloud," a vast near-spherical arsenal of frozen chunks top top the remote fringes that the solar system. They room loosely bound to the Sun, and now and also then the gravity of a remote star is believed to slightly adjust the movement of some and send them sunwards. They come to be visible together comets when sunlight evaporates several of their surface ar to produce the comet"s glow and tail. Periodic comets to be once believed to have started together non-returning ones but to have actually been diverted by the pull of one of the larger planets. Castle are currently widely organized to arrive from the Kuiper belt as a class of objects recognized as Centaurs.

Early History, False Leads

As detailed earlier, Aristarchus of Samos proposed the the planet revolved roughly the Sun, yet the idea was rejected by later Greek astronomers, in certain by Hipparchus. Ptolemy, life in Egypt in the second century AD, express the consensus when he argued that all resolved stars to be on some remote sphere i m sorry rotated roughly the Earth. Ptolemy tried to assemble and write under all the was well-known in his day about the heavens in "The good Treatise," now known as the "Almagest," a corruption the its Arab name. (An annotated translate into by G.J. Toomre was published in 1984 by Princeton college Press and is now available in paperback because that $39.50. Watch p. 120, Nature vol. 397, 14 January 1999.)

To describe the activity of planets, Ptolemy provided a theory which began with Hipparchus. The work of Aristarchus and also Hipparchus had already established the the Moon moved around Earth. Ptolemy assumed that the Sun, planets and the distant stars (whatever those were) additionally moved roughly the planet . To the Greeks, the circle represented perfection, and also Ptolemy suspect Moon, Sun and also stars moved in circles too. Due to the fact that the activity was sometimes slower, sometimes quicker , the assumed that these one were centered some distance away from the Earth.

when the sunlight moved approximately Earth, planets relocated on circles around points the rotated approximately Earth when a year (today we realize Venus and Mercury in reality orbit the Sun, i m sorry is why their positions never ever stray much from it). However what around Mars, Jupiter and Saturn? Cleverly, Ptolemy suggest that like Venus and Mercury, they likewise rotated approximately points in the skies that were carried approximately Earth, however with much longer periods. The facility carrying each of those planets accounted because that the planet"s typical motion, however to this the planet"s own motion roughly its facility had to it is in added, and also sometimes the sum of the two made the planet show up (for a while) to breakthrough backwards. The backtracking that the planets now hence explained.

The main an ideas behind beforehand models that planetary movement was astrology, the superstitious divining of human fate from location of planets at key moments, e.g. Birth. The models left open the inquiries of what
the planets were, what the Sun and also Moon were, and why they displayed their weird motions. Worse, lock were additionally somewhat inaccurate.

however Ptolemy"s see of the solar system conquered European astronomy for over 1000 years. One factor was that astronomy virtually stopped emerging during the decline and fall of the roman inn Empire and also during the "dark ages" the followed. The examine of the heavens ongoing in the Arab world, under Arab rulers, yet of all the achievements of Arab astronomers, the one which exerted the biggest influence was the preservation and translation that Ptolemy"s books, and also thus of their erroneous views.

Teachers using this web page and also the one complying with will uncover a associated lesson arrangement at Lsolsys.htm that belongs come a set of lesson plans whose home page is at Lintro.htm.Questions indigenous Users: *** Ptolemaic theory of Planet motion

The very first of 14 part on members of the solar system: (P-1) The Solar System--Links and Tables Or else, the key story proceeds at: #9c The discovery of the Solar device Timeline Glossary back to the grasp ListAuthor and also Curator: Dr. David P. SternMail to Dr.Stern: stargaze("at" symbol) .Last updated: 2 April 2014