The synodic period of the Moon is commonly referred to as what?

• A. Sidereal period
• B. Lunar month
• C. Orbital period
• D. Solar month

Answer: (B)

The key idea is the cycle of the Moon’s visible phases as seen from Earth. The synodic period is the time it takes for the Moon to go through a full sequence of phases—from one new moon back to the next new moon. This is commonly called the lunar month (also known as a lunation). It lasts about 29.5 days because, while the Moon completes its orbit around Earth in about 27.3 days (the sidereal month), Earth is orbiting the Sun as well, so the Sun’s direction changes and the Moon must travel a bit farther to reach the same phase.

That makes sense why the other terms aren’t used here: the sidereal period refers to the Moon’s orbit relative to distant stars, not the phase cycle; orbital period is a general term for any body’s revolution and doesn’t specify the phase; a solar month isn’t a standard term for the Moon’s cycle. So the best fit for the synodic period is the lunar month.