The Sidereal and Synodic Months
The sidereal month is the time the Moon takes to complete one full revolution around the Earth with respect to the background stars. However, because the Earth is constantly moving along its orbit about the Sun, the Moon must travel slightly more than 360° to get from one new moon to the next. Thus, the synodic month, or lunar month, is longer than the sidereal month. A sidereal month lasts 27.322 days, while a synodic month lasts 29.531 days.
COPYRIGHT: Freedman and Kaufmann, Universe, Seventh Edition, published by W. H. Freeman & Co. © 2004 W. H. Freeman & Co., and Sumanas, Inc.