Perspective
What a Lunation Is, and Why the Length Varies
A lunation is one full moon cycle, new to new, but it's never exactly 29.5 days. Here's why the length shifts, and how we account for it in our calendar.
A lunation is the technical name for one full cycle of the moon, new moon to new moon. It’s also where the word “month” comes from. The number most people repeat, 29.5 days, is only the average. No single lunation actually lasts exactly that long.
The Real Range
The mean length of a lunation is 29.53059 days, about 29 days, 12 hours, and 44 minutes. But because the moon’s orbit is elliptical rather than perfectly circular, individual lunations run shorter or longer than that average. The shortest lunation of the 21st century runs about 29 days and 6 hours. The longest stretches to nearly 29 days and 20 hours, a difference of more than 13 hours between the fastest and slowest cycle.
Why the Moon Doesn’t Keep a Steady Pace
When the moon is closer to Earth, at perigee, it moves faster along its orbit and a new moon-to-new moon cycle runs shorter. When it’s farther away, at apogee, the pace slows and the cycle stretches out. Astronomers track this with the Brown lunation number system, a running count of every lunation since January 17, 1923, precise enough to identify any single cycle by number.
Why This Matters for a Calendar Built on the Moon
A calendar that just divides the year into fixed 29.5-day blocks will drift out of sync with the actual sky within a few cycles. That’s why our own calendar doesn’t work off an average. We calculate the moon’s real position for every single day, not a rounded number, so the dates in our calendar track what’s actually happening overhead instead of a tidy approximation.