Page 302 - NS-2 Textbook

The Planets

There are nine traditional known planets in our solar sys- The planets Venus, Mars, Jupiter, and Saturn are
tem, plus several recently discovered and as yet wmamed sometinles ca!led the "Big Four" because they are so eas-
possible additional ones in the Kuiper comet belt in the ily visible. Since its orbit is nearer the Sun than Earth's
outer reaches of the solar system. In order outward from orbit, Venus can oniy be seen in the western sky just after
the Sun, they are Mercwy, Venus, Earth, Mars, Jupitel~ Sat- sunset or in the eastern sky before sunrise, because it is
urn, Uranus, Neptune, and Pluto. Planets circle the SWl in always near the Sun as we look at it. It is thus called the
regular orbits, and in iliat respect they are similar to Earth. evening star or 1Il0ming star. Mars, Jupitel~ and Saturn
Venus and Mars have some additional similarities to Earili, have orbits that lie outside that of Earth. Thus they can
but the other planets are quite different. MerctllY scormes be seen all night. The Big Four are brighter than any of
under the intense rays of the SW1. TIle outer planets are the stars and do not twinkle as stars do. Because of their
sh'ange cold worlds, surrounded by poisonous ainlos- relative nearness, they appear as discs instead of points
pheric gases and chemicals uncommon on Earth. of light. The planets, like our Moon, shine only by re-
The planets are wanderers in the sky; the word flected sunlight.
"planet" actually means "wanderer." They are ca!led that TIle tinle it takes a planet to go around the Sun is
because they are constantly moving about the SWl in called its orbital period. Sometinles the planets, as seen
their orbits. Since they are moving, it is difficult to keep from Earth, seem to go backward in their orbits-that is,
h'ack of them without some sort of chart. A chart that east to west. This backing up, or retrograde Illation, is eas-
serves as a tinletable for the movement and location of ily explained. The best example of obsen'able retrograde
the planets is called an almfl1mc or an ephemeris. motion is with the planet Mars. Mars travels slower than
The planets all orbit the Sun in the same direction Earth does, since it is farther from the Sun. So when
and generally in the same plane. Earth's orbit about Earth comes along on its orbit, it catches up to Mars and
the Sun, called the plane of the ecliptic, is the usual refer- overtakes it, like a car passing another on a highway.
ence to which all the oilier orbital planes are compared. Mars then seems to be moving in retrograde motion
The planets' orbits around the Sun are each in the shape (backward) as viewed from Earth.
of an ellipse (an egg shape), just a slight variation from a All the planets except MercUlY and Venus have satel-
circle. The Sun is located at one focus point of these el- lite moons. Earth and Pluto each have one, Mars has two,
lipses. The gravitational pull of the Sun keeps the planets and the large outer planets each have many, with addi-
in their orbits. Were it not for this gravity, the planets tional ones being rapidly discovered. Many of the outer
would continue moving in a straight line. You might planets' moons were discovered and photographed by
compare the orbital plane to an old phonograph record- the Pioneer and Voyager spacecraft in the 1980s, by the
the Sun is at the center, and each planet's orbit falls into Galileo spacecraft in the 1990s, and more recently by the
its own groove ouhvard from the center. Cassill; spacecraft orbiting Sahlrn. Some of these were
As the planets travel in their elliptical paths, they also observed by the Hubble Space Telescope, as well as by
sweep out equal pie-shaped areas in their orbital planes several of the larger terrestrial telescopes.
with equal tinles. Thus, when the planets are closer to the
Sun, they are traveling faster in ilieir orbits. When they
are farther away, they are traveling slowe" MERCURY
Although planets are much smaller than stars, they
are also much closer to us, so that a telescope can mag- Mercury is the smallest of the inner planets in the solar
nify them. Five planets can be seen without a telescope: system. Its temperature is so high on the sunny side that
MerclU'YJ Venus, Mars, JupiterJ and SatLU'n. Uranus is just it has no atmosphere. The temperature is 800 degrees F
at the limit of visibility, and Neptune, Pluto, and the new on the lighted side and -300 degrees F on the dark side.
tenth planet can be seen only with powerful telescopes. Since there is no atmosphere, there is no erosion.

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