This small, rocky planet has almost no atmosphere. Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth! Also, there is no "greenhouse effect" on Mercury. When the Sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.

Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 88 Earth days or an entire Mercurian year), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer.

Mercury is about 3,031 miles (4,878 km) in diameter. It is the smallest planet in our Solar System (Pluto, now not considered to be a planet, is smaller). Mercury is only slightly larger than our moon. Mercury is a heavily cratered planet; its surface is similar to the surface of the Earth's moon.

There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet's orbit. Since Mercury's axis is directly perpendicular to its motion (it is not tilted), it has no seasons.

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