Approximately how many days does the Moon take to complete one rotation?

Approximately how many days does the Moon take to complete one rotation?

• A. 24 days
• B. 27 days
• C. 30 days
• D. 31 days

Answer: (B)

The Moon spins on its axis in roughly the same time it takes to orbit Earth, about 27 days. This is because the Moon is tidally locked to Earth, so its rotation period has slowed to match its orbital period. As a result, one face continually turns toward Earth, completing a rotation in about 27.3 days relative to distant stars. If you were thinking in terms of the Moon’s phases, that cycle is longer—about 29.5 days—but the actual rotation is the sidereal ~27 days. The other numbers don’t match the Moon’s true spin rate.

The Moon spins on its axis in roughly the same time it takes to orbit Earth, about 27 days. This is because the Moon is tidally locked to Earth, so its rotation period has slowed to match its orbital period. As a result, one face continually turns toward Earth, completing a rotation in about 27.3 days relative to distant stars. If you were thinking in terms of the Moon’s phases, that cycle is longer—about 29.5 days—but the actual rotation is the sidereal ~27 days. The other numbers don’t match the Moon’s true spin rate.