Mars is the fourth planet from the sun and orbits the sun at a distance of 141 million miles. Mars is named for the Roman god of war because it appears fiery and red in the night sky. Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth's mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth's moon. .
The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth. The time it takes Mars to rotate once on its axis, is about a half an hour longer than an Earth day. Its year is about two Earth years long. Mars has two moons, Phobos and Deimos, which are named after the dogs of the Roman god Mars. These tiny satellites are heavily cratered dark chunks of rock and may be asteroids captured by the gravitational pull of Mars. Phobos orbits Mars once in less than one day, so it appears to rise in the west and set in the east, usually twice each day. Deimos has the more ordinary habit of rising in the east and setting in the west.
Mars appears bright and almost like a star in the night sky. Because of the movements of Earth and Mars around the sun, Mars appears to move backward in the sky for a short time. As Mars and Earth orbit the sun, the distance between them varies from about 47 million miles at their closest point to about 233 million miles when the planets are on opposite sides of the sun. This change in distance causes the size of Mars to vary by a factor of 5 and its brightness to vary by a factor of 25.
When Mars is viewed through a telescope, it looks like a red and orange disk. An observer can easily see white ice caps at the north and south poles of Mars. These caps grow and shrink throughout the Martian year, just as the polar caps of Earth do. The darker areas of Mars's surface may look greenish, but this is an optical illusion caused by the contrast in color between the dark patches and brighter areas.
