How long does it take the Moon's phases to repeat?

How long does it take the Moon's phases to repeat?

• A. 29.5 days
• B. 1 year
• C. 30 days
• D. 27 days

Answer: (A)

The Moon’s phase cycle is called the synodic month, the time between successive New Moons (or other same phases) as seen from Earth. It takes about 29.53 days for the Moon to return to the same Sun–Earth alignment and show the same phase again. That extra bit beyond its 27.3-day orbital period around Earth happens because Earth is moving around the Sun, so the Moon must “catch up” to the Sun–Earth line to line up the same way. So the best answer is about 29.5 days. The 27-day figure is the sidereal month (Moon’s orbit relative to the stars, not the phase cycle), while 30 days is a rough approximation and 1 year is the time Earth takes to orbit the Sun, not the Moon’s phase cycle.

The Moon’s phase cycle is called the synodic month, the time between successive New Moons (or other same phases) as seen from Earth. It takes about 29.53 days for the Moon to return to the same Sun–Earth alignment and show the same phase again. That extra bit beyond its 27.3-day orbital period around Earth happens because Earth is moving around the Sun, so the Moon must “catch up” to the Sun–Earth line to line up the same way.

So the best answer is about 29.5 days. The 27-day figure is the sidereal month (Moon’s orbit relative to the stars, not the phase cycle), while 30 days is a rough approximation and 1 year is the time Earth takes to orbit the Sun, not the Moon’s phase cycle.