The mean orbital period of the Moon about the Earth with respect to the vernal equinox. Its length is 27.321 58 days. This definition differs from that of the sidereal month only through the inclusion of the precession of the equinoxes. The difference is only 7 s of time, but this very small difference will build up to one complete month over the 25 800-year cycle of precession.
Subjects: Astronomy and Astrophysics.