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How Long Is a Lunar Cycle?
From one astronomical New Moon to the next takes about 29.53 days on average. This interval is the synodic month—the cycle of phases seen from Earth.
A lunar phase cycle averages 29 days, 12 hours, and 44 minutes, but individual cycles vary because the Moon’s orbit is elliptical and the whole Earth–Moon system moves around the Sun.
The synodic month
The Moon completes one orbit relative to the distant stars in about 27.3 days. Yet Earth travels farther around the Sun during that time, so the Moon must move a little farther to return to the same Sun–Earth–Moon geometry. That longer, roughly 29.53-day interval is the synodic month.
The four major phases are not guaranteed to fall exactly 7.38 days apart. The Moon changes speed along its elliptical orbit, and solar gravity perturbs its motion, so real event times must be calculated rather than evenly spaced on a calendar.
Lunar age is elapsed time, not a fixed percentage
Moon Exactly defines lunar age as the elapsed time since the preceding astronomical New Moon. It searches for that actual event and subtracts its UTC time from the selected instant. This avoids treating every lunation as an identical 29.53-day clock.
Phase angle and lunar age move together, but they are not interchangeable. The angle measures geometry; age measures time. Orbital speed variations are why two dates with similar ages can have slightly different angles or illumination values.
What a calendar can and cannot show
A calendar assigns a phase to a civil day, but each major phase occurs at one precise instant. The local date can differ across time zones when the event is near midnight. Moon Exactly labels exact event times in UTC and evaluates date-only calculator inputs at noon UTC for consistency.
Questions, answered
Frequently asked questions
Is every lunar cycle exactly 29.53 days?
No. 29.53 days is a mean; individual New-Moon-to-New-Moon intervals vary.
Why is the star-relative orbit shorter?
While the Moon orbits Earth, Earth also advances around the Sun, so the Moon travels farther before the same phase geometry returns.