The lunar cycle takes about how many days to complete its cycle (rounded to the nearest whole day)?

The lunar cycle takes about how many days to complete its cycle (rounded to the nearest whole day)?

• A. 28 days
• B. 29 days
• C. 30 days
• D. 32 days

Answer: (C)

Think about what the lunar cycle refers to: the sequence of Moon phases from one New Moon to the next New Moon. That synodic month is about 29.53 days. Since 29.53 days is roughly 29 and a half days, rounding to the nearest whole day gives 30 days. The Moon’s orbital period relative to the stars (about 27.3 days) is shorter, but that’s not the cycle of phases we observe from Earth. So the best rounded value for the lunar cycle is 30 days.

Think about what the lunar cycle refers to: the sequence of Moon phases from one New Moon to the next New Moon. That synodic month is about 29.53 days. Since 29.53 days is roughly 29 and a half days, rounding to the nearest whole day gives 30 days. The Moon’s orbital period relative to the stars (about 27.3 days) is shorter, but that’s not the cycle of phases we observe from Earth. So the best rounded value for the lunar cycle is 30 days.