What actually happens at a full moon

Before we get to what the full moon can mean for you, it helps to know a little about what's happening in the sky.

"Full" is a moment rather than a night

The moon is full at the exact moment it sits on the opposite side of Earth from the sun. At that moment, the whole side facing us is lit up. That moment has a precise time, down to the minute, and it often lands somewhere inconvenient, like 4:12 in the morning.

Luckily, your eyes are more forgiving. The moon looks full for about three nights: the night before, the night itself and the night after, because the thin edge of shadow is too small to notice. If you miss the exact night, you haven't missed the full moon.

Why there are twelve or thirteen a year

It takes the moon about 29.5 days to go from one full moon to the next, and that doesn't fit evenly into a 365-day year. So a year has a little more than twelve full moons, and about every two and a half years an extra one shows up, giving one month or season two full moons. That extra moon is what people call a blue moon, though it doesn't change color. You can read more about 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 every month. That doesn't happen, because the moon's path around Earth is tilted by about five degrees. At most full moons it passes a little above or below Earth's shadow. An eclipse happens only when a full moon falls at one of the points where those paths cross, which gives us a couple of chances a year instead of twelve.

Supermoons

The moon's path around Earth is a slightly stretched circle, so its distance from us changes by about 31,000 miles over a month. A full moon that happens when the moon is near its closest point is called a supermoon. At its biggest, it looks about 14% wider and up to 30% brighter than a full moon at its farthest point. That sounds like a lot, but it's surprisingly hard to spot by eye, since there's nothing in the sky to compare it to.

When the moon looks huge near the horizon, that's something else: the moon illusion. It's a trick your brain plays, and it can happen at any phase. Take a photo and the moon goes back to its usual size.

Does it affect people?

The honest answer is that the evidence is weak and mixed. A few small studies found people slept a bit less or more lightly around the full moon, but larger studies trying to repeat those results mostly didn't find a clear effect. Claims that full moons bring more hospital visits, births or crime have been checked many times and don't hold up.

The moon does cause ocean tides, but its pull on something as small as a person is tiny compared with its pull on an ocean. The popular idea that the moon moves us because our bodies are mostly water sounds nice, but tides don't work that way.

So why mark it at all?

The full moon is one of the most dependable, visible clocks we have, and nobody has to keep it running. It comes on schedule, and it's the same moon everyone else is looking up at. That makes it a lovely anchor for a monthly practice, which is the idea behind building a full moon ritual you'll actually keep.

We mail a postcard for every full moon, timed to arrive close to the night itself. Start with the next one.