What actually happens at a full moon

A useful amount of astronomy, before any of the symbolism: here's what is physically going on when the calendar says full moon.

"Full" is a moment, not a night

The moon is full at the instant it sits opposite the sun as seen from Earth, so the face pointed at us is completely lit. That instant has a timestamp — a specific minute, in a specific time zone — and it's often somewhere unhelpful like 4:12 in the morning.

What you see is more forgiving. The moon looks essentially full for about three nights, the one before, the night of, and the one after, because the sliver of shadow is too thin to notice with the naked eye. If you miss the exact night, you haven't missed the moon.

Why there are 12 or 13 a year

A full cycle from one full moon to the next — the synodic month — runs about 29.5 days, which doesn't divide evenly into a 365-day year. You get about 12.37 full moons annually, so roughly every two and a half years a thirteenth turns up, and one calendar month or one season ends up with two. That extra one is what people call a blue moon; it has nothing to do with colour. More on the names in every full moon name, month by month.

Why it isn't a lunar eclipse every month

If the sun, Earth, and moon lined up perfectly at every full moon, the moon would pass through Earth's shadow monthly. It doesn't, because the moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun, so at most full moons the moon passes above or below the shadow. Eclipses happen only when a full moon coincides with the moon crossing the plane of Earth's orbit — which gives you a couple of chances a year rather than twelve.

Supermoons

The moon's orbit is an ellipse, so its distance from Earth varies by roughly 50,000 kilometres over a month. A full moon that happens near perigee — the closest point — is what gets called a supermoon. At its largest it appears about 14% wider and up to 30% brighter than a full moon at the far point of the orbit. That sounds dramatic and is genuinely hard to notice by eye, because you have nothing to compare it against in the moment.

The moon looking enormous on the horizon is a separate thing entirely: the moon illusion, a trick of perception that happens at any phase and any distance. Photograph it and the illusion vanishes.

Does it affect people?

The honest answer: the evidence is thin and inconsistent. A handful of small studies have reported slightly shorter or lighter sleep around the full moon, and larger attempts to replicate them have generally found no reliable effect. Claims about full moons and hospital admissions, births, or crime rates have been examined repeatedly and don't hold up.

Tides are real, and they are genuinely lunar — but the moon's tidal pull on a human-sized body is vanishingly small compared to its pull on an ocean-sized one. The often-repeated "you're mostly water, so the moon moves you" isn't how tides work.

So why mark it at all?

Because it's the most reliable, most visible clock we have that isn't ours. It arrives on schedule, it's the same moon everyone else is looking at, and nobody has to maintain it. That's an unusually good anchor for a monthly practice — which is the entire premise of building a full moon ritual you'll actually keep.

Luna Post mails a postcard for every one of them, timed to arrive around the night itself. Start with the next.