Note 168 — BOOK II. AN ACCOUNT OF THE WORLD AND THE ELEMENTS.

Astronomers describe two different revolutions or periods of the moon; the synodical and the sidereal. The synodical marks the time in which the moon passes from one conjunction with the sun to the next conjunction, or other similar position with respect to the sun. The sidereal period is the time in which the moon returns to the same position with respect to the stars, or in which it makes a complete revolution round the earth. These numbers are, for the synodical period, 29ᵈ 12ʰ 44ᵐ 2·87ˢ, and for the sidereal, 27ᵈ 7ʰ 43ᵐ 11·5ˢ; Herschel, pp. 213, 224.