Earthrise - Potential Earthrises As Seen From The Moon's Surface

Potential Earthrises As Seen From The Moon's Surface

An earthrise that might be witnessed from the surface of the Moon would be quite unlike sunrises on Earth. Because the Moon is tidally locked with the Earth, one side of the Moon always faces toward Earth. Interpretation of this fact would lead one to believe that the Earth's position is fixed on the lunar sky and no earthrises can occur, however, the Moon librates slightly, which causes the Earth to draw a Lissajous figure on the sky. This figure fits inside a rectangle 15°48' wide and 13°20' high (in angular dimensions), while the angular diameter of the Earth as seen from Moon is only about 2°. This means that earthrises are visible near the edge of the Earth-observable surface of the Moon (about 20% of the surface). Since a full libration cycle takes about 27 days, earthrises are very slow, and it takes about 48 hours for Earth to clear its diameter. During the course of the month-long lunar orbit, an observer would additionally witness a succession of "Earth phases", much like the lunar phases seen from Earth. That is what accounts for the half-illuminated globe seen in the photograph.