A Peterson Field Guide to Stars and Planets (Peterson Field Guides) - Softcover

The moon is often the most prominent object in the nighttime sky. The moon is somewhat more than one-quarter the diameter of the earth. This makes it the largest substantial satellite (moon) in the solar system in comparison to its parent planet. (Three moons of Jupiter and one each of Neptune and Saturn are physically larger than our moon; Pluto’s small moon Charon is nearly half Pluto’s size.) The moon orbits the earth every 27 1/3 days with respect to the stars. But during that time, the earth and moon have moved as a system about 1/12 of the way in their yearly orbit around the sun. So if the moon at a certain point in its orbit is directly between the earth and the sun, 27 1/3 days later it has not quite returned to that point directly between the earth and the sun. The moon must orbit the earth a bit farther to get back to the same place with respect to the line between the earth and the sun. The moon reaches this point in a couple of days, making the synodic period of the moon equal to 29 1/2 days. (The synodic period is the interval between two successive conjunctions — coming to the same celestial longitude — of two celestial bodies, in this case conjunctions of the moon and sun as observed from the earth.) It is the synodic months that are taken into account in lunar calendars.