Moon Phase Calculator
Find the moon's phase and illumination for any date, computed from real lunar astronomy rather than a lookup table.
The moon orbits in 27.3 days and takes 29.5 to get back to full
Those are two different months and the gap between them is not an inaccuracy. The moon completes an orbit in 27.321661 days, timed against the fixed stars. But new moon to new moon is 29.530589 days — 2.21 days longer.
A phase depends on where the moon sits relative to the sun, and the Earth has moved about 27 degrees around its own orbit during that month. The moon has to travel that bit further to line up again. The relationship is exact:
1 ÷ synodic = 1 ÷ sidereal − 1 ÷ year
| Predicted from the orbital periods | 29.530588 days |
| Measured phase cycle | 29.530589 days |
| Difference | 0.1 seconds |
Two days of apparent discrepancy fall out of one subtraction, to within a tenth of a second. And it is a real constraint rather than an identity — hold the Earth still in the formula and it collapses to the orbital period exactly, which is precisely the physical claim those two extra days encode.
And why nineteen years keeps turning up in calendars
Twelve synodic months come to 354.37 days — 10.63 days short of a common year. That single number explains a great deal of calendar history. A purely lunar calendar slides backwards through the seasons by about eleven days a year and goes right round in roughly 34 years, which is why Ramadan moves through summer and winter within a lifetime.
Thirteen months overshoots by nearly nineteen days, so neither count fits and any calendar wanting both lunar months and stable seasons has to alternate. The question becomes: over how many years do whole months and whole years nearly coincide?
| Span | Whole months | Off by |
|---|---|---|
| 8 years | 99 | +1.588 days |
| 11 years | 136 | -1.507 days |
| 19 years | 235 | +0.081 days |
| 33 years | 408 | -4.522 days |
Nineteen years wins by a wide margin — 235 months to within 1.9 hours, where every other candidate under thirty-five years is at least ten times worse. Searching every span up to forty years independently picks the same one.
That is the Metonic cycle. It is why the Hebrew calendar inserts seven leap months every nineteen years, and why the date of Easter — which is defined off the moon — repeats on a nineteen-year pattern. A two-hour error accumulated over two decades is close enough that civilisations built their calendars on it and did not need to revisit the arithmetic for a very long time.
One small consequence for reading the display above: the 8 named phases are 3.69 days each, so "full moon" as a label covers most of a week either side of the actual moment. The moon looks full for several nights, and the date given for it is the instant rather than the appearance.
How to use
- Pick a date, or leave it as today.
- Read the phase name and illuminated percentage.
- Check moonrise and moonset for observing.
- Note that the phase looks different from each hemisphere.
Frequently asked questions
How long is a lunar cycle?
About 29.53 days from one new moon to the next — the synodic month. It differs from the 27.3-day orbital period because the Earth also moves around the Sun, so the Moon must travel a little further to return to the same phase.
Why does the Moon look different from the southern hemisphere?
Because you are effectively upside down relative to a northern observer, so the illuminated side appears on the opposite edge. A waxing crescent lit on the right in London is lit on the left in Sydney, which regularly confuses people who learned one and travelled to the other.
What is a blue moon?
Two definitions, both in use. The older seasonal one means the third full moon in a season containing four. The modern and more common one — a second full moon in a calendar month — originated in a 1946 magazine misreading of the older rule and stuck anyway.
What is a supermoon?
A full moon near the closest point of its elliptical orbit, so it appears somewhat larger and brighter. The difference is real but modest — around 14 per cent in diameter at the extremes — and far less dramatic than photographs suggest, since those usually rely on a long lens near the horizon.
Why does the Moon look huge on the horizon?
The moon illusion, which is a perceptual effect rather than anything optical. Measured, the Moon is the same size overhead as on the horizon, and photographing both proves it. The explanation is still debated, which is remarkable for something noticed for thousands of years.
Does the Moon affect people or weather?
There is a real effect on tides, and essentially no credible evidence for effects on human behaviour, birth rates or hospital admissions, despite the persistence of the belief. Studies looking for it repeatedly find nothing once the data is analysed properly.
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