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(9b) The Planets



8. The Round Earth

  8a. The Horizon

  8b. Parallax

8c. Moon dist. (1)

8d. Moon dist. (2)

9a. Earth orbits Sun?

9b. The Planets

9c. Copernicus
        to Galileo

10. Kepler's Laws

Kepler's Laws
        (For teachers)

10a. Scale of Solar Sys.

11. Graphs & Ellipses

11a. Ellipses
        and First Law

12. Second Law
    Most stars we observe form fixed constellations in the sky, undergoing daily motion (e.g. rising and setting) but maintaining fixed positions relative to each other--like the stars of Orion, or the Big Dipper. The ancients however noted that 5 stars constantly moved--all following close to the paths of the Sun and Moon across the heavens, i.e., close to the ecliptic. The Greeks called them planets, i.e. wanderes, a name still used.

Venus and Mercury

    The five planets known to the ancients were named after principal Greek gods, later replaced by their Roman equivalents: Mercury, Venus, Mars, Jupiter and Saturn. They were relatively bright--Venus and Jupiter can be brighter than any fixed star--though their brightness seemed to vary. Venus and Mercury never appear far from the Sun and (outside the polar regions, at least) are only visible just after sunset or before sunrise, suggesting that those planets were confined near the Sun. The Greeks called Venus "Hesperus" when it appeared as the evening star and "Phosphorus" when as morning star it rose before sunrise, though they realized both were the same object. Mercury, which is fainter and closer to the Sun, is particularly hard to detect by eye, and only when its visible position is far from the Sun's.

    All planets seemed to move past background the stars in the same direction as the Moon (and of the Sun)--with one strange variation: sometimes their apparent motion is temporarily reversed ("retrograde motion"). That is most evident with Mercury and Venus, which shuttle back and forth across the position of the Sun. As the Sun moves among the stars--along the constellations of the zodiac--these planets sometimes move the same way and add their motion to that of the Sun, but sometimes their apparent motion opposes the one of the Sun, causing them to seem to move backwards or "retrograde."

Mars, Jupiter and Saturn

The other three planets visible to the eye can be seen anywhere along the ecliptic--even at midnight, directly opposite the Sun, which was when they appear brightest. Mars seems to move the fastest, Jupiter next, and Saturn the slowest. But all exhibit that puzzling quirk--near the point of their celestial path exactly opposite the Sun ("opposition"), their motion among the stars temporarily turns around.

    Schematic drawing of the apparent reversal of motion (retrograde motion) observed with Mars.
    Positions 1...7 of the Earth correspond to positions 1...7 of Mars, which moves more slowly. As the Earth overtakes Mars (positions 4 and 5) the planet's position in the sky moves backwards.

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 set of 14 sections added to "Stargazers" in 2008 describes planets of the solar system, but for completeness, here is a quick summary of the components of the solar system. A total of 8 major planets orbit the Sun, including Uranus and Neptune which are too dim for the unaided eye, and also Earth (Pluto is no longer counted here). Four distinct classes of objects are usually recognized:    
  1.   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.

  2.   Asteroids or minor planets, most but not all between Mars and Jupiter. Ranging in diameter up to about 500 km.

  3.   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.

  4.   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. The work of Aristarchus and Hipparchus had already established that the Moon moved around Earth.   Ptolemy assumed that the Sun, planets and the distant stars (whatever those were) also moved around the Earth [but not everyone agreed ]. To the Greeks, the circle represented perfection, and Ptolemy assumed Moon, Sun and stars moved in circles too. Since the motion was sometimes slower, sometimes faster [as later expressed in Kepler's 2nd law, sect.#12a], he assumed that these circles were centered some distance away from the Earth.

    While the Sun moved around Earth, planets moved on circles around points that rotated around Earth once a year (today we realize Venus and Mercury actually orbit the Sun, which is why their positions never stray far from it). But what about Mars, Jupiter and Saturn? Cleverly, Ptolemy proposed that like Venus and Mercury, they also rotated around points in the sky that were carried around Earth, but with longer periods. The center carrying each of those planets accounted for the planet's average motion, but to this the planet's own motion around its center had to be added, and sometimes the sum of the two made the planet appear (for a while) to advance backwards. The backtracking of the planets now thus explained.

    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 questions of what the planets were, what the Sun and Moon were, and why they displayed their strange motions. Worse, they were also somewhat inaccurate.

    Yet Ptolemy's view of the solar system dominated European astronomy for over 1000 years. One reason was that astronomy almost stopped developing 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

The first of 14 sections on members of the solar system:
(P-1) The Solar System--Links and Tables

        Or else, the main story continues at:
#9c   The Discovery of the Solar System

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Author and Curator:   Dr. David P. Stern
     Mail to Dr.Stern:   stargaze("at" symbol) .

Last updated: 2 April 2014