Earth observations from the Moon's surface: dependence on lunar libration

Abstract

Abstract. Observing the Earth from the Moon's surface has important scientific advantages. The angular diameter of the Earth as seen from the Moon's surface is 1.8–2.0∘ (the angular size varies due to the change in the Earth–Moon distance). The libration of the Moon in latitude reaches an amplitude of 6.68∘ and has a main period of 27.21 d (or 653.1 h). The libration of the Moon in longitude, reaching an amplitude of 7.9∘, has a period of 27.55 d (or 661.3 h). This causes the center of the Earth to 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 within a period of 6 years. This apparent librational movement of the Earth in the Moon's sky complicates observations of the Earth. This paper proposes that this disadvantage be turned into an advantage and that a multi-slit spectrometer be placed on the Moon's 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.