SESK: Stuff Everyone Should Know · sesk.info/astro-synodic-month
About how long is one complete lunar phase cycle?
About 29.5 days, called the synodic month.
- 14.8 days
- 27.3 days
- 29.5 days ✔
Why: A phase cycle measures the Moon returning to the same Sun-Earth geometry. It takes longer than one orbit relative to the stars because Earth moves along its own orbit during the month.
Huh, didn't know that: The Moon completes an orbit relative to distant stars in about 27.3 days, shorter than its phase cycle.
The Moon's monthly schedule is full of phases.
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References
Answer: 29.5 days. A phase cycle measures the Moon returning to the same Sun-Earth geometry. It takes longer than one orbit relative to the stars because Earth moves along its own orbit during the month.
- Moon Phases science.nasa.gov/moon/moon-phases
The cycle repeats about once a month (every 29.5 days).
Checked 2026-10-10.
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