- 2 Astronomy a small planet that is the closest to the sun in the solar system, sometimes visible to the naked eye just after sunset.
Mercury orbits within the orbit of Venus at an average distance of 36 million miles (57.9 million km) from the sun. With a diameter of 3,031 miles (4,878 km), it is only a third larger than earth’s moon, which it resembles in having a heavily cratered surface. Its ‘day’ (equivalent to 58.65 Earth days) is precisely two thirds the length of its ‘year’ (87.97 Earth days). Daytime temperatures average 338°F (170°C). There is no atmosphere and the planet has no satellites