What is the sidereal period of the Moon?

• A. The time it takes the Moon to orbit the Earth with fixed stars as reference points (about 27 days)
• B. The time it takes the Moon to return to the same phase as observed from Earth (about 29 days)
• C. The time it takes the Earth to orbit the Sun
• D. The time it takes the Moon to align with the Sun every 24 hours

Answer: (A)

The sidereal period is the Moon’s orbital period measured against the background of fixed stars. It tells you how long it takes the Moon to come back to the same star position in its orbit, which is about 27.3 days (often rounded to 27 days). This value is shorter than the cycle of Moon phases because the Earth–Moon system is moving around the Sun as the Moon travels, so the Moon must cover a bit more angle to reach the same phase relative to the Sun, leading to the longer ~29.5-day synodic period. The Earth’s year (365 days) is unrelated to the Moon’s orbital period, and a 24-hour cycle would describe a daily solar alignment, not the Moon’s orbit.