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The Terrestrial Planets:

Mercury, Venus, Earth, The Moon, and Mars

The terrestrial planets make up the inner solar system. They form a related group of rocky worlds that are very different from the gas planets of the outer solar system and the icy bodies of the Kuiper belt. Although Earth's Moon is not technically a planet, it is usually organized among the terrestrial planets because its size, composition, and history are related to those of Mercury, Venus, Earth, and Mars.

Diameter - Distance from the Sun - Day Length - Year Length - Force of Gravity - Global Average Temperature

Diameter

Venus and Earth are very similar in size. Mars is half their size, and Mercury is even smaller (it's about the same size as Saturn's moon Titan and Jupiter's moon Ganymede). The Moon is one quarter the diameter of Earth, or one half the diameter of Mars.

Mercury	Venus	Earth	The Moon	Mars
4,900 km (3,100 mi)	12,100 km (7,500 mi)	13,000 km (8,100 mi)	3,500 km (2,200 mi)	6,800 km (4,200 mi)

Distance from the Sun (Average)

On average, Mercury is only about a third as far from the Sun as Earth is. That makes the Sun 11 times hotter at Mercury than at Earth. Mars's distance from the Sun is 50% greater than Earth's, which makes the Sun only half as strong there. Venus' and Earth's orbits are almost perfectly circular, while Mercury's and Mars' are noticeably elliptical. That makes Mercury's and Mars' distance from the Sun vary more over the course of one year than Earth's and Venus'.

Credit: JPL Solar System Simulator (Dave Seal)

Solar Day Length (Noon to Noon)

Mars days and Earth days are almost exactly the same length (Mars days are just 37 minutes and 22 seconds longer than Earth days). One solar day on the Moon is about the same time it takes to orbit Earth, or about a month. Mercury and Venus have very long solar days because they both rotate very slowly. Venus' rotation is the slowest of any planet in the solar system, and it rotates retrograde, so the Sun rises in the west and sets in the east.

Year Length

The closer to the Sun a planet is, the faster it travels in its orbit. Mercury goes around the Sun four times for every one time Earth does. Venus goes around almost twice for every Earth year. Mars takes almost twice as long as Earth to orbit the Sun. Mars and Earth's relatively fast rotation gives them hundreds of solar days for every one of their years. But Mercury's and Venus' slow rotations mean that Venus has less than two solar days per year, while Mercury's solar days are actually twice as long as its years!

Force of Gravity

Earth and Venus have similar gravity. So do Mercury and Mars, even though Mercury is much smaller -- it is made of denser stuff. You would weigh about 1/3 as much on Mercury and Mars as you do on Earth and Venus. On the Moon you'd weigh only 1/6 as much. The low gravity on the Moon made walking difficult for the astronauts -- they found hopping or skipping to be easier. No one has yet tried walking on Mercury or Mars.

Global Average Temperature

Mercury records the greatest extremes of temperature of any place in the solar system because of its lack of atmosphere, closeness to the Sun, and long solar day. The Moon also has fairly extreme temperatures. Venus has the hottest surface of any terrestiral planet due to its insulating atmosphere and runaway greenhouse effect. Earth's oceans keep its temperature buffered and relatively constant from day to night, just a little above the freezing temperature of water. Mars gets cold at night, but its temperature range isn't all that much different from some of the coldest places on Earth.