This volume, together with Volumes 22 and 23 of the same series, contains Euler's contributions to the theory of the movement of the sun and especially of the moon. Time and again Euler worked on the program of applying Newton's principles of mechanics to improve our unterstanding of the movement of the celestial bodies. Euler's monumental "Theoria motuum Lunae nova methodo pertractata" of 1772 (his "Second Theory of the Moon") forms Volume 22, whereas Volume 23 mainly contains his early Astronomical Tables and his "First Theory of the Moon" of 1753. In the present volume the reader will find early papers by Euler pertaining to the preparation of his Astronomical Tables, a series of articles written between his two Theories of the Moon, and finally three substantial essays closely related to his "Second Theory of the Moon". The texts are reprinted in the original language most commonly French or Latin.