Mercury. Computer artwork of the planet Mercury (left), partially illuminated by the Sun (upper right). Mercury is the closest planet to the Sun, being around 58 million kilometres away, less than half the distance of Earth from the Sun. With a diameter of 4880 kilometres, it is also less than half the size of the Earth. Mercury is a rocky planet that takes 88 days to complete its orbit. Its rotational period has been affected by tidal forces from the Sun, being slowed to 59 days. This means that Mercury rotates three times in two of its years, with sunrise occurring once every two Mercury years. Temperatures range from 400 degrees Celsius (day) to -170 degrees Celsius (night).