The moon phases come from one simple piece of geometry: at any moment the Sun lights exactly half of the Moon — just as it always lights half of the Earth. As the Moon orbits the Earth, we look at that lit half from a slightly different angle each day. When the Moon sits between the Sun and the Earth, it turns its dark side towards us: new moon. When it sits on the side of the Earth facing away from the Sun, we see the full lit half: full moon. In between, the visible share grows and shrinks — from a thin crescent through the half moon to an almost full disc and back again. One complete cycle from new moon to new moon takes about 29.5 days. If you would rather look up tonight's phase than simulate it, the live page Moon Phase Today has the current numbers.
The most common misconception: Earth's shadow
Many people assume that the Earth's shadow carves the crescent out of the lunar disc. That is wrong: the Earth's shadow only touches the Moon during a lunar eclipse — a rare event in which the Sun, Earth and Moon line up almost exactly. The everyday moon phases are pure perspective: depending on where the Moon stands, we see more or less of its permanently lit day side. The boundary between light and shadow on the Moon is called the terminator — craters throw their longest shadows there, which is why the Moon looks most three-dimensional along that line through binoculars.
Why does the Moon stand somewhere else every day?
The Moon moves about 12 to 13 degrees eastwards along its orbit each day. The consequence: it rises roughly 50 minutes later on average than it did the day before, and at the same clock time it sits in a different part of the sky. Watch the Moon on several evenings in a row and it appears to wander through the constellations. This daily offset is the very same effect that turns a thin crescent into a half moon after a good week, and into a full moon after two.
Why do we always see the same side?
The Moon rotates exactly once around its own axis while it orbits the Earth once. This synchronous rotation was shaped over billions of years by the Earth's tidal forces. The result: from Earth we always see the same lunar face with its familiar dark plains, the maria. The far side of the Moon was not photographed until 1959, by a space probe — and it is by no means "dark": it is lit by the Sun just as regularly as the side facing us.
High or low: the lunar path through the year
Across the sky the Moon roughly follows the ecliptic — the line the Sun travels along too. Because the full moon always stands directly opposite the Sun, the two swap roles: in winter the full moon climbs high across the sky like the summer Sun and rises far in the north-east; in summer it stays low above the horizon like the winter Sun. Compare a full moon in January with one in June and you will see completely different rising points and path heights — the horizon view of this simulator lets you follow exactly that with real London data for 2027.
Synodic and sidereal month
The Moon needs 27.3 days for one complete orbit around the Earth — the sidereal month, measured against the fixed stars. From new moon to new moon, however, 29.5 days pass — the synodic month. The difference arises because the Earth itself moves a stretch further around the Sun during one lunar orbit: the Moon has to catch up for a good two days before it stands in the same position relative to the Sun again. For the moon phases — and therefore for every lunar calendar — only the synodic cycle counts.