Phases of the moon: what's really happening up there

Everyone knows the moon has phases. Fewer people know exactly why, and almost nobody gets a satisfying explanation in school. The truth comes down to a single fact that, once you see it, you never forget.

The key fact

The sun is always lighting exactly half of the moon.

That half is whichever side is facing the sun, and it doesn’t change. Always half-lit, always half-dark, every moment of every orbit.

What changes is how much of the lit half we, on Earth, can see.

That’s it. Every phase you’ll ever observe comes from Earth’s view of that lit hemisphere as the moon moves around us.

The eight phases

The moon takes about 29.5 days to go around Earth once. During that orbit, the angle between Earth, moon, and sun changes continuously. We name the most recognisable angles:

New moon. The moon sits between Earth and the sun. Its lit side faces the sun, so from our view we’re looking at the dark side. We can’t see it. It rises and sets with the sun.

Waxing crescent. The moon has moved a little along its orbit, and we start to see a thin sliver of the lit side. Curving like a smile, or a frown, depending on your hemisphere and the season. The rest of the moon is lit only by earthshine: sunlight bouncing off Earth’s clouds and oceans and back onto the moon’s dark side.

First quarter. A quarter of the way around. Half the face we see is lit. Looks like a half-moon, but we’re actually seeing a quarter of the moon’s total surface.

Waxing gibbous. More than half. Less than full. The lit area fattens each night.

Full moon. The moon is on the opposite side of Earth from the sun, and we see the entire lit face. This is also the only phase where lunar eclipses can happen, when Earth’s shadow falls on the fully-lit face.

Waning gibbous. Past full, heading back. The lit side shrinks from the opposite edge this time.

Last quarter. Half-lit again, but from the other side compared to first quarter.

Waning crescent. A sliver, curving the opposite way from the waxing crescent.

Then back to new, and the whole thing repeats.

Which side is lit, and what that means

In the northern hemisphere: if the right side of the moon is lit, it’s waxing (growing). If the left side is lit, it’s waning (shrinking). Another way to remember it: waxing moons set after the sun, waning moons rise after midnight.

Southern hemisphere observers see the opposite, because they’re essentially looking at the sky upside-down relative to the north.

For teaching a kid: in the northern hemisphere, a waxing moon makes a “D” shape, a waning one makes a “C.” Pick whatever mnemonic sticks.

Why we always see the same face

Here’s a detail that surprises people. We always see the same face of the moon. There’s no regular “far side” show in our sky.

The moon rotates on its axis in exactly the same time it takes to orbit Earth: 29.5 days. This is called tidal locking, and it’s incredibly common in the solar system. Most large moons are tidally locked to their planets.

The result: from Earth, the moon always shows the same familiar pattern of dark “seas” (ancient lava plains, called maria) and brighter highlands. The far side, often called the “dark side” even though it gets just as much sunlight as the near side, was never seen by human eyes until the Soviet Luna 3 mission photographed it in 1959.

The maria are what makes the moon’s face look like a face (or a rabbit, or a woman, or a hare, depending on your culture). The bright highlands are more heavily cratered and older.

The month that isn’t quite a month

The word month comes from moon. One month was originally one full cycle of moon phases, a lunation, which lasts about 29.5 days. Most calendars in human history were some form of lunar calendar. That’s why Islamic, Hebrew, and traditional Chinese calendars all have months that drift through the Gregorian calendar.

The Gregorian calendar is a solar calendar, tied to Earth’s orbit around the sun (365.25 days), with months fixed at 28 to 31 days for accounting convenience. As a result, the moon phase drifts across our calendar. A full moon on April 3rd one year might fall on April 23rd the next. It’s not a bug. It’s what happens when two different orbital periods are forced to share a calendar.

Blue moons and supermoons

A blue moon, in modern usage, is either the second full moon in a calendar month, or the third full moon in a season that has four. Either way, it happens roughly once every 2 to 3 years. The moon does not actually appear blue. “Once in a blue moon” just refers to how rare the calendar quirk is.

A supermoon is a full moon occurring near the point in the moon’s orbit where it’s closest to Earth (perigee). The moon’s orbit isn’t a perfect circle. Its distance varies by about 50,000 km. At perigee, the moon looks about 14% larger and 30% brighter than at its farthest (apogee). Real effect, just smaller than most people expect.

One suggestion for tonight

Next time you notice the moon, take thirty seconds to work out which way round it is. Where’s the sun right now? Is the moon ahead of the sun’s path (waxing) or behind it (waning)? If you can answer that, you’re reading the sky, not just seeing it.

Everyone looks at the moon. A smaller, quieter group of people understands what they’re looking at. Both are worthwhile. One is more interesting.