Quick Astronomy Guide
The moon actually runs on two different clocks at once, and mixing them up is the easiest way to get your moon phase math wrong.
The moon's two cycles, and why they're not the same number
A sidereal month, the time it takes the moon to complete one full orbit relative to the fixed stars, is about 27.3 days. A synodic month, the time from one new moon to the next as seen from Earth, is about 29.5 days. Those two numbers describe the same object but answer different questions, and almost every calendar or app that tracks moon phases is using the synodic month, since that's the cycle that actually determines what shape the moon appears to be from where you're standing.
The gap between the two exists because Earth is also moving. By the time the moon finishes one full orbit around Earth, Earth itself has traveled roughly a twelfth of the way around the sun, so the moon has to travel a little further in its orbit to catch back up to the same sun-Earth-moon alignment that produces a new moon. That extra distance adds about two extra days to the cycle.
Why this trips people up
Because 29.5 doesn't divide evenly into a 365-day year, lunar phases fall on different calendar dates every year, which is why a full moon can land on the 3rd of the month one year and the 22nd the next. It's also why lunar calendars, still used to set dates for holidays in several cultures and religions, require a periodic extra month to stay roughly aligned with the solar year.
- New moon to full moon takes about 14.75 days, half of a 29.5-day synodic month.
- The moon's orbit isn't a perfect circle, so the exact interval between phases varies by a few hours cycle to cycle.
- A blue moon, a second full moon in a single calendar month, happens because 29.5 days is shorter than most calendar months.
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Shop All T-Shirts →Frequently Asked Questions
Which lunar cycle do moon phase calendars actually use?
The synodic month, about 29.5 days, since that's the cycle tied to how the moon actually appears from Earth.
Why isn't the moon's orbit a fixed 29.5 days every single time?
Its orbit is elliptical rather than circular, and Earth's own orbital speed varies slightly, so the exact synodic period fluctuates by a few hours from cycle to cycle.
What's the difference between a sidereal and synodic month in plain terms?
Sidereal measures the moon's orbit against the stars; synodic measures it against the sun's position as seen from Earth, which is the one that determines moon phases.
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