The shadow isn’t Earth’s
The Moon doesn’t shine on its own. It works like a giant, dusty mirror for sunlight, and just like Earth it always has a day side and a night side. The dark part we see is the Moon’s own night.
Many people think Earth’s shadow creeps across the Moon. But Earth’s shadow only reaches it during a lunar eclipse, and that only happens when the Moon is full.
Myth: Earth’s shadow creates the phases. Then the Moon would have to sit in the shadow all month.
In fact: the side facing the Sun is always lit. We just see it from different angles.
Eight phases, one cycle
From new moon to new moon takes 29.5 days on average. The cycle is usually split into eight phases, but in reality it is a smooth change: the terminator moves a little every hour.
Waxing or waning?
In the Northern Hemisphere there is a simple rule: if the Moon is lit on the right it is waxing, if it is lit on the left it is waning. In the Southern Hemisphere it is the other way round. The waxing Moon is seen in the evening, the waning Moon in the morning.
29.5 or 27.3 days?
Measured against the stars, the Moon orbits Earth in 27.3 days. But meanwhile Earth has moved about 27° along its orbit around the Sun. The Moon has to travel just over two more days to return to the same position relative to the Sun, and only then is it new moon again.
When is the Moon visible?
The phase also tells you the time of day. The full moon sits opposite the Sun, so it rises around sunset. The new moon follows the Sun across the sky and is lost in daylight.
| Phase | Rises | Best seen | Sets |
|---|---|---|---|
| New Moon | ~06 | Not visible | ~18 |
| First quarter | ~12 | Afternoon and evening | ~24 |
| Full Moon | ~18 | All night | ~06 |
| Last quarter | ~24 | Night and morning | ~12 |
| Waning crescent | ~03 | Dawn in the east | ~15 |
The times are approximate and vary a lot with season and latitude. Winter full moons ride high across the sky, while summer ones barely clear the southern horizon.