The synodic period of the Moon, about 29 days, is also known as what?

• A. The sidereal period
• B. The lunar month
• C. The orbital period
• D. The solar month

Answer: (B)

The synodic period is the cycle of the Moon’s visible phases as seen from Earth, from one new Moon to the next. This phase cycle takes about 29.5 days, which is why it’s called the lunar month—the duration of the Moon’s monthly phase change. The Moon’s true orbital motion around Earth relative to the stars—the sidereal month—lasts about 27.3 days, and the orbital period refers to that same revolution around Earth. Because Earth is moving around the Sun, it takes a bit longer for the Moon to reach the same phase again, making the synodic month longer than the sidereal/orbital period. The solar month is a calendar notion tied to the Sun and not the Moon’s phase. So the best label for the synodic period is the lunar month.