The Moon's calendar, live

The words moon and month share one root, and for most of history they meant nearly the same thing. A month was the time from one new crescent to the next. Calendars still split three ways over how to handle it: the Islamic Hijri calendar follows the Moon alone, so its 354- or 355-day year slides through the seasons; the Hindu, Hebrew and Chinese calendars are lunisolar and add a leap month — an adhika masa in India — roughly 7 times every 19 years to stay in step with the Sun; and the Gregorian calendar dropped the Moon entirely, keeping only the word.

In 2026 that leap month is happening: an Adhika Jyeshtha ran from mid-May to mid-June, which is why the table below shows two Jyeshtha full moons.

Measure "one orbit of the Moon" against different reference points and you get five different answers. Each one controls something you can see.

Eclipses happen when the synodic and draconic months line up — a full or new moon right at a node. 223 synodic months (6,585.32 days) are almost exactly 242 draconic months and 239 anomalistic months, so an eclipse repeats with nearly the same shape after 18 years, 11 days and 8 hours. This is the saros, known to Babylonian astronomers more than 2,500 years ago.

Every full moon of the next two years, computed from the Moon's and Sun's positions. The second column gives the traditional North American name; the third names the Hindu lunar month the Purnima falls in, with the main observance that usually lands on it. Festival days are set by local sunrise, so they can fall a day either side of the UTC moment shown.

The Moon keeps one face towards Earth, so it spins once per orbit. A day-night cycle there lasts a whole synodic month: about 14.8 Earth days of sunlight followed by 14.8 days of darkness. If we cut that lunar day into 24 "lunar hours", each lasts about 29.5 Earth hours.

Apollo 11 landed shortly after local sunrise on purpose: long, low shadows made craters and boulders stand out, and the surface had not yet heated up. Every Apollo landing was timed for the lunar morning.

Clocks on the Moon really do run faster than clocks on Earth. Einstein's general relativity says time passes more quickly where gravity is weaker, and the Moon's surface sits in a shallower gravity well. Add the effect of the Moon's orbital motion and a lunar clock gains roughly 56 microseconds every day on an Earth clock (estimates range up to about 59 µs depending on what's included).

That sounds tiny, but light travels about 17 km in 56 microseconds — enough to wreck a lunar navigation system that works like GPS. On 2 April 2024 the White House directed NASA to work out a standard called Coordinated Lunar Time (LTC), with a strategy due by the end of 2026, ahead of crewed Artemis landings.

Mirrors left on the surface by Apollo 11, 14 and 15 — and two Soviet Lunokhod rovers — still bounce back laser pulses fired from Earth. Timing the round trip to a few millimetres shows the Moon drifting away by about 3.8 cm a year, as tides transfer Earth's spin energy into the Moon's orbit. The same process is slowly lengthening our day. Some 620 million years ago, rock layers suggest, a day lasted about 21.9 hours and a year had over 400 of them.

The dark patches are called maria, Latin for seas, because early telescope observers thought they might be oceans. The Jesuit astronomer Giovanni Riccioli gave them their poetic names in 1651 — the Sea of Tranquillity, the Ocean of Storms, the Sea of Crises, the Bay of Rainbows — and his system is the one still used. They are actually basalt, lava that flooded giant impact basins 3–4 billion years ago. They're darker and smoother than the highlands, and nearly all of them are on the side facing Earth, where the crust is thinner.