.

             The planets are also classified by its position to the Earth. The planets are placed in three groups when using this method, inferior, Earth and superior. The inferior planets are Mercury and Venus. They are located closer to the Sun than Earth, and they show phases like the moon"s when viewed from the Earth. The superior planets are Mars through Pluto. They are further away from the Sun than the earth is, and they always appear full or nearly so. .

             The last method used to classify the nine planets is by history. This method is also .

             classified into three groups. The groups are classical, modern and Earth. The classical planets are Mercury, Venus, Jupiter and Saturn, which have been known since prehistoric times. The modern planets are Uranus, Neptune and Pluto. The modern planets were discovered in modern times, and are visible with only a telescope. Then there is Earth, once again in a category all its own. .

             The inner planets, which are Earth, Mars, Mercury and Venus, are similar in several ways. Each of these planets are terrestrial. Terrestrial meaning, planets that are made .

             mostly of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites. They are also the smallest planets in size, each under 13,000 km. in diameter. All of the inner planets are, by position, relative to the Sun by being closer than the outer planets will ever be. .

             The outer planets, which are Jupiter, Saturn, Uranus, Neptune and Pluto, are similar in many ways. Each of these planets are made of gas. Gas meaning, planets that are composed primarily of hydrogen and helium and generally have less densities, rapid rotation, deep atmospheres, rings and a lot of satellites. They are also the largest planets in size, each over 48,000 km. in diameter. All of the outer planets are, by position, relative to the Sun by being farther away than the inner planets will ever be.