The closest planet to the Sun, Mercury is also the smallest. It takes 88 days to orbit the Sun, with a surface temperature of 400° C. The side facing away from the Sun has a temperature of -180° C. The surface is very similar to that of the Moon, with many craters and mountain ranges.

It is in a highly eccentric orbit around the Sun. At its closest (perihelion), it is only 46 million km from the Sun. At its most distant (aphelion), it is 70 million km. Its orbit perihelion moves backward (precesses) around the Sun at a very slow rate.

Mercury always appears near the Sun when viewed from Earth, but is rarely seen because of the Sun's glare. Many astronomers have never seen it because it's only visible a few days out of the year, and does not rise very far above the horizon. Mercury can only be seen from Earth just before sunrise or just after sunset, but not in the middle of the night.

Mercury is very difficult to study from the Earth because it is always so close to the Sun. Even at elongation, it is never more than 28 degrees from the Sun in our sky.