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

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 1.8-2.0g (the angular size varies due to the change in the Earth-Moon distance). The libration of the Moon in latitude reaches an amplitude of 6.68g and has a main period of 27.21gd (or 653.1gh). The libration of the Moon in longitude, reaching an amplitude of 7.9g , has a period of 27.55gd (or 661.3gh). This causes the center of the Earth to move in the Moon's sky in a rectangle measuring 13.4g g×g15.8g . 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.