Mercury. Artwork of the planet Mercury, the closest planet to the Sun. Mercury is a small rocky (terrestrial) planet which orbits the Sun at a distance of only 58 million kilometres. It takes 88 days to complete one orbit. Its rotation has been affected by the gravitational forces of the Sun. It rotates in just under 59 days, and experiences exactly three rotations for every two years. This complicated system means that the period between successive sunrises on Mercury is two of its years. This, and its lack of an atmo- sphere, leads to extremes of temperature at its surface. In the day it may reach over 400 degrees Celsius, falling to -170 degrees at night.