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Synodic month

What is a synodic month?

A synodic month is the time from one full moon to the next, about 29.5 days. It is longer than the Moon’s trip around Earth relative to the stars, because Earth has moved and the Moon must catch up to the same angle with the Sun.

One lap around Earth relative to the stars is not yet the same phase. The Sun’s direction has shifted because Earth moved.

Two months, one Moon

Sidereal month: back to the same stars, about 27.3 days. Synodic month: back to the same phase, about 29.5 days. Calendars and the word “month” follow the phases, so they follow the longer one.

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Why moonrise slips

Earth must turn extra to catch the Moon, which has moved east. That extra is about 50 minutes a day — the same arithmetic as the synodic month spread over 24 hours.

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Related questions

Why does the Moon change shape? The Moon makes no light of its own. One half is always lit by the Sun. Phases are your changing view of that lit half as the Moon goes around Earth — about 29.5 days from full moon to full moon.

Why does moonrise get later each night? The Moon moves east along its orbit while Earth turns. Earth has to spin extra — about 50 minutes — to bring the Moon back to the same place on your sky. That extra is why moonrise (and moonset) slip later day by day.

Why isn’t there an eclipse every month? The Moon’s path around Earth is tipped about 5° to Earth’s path around the Sun. Most months the full Moon rides above or below Earth’s shadow, and the new Moon rides above or below the Sun. An eclipse needs the Moon at a crossing, not just at a phase.

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