Mars is the fourth planet from the Sun and the next beyond the Earth. It has always been particularly fascinating to astronomers, perhaps because of its similarities with Earth and the constant changes in the appearances of its surface. Mars is about 1 times as far away from the Sun as the Earth, so it does not receive as much heat. Nevertheless, it is the only planet with an atmosphere and temperatures anything like Earth's.

Mars is about half the size of the Earth but only has one-tenth the Earth's mass. So its gravity is much less than Earth's. Its orbit round the Sun is quite elliptical (oval-shaped), so its distance from the Sun varies, as does its distance from Earth. When Mars is at a close point to Earth, it is said to be "at opposition". Some are closer than other.

The Moons of Mars
Mars has two very small moons, which were discovered by the American astronomer Asaph Hall in 1877. They have Greek names: Phobos (which means "fear") and Deimos ("terror"). Photographs taken from orbiting spacecraft show that both have irregular shapes and dark heavily cratered surfaces. Phobos has an average diameter of only 22 kilometers (14 miles), and Deimos is even smaller, measuring about 14 kilometers (9 miles) across.

Facts About Mars
Average Distance From Sun: 228 million-km (141 million miles)
Length of Year: 687 Earth days
Length of Martian Day: 24 hours 39 minutes 36 seconds
Diameter: 6,787 km (4,218 miles)
Mass: 0.1074 (Earth=1)
Density: 3.933 (water=1)
Average Surface Gravity: 0.38 (Earth=1)
Average Surface Temperature: -23C (-9.4F)

Observations of Mars
Because of its fearsome red colour, the ancients gave the name Mars to this planet in honor of the Roman god of war. The first telescopic observations of it were made by the Italian astronomer Galileo in 1610, while the Dutch scientist Christiaan Huygens is generally believed to have produced the first accurate drawings in 1659. Huygens showed that Mars rotates on its axis, and another Italian astronomer, Giovanni Domenico Cassini, discovered Mars's polar ice caps in about 1666. It was the 18th-century German-born British astronomer Sir William Herschel who detected Mars's thin atmosphere and its changing seasons.

1988 Encyclopaedia Britannica, Inc.

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