Why Does the Moon Have Phases?

The short answer

Three facts explain every phase:

• Half of the Moon is always lit. Like any ball in sunlight, the Moon has a day side facing the Sun and a night side facing away. That never changes.
• The Moon goes around Earth. It completes one orbit in about 27 days, so its position relative to the Sun keeps moving.
• We see it from Earth. How much of the day side faces us depends on where the Moon is in that orbit. When it sits between us and the Sun, the day side faces away and we see a new moon. When Earth is in the middle, the whole day side faces us and the Moon is full.

Tonight the Moon is a waning gibbous. The next new moon is Oct 10, 2026 and the next full moon is Oct 26, 2026.

Reading the diagram

Start at the left, where the Moon sits between Earth and the Sun. Its lit half faces the Sun, which means its dark half faces us: that is the new moon, and we cannot see it at all. A week later the Moon has moved a quarter of the way round. Now we look at it side-on and see half of the lit half, a half Moon that we call first quarter because a quarter of the orbit is done. Another week and the Moon is on the far side of Earth from the Sun. Its whole lit face points at us: full moon. The second half of the orbit runs the sequence in reverse, through last quarter and back to new.

In between those four points the Moon shows the crescent and gibbous shapes. A crescent is less than half lit, a gibbous Moon more than half. Waxing means growing, the two weeks from new to full; waning means shrinking, the two weeks from full back to new.

Why the cycle takes 29.5 days, not 27

The Moon orbits Earth once every 27.3 days measured against the stars. But while it does so, Earth has carried on around the Sun, so the Sun has shifted a little in the sky. The Moon needs roughly two extra days to catch up and line up with the Sun again. That is why the cycle of phases, from one new moon to the next, averages 29.53 days. It is also why the phase dates drift about 11 days earlier each calendar year, and why some years have 13 full moons instead of 12.

It is not Earth's shadow

The most common misconception is that the phases are Earth's shadow falling on the Moon. They are not. For most of the month Earth's shadow points nowhere near the Moon. The dark part of a crescent is simply the Moon's own night side, the half turned away from the Sun. You can prove it with a ball and a lamp in a dark room: walk the ball around your head and you will see it run through every phase, with nothing casting a shadow on it.

Earth's shadow does reach the Moon occasionally, and when it does the result is a lunar eclipse. That can only happen at full moon, and only two or three times a year, because the Moon's orbit is tilted about 5 degrees to Earth's path around the Sun. Most months the full Moon passes above or below the shadow and misses it, just as most new moons pass above or below the Sun instead of causing a solar eclipse.

Waxing or waning? How to tell at a glance

The lit side of the Moon always faces the Sun. From the northern hemisphere, a Moon lit on the right is waxing: it will be fuller tomorrow. A Moon lit on the left is waning. A memory aid is that a waxing crescent is shaped like a D and a waning crescent like a C, or that the Moon "lies" to you: the C-shaped one is not coming but going.

From the southern hemisphere the sky is viewed the other way up, so the rule flips: right-lit means waning and left-lit means waxing. Close to the equator the crescent lies on its back like a bowl or a smile, because the Sun is straight below the horizon rather than off to one side. The moon calendar draws the Moon the right way round for whichever city you choose.

Why you can see the Moon in the daytime

The Moon is up for about 12 hours in every 24, and only at full moon are those hours the same as the night. A first quarter Moon rises around midday and is easy to spot in the afternoon sky; a last quarter Moon is still up in the morning after sunrise. Only the new moon is truly a night-time no-show, and that is because it is beside the Sun in the daytime sky.

Is the phase the same everywhere on Earth?

Yes. The phase depends only on where the Moon is in its orbit, so everyone on Earth sees the same phase on the same night. The exact instant of full moon or new moon is a single moment worldwide; what differs is the clock time and sometimes the calendar date. A full moon at 02:00 UTC is the evening of the day before in the Americas and the following morning in Australia. Every time on this site is shown in your own time zone once your browser converts it.

Common questions

Why does the Moon always show the same face?

The Moon spins once on its axis in the same time it takes to orbit Earth, a state called tidal locking. So the same side always faces us, and the phases play out across that one face. The far side gets just as much sunlight; we simply never see it.

Why does a full moon look so much brighter than a half moon?

A first quarter Moon shows half the area of a full Moon but gives less than a tenth of the light. Near full, sunlight hits the surface head-on and the rough ground casts no shadows, so the whole disc brightens sharply in the last day or two, an effect called the opposition surge.

What is a supermoon?

The Moon's orbit is slightly oval, so its distance from Earth varies by about 50,000 km each month. A full or new moon that happens near the closest point is called a supermoon and looks a little larger and brighter than average. The full moon calendar flags every one.

The eight phases in order

See tonight's phase, moonrise and moonset for your city on the moon phase calendar, or jump to the next new moon and next full moon.