The days are long on Mercury

The days are long on Mercury

. Because of its spin-orbit coupling, Mercury rotates once every 58.6 Earth days, and orbits the Sun in 88.0 Earth days. After two orbits the planet has rotated three times, but from Mercury's surface the Sun appears to have moved only once around the planet. So sunrise is repeated at a given point on the planet's surface (black dot) every two orbits, and a solar day on Mercury lasts two of the planet's years and 176 Earth days. The labels on this figure refer to Earth days.

Copyright 2010, Professor Kenneth R. Lang, Tufts University