Using the calculator
Type a date, such as a birthday, a wedding day or tonight. You get the phase's name, how much of the Moon was lit, how many days it was since the new moon, and the dates of the new and full moons around it. Show the steps draws the Moon as it looked.
It works for any date from 1583 to 2200. Numbers-only dates like `03/04/1990` are read with the Date order setting.
How the phase is worked out
The calculator finds the exact times of the new moon, first quarter, full moon and last quarter around your date, using the method in Jean Meeus's Astronomical Algorithms. That method corrects the average month with dozens of small terms for the shapes of the Earth's and Moon's orbits, and lands within a minute or two of the published times.
A day takes the name of a principal phase when that phase happens during it, so the day of the full moon is called Full moon even if the exact moment was at 3 a.m. Days in between take the name of their stretch: waxing crescent, waxing gibbous, waning gibbous or waning crescent.
The times are for Greenwich (UTC). If a phase fell close to midnight, it may have been the day before or after where you were.
Why the month isn't 29.5 days
On average, new moon to new moon takes 29.530588861 days. Any single month can be as short as 29.274 days or as long as 29.829, because the Earth and the Moon both move faster near the closest points of their elliptical orbits. Counting forward from a known new moon by the average can be more than half a day out, which is the difference between a full moon and the day after.
The drawing shows the Moon from the northern hemisphere, lit on the right as it waxes. From the southern hemisphere it's the other way round.
The meteorologist's algorithms
Jean Meeus, born in Belgium in 1928, worked as a meteorologist at Brussels Airport from 1953 until he retired in 1993, and calculated the sky in his spare time. His Astronomical Algorithms, first published in 1991 with a second edition in 1998, sets out methods like this one in a form anyone can program. The International Astronomical Union named the asteroid 2213 Meeus after him in 1981.
Questions
What was the moon on my birthday?
Type your birth date above. You'll get the phase, how much was lit and a drawing of the Moon that day.
How accurate is it?
The times of new and full moons are accurate to a minute or two. The percentage lit is an estimate to within a few per cent.
Why might a moon calendar give a different day?
Calendars use local time. This calculator uses Greenwich time, so a phase close to midnight can land on the neighbouring date.
What's the difference between waxing and waning?
Waxing means growing, from new moon to full. Waning means shrinking, from full back to new.
Sources
- Jean Meeus, Astronomical Algorithms (2nd ed., Willmann-Bell, 1998), chapter 49
- Celestial Programming: Moon phases (public domain implementation)
- Wikipedia: Lunar month
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