An Overview of the Solar System
The solar system consists of the Sun; the nine planets, over 100 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are also many more planetary satellites that have been discovered but not yet been officially named.) The inner solar system contains the Sun, Mercury, Venus, Earth and Mars:
The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. Pluto's orbit deviates the most from the plane of the ecliptic with an inclination of 17 degrees. The above diagrams show the relative sizes of the orbits of the nine planets from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.
The above composite shows the nine planets with approximately correct relative sizes.
One way to help visualize the relative sizes in the solar system is to imagine a model in which it is reduced in size by a factor of a billion (1e9). Then the Earth is about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40000 km away.
Not shown in the above illustrations are the numerous smaller bodies that inhabit the solar system: the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; and the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids.
The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits).
Unfortunately, the solar system has been found to be more complicated than this would suggest:
Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions. The bottom line is that many of the bodies are unique; our present understanding is insufficient to establish clear categories. In the pages that follow, I will use the conventional categorizations.
The nine bodies conventionally referred to as planets are often further classified in several ways:
by position relative to the Sun:
by position relative to Earth:
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