Earth Observations from the Moon surface: dependence on lunar libration

Abstract. Observing the Earth from the Moon has important scientific advantages. The angular diameter of the Earth as seen from the Moon surface is 1.9° ± 0.1°(the angular size varies due to the change in the distance between the Earth and the Moon). The libration of the Moon in latitude reaches an amplitude of 6.68° and has a main period of 27.21 days (or 653.1 hours). The libration of the Moon in longitude, reaching 7.9°, has a period of 27.55 days (or 661.3 hours). This causes the center of the Earth move in the Moon’s sky in a rectangle measuring 13.4°× 15.8°. The trajectory of the Earth's motion in this rectangle changes its shape with a period of 6 years. This apparent librational movement of the Earth in the Moon’s sky complicates observations of the Earth. The paper proposes to turn this disadvantage into an advantage and place a multi-slit spectrometer on the Moon surface on a fixed platform. The libration motion and the daily rotation of the Earth will act as a natural replacement for the scanning mechanism.