Today we undestand all that very well (see image above). Planets are spherical objects like Earth--Venus, Mercury and Mars are smaller, Jupiter and Saturn much bigger. Earth is a planet too and others exist as well (too faint to be seen without a telescopes), all orbiting the sun on or near the plane of the ecliptic. Their speed however varies--the closer to the Sun, the faster (see section 10a and in particular Kepler's third law). Therefore, when the three outer planets are near opposition, the Earth orbiting closer to the Sun overtakes them, and they
seem to move backwards.
The retrograde motion of the two inner planets has a similar cause. Being closer to the Sun, they overtake the Earth in their motion.
Summary of what is now known about Planets
A section is being added to "Stargazers" about planets of the solar system, but for completeness, here is a quick summary of the components of the solar system--besides the fact that all of them orbit the Sun, including Earth and two major planets too dim for ancient astronomers to have seen. Four distinct classes of objects are usually recognized:
Major planets, in order of distance from the Sun--Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. All but the inner two have satellites, and all four outer ones have rings as well, composed of small orbiting chunks of matter.
Asteroids or minor planets, most but not all between Mars and Jupiter. Ranging in diameter up to about 500 km.
The "Kuiper Belt" of icy objects outside the orbit of Neptune, of which the best known (though as of now only the second largest) is Pluto, discovered in 1930 and about the size of our Moon. The belt is named after the Belgian astronomer Gerard Kuiper, may extend to twice the distance of Neptune and is estimated to consist of as many as 100,000 objects (about 1000 of them identified so far), many only 100 km across or smaller.
Comets, traditionally divided into "non returning" (official name, "long period comets") and "periodic" ones." Non-returning comets are believed to come from the "Oort cloud," a huge near-spherical collection of frozen chunks on the distant fringes of the solar system. They are loosely bound to the Sun, and now and then the gravity of a distant star is believed to slightly change the motion of some and send them sunwards. They become visible as comets when sunlight evaporates some of their surface to create the comet's glow and tail.
Periodic comets were once believed to have started as non-returning ones but to have been diverted by the pull of one of the larger planets. They are now widely held to arrive from the Kuiper belt as a class of objects known as Centaurs.
Early History, False Leads
As noted earlier, Aristarchus of Samos proposed that the Earth revolved around the Sun, but the idea was rejected by later Greek astronomers, in particular by Hipparchus. Ptolemy, living in Egypt in the 2nd century AD, expressed the consensus when he argued that all fixed stars were on some distant sphere which rotated around the Earth. Ptolemy tried to assemble and write down all that was known in his day about the heavens in "The Great Treatise," now known as the "Almagest," a corruption of its Arab name. (An annotated translation by G.J. Toomre was published in 1984 by Princeton University Press and is now available in paperback for $39.50. See p. 120, Nature vol. 397, 14 January 1999.)
To explain the motion of planets, Ptolemy used a theory which started with Hipparchus. Following the work of Aristarchus and Hipparchus, it was already accepted that the Moon moved around Earth. Ptolemy assumed that the Sun, planets and the distant stars (whatever those were) also moved around the Earth (For a dissenting opinion, see here). To the Greeks, the circle represented perfection, and Ptolemy assumed Moon, Sun and stars moved in circles too. Since the motion was not exactly uniform (later explained by Kepler's laws, sect.#12a), he assumed that these circles were centered some distance away from the Earth.
While the Sun moved around Earth, Venus and Mercury obviously moved around it, on circles of their own, centered near the Sun. But what about Mars, Jupiter and Saturn? Cleverly, Ptolemy proposed that like Venus and Mercury, each of them also rotated around a point in the sky that orbited around Earth like the Sun, except that those points were empty. The backtracking of the planets now looked similar to the backtracking of Venus and Mercury. The center carrying each of those planets accounted for the planet's regular motion, but to this the planet's own motion around that center had to be added, and sometimes the sum of the two made the planet appear (for a while) to advance backwards.
The main motivation behind early models of planetary motion was astrology, the (superstitious) divining of human fate from positions of planets at pivotal moments, e.g. birth. The models left open the question what the planets, the Sun and Moon were, and why they displayed their strange motions. But worse, they also were inaccurate.
Yet Ptolemy's view of the solar system dominated European astronomy for over 1000 years. One reason was that astronomy almost stopped in its tracks during the decline and fall of the Roman Empire and during the "dark ages" that followed. The study of the heavens continued in the Arab world, under Arab rulers, but of all the achievements of Arab astronomers, the one which exerted the greatest influence was the preservation and translation of Ptolemy's books, and thus of their erroneous views.
Teachers using this web page and the one following will find a related lesson plan at Lsolsys.htm
It belongs to a set of lesson plans whose home page is at Lintro.htm.
Questions from Users:
Ptolemaic Theory of Planet Motion