Phases of the Moon Quiz
Why the Moon has phases
The Moon makes no light of its own. It shines because it reflects sunlight, and at any moment the Sun lights up one half of it, the half facing the Sun.
As the Moon travels around Earth, we see that sunlit half from a changing angle. Sometimes we face it fully, sometimes only a sliver, and sometimes almost none of it. That changing view is what we call the phases.
A full cycle, from one new moon to the next, takes about 29.5 days. That's a little longer than the 27.3 days the Moon needs to circle Earth, because Earth moves along its own orbit at the same time. Earth's shadow has nothing to do with the phases: it only falls on the Moon during a lunar eclipse.
The eight phases
At new moon, the sunlit side faces away from us and the Moon is lost in the Sun's glare. Over the next week a thin crescent appears and grows. 'Waxing' means growing; 'waning' means shrinking.
First quarter comes about a week after new moon. Half of the Moon's face looks lit, but it's called a quarter because the Moon is a quarter of the way around its orbit. Next comes the gibbous phase, when more than half is lit, and then the full moon, which rises around sunset and shines all night.
After full moon the order runs backward: waning gibbous, last quarter and waning crescent. From the Southern Hemisphere the Moon looks flipped, so a waxing crescent is lit on the left instead of the right.
Phases, tides and eclipses
The Moon's pull, together with the Sun's, raises the ocean tides. When Sun, Earth and Moon line up, at new moon and full moon, their pulls add together and the tides are at their strongest. These are called spring tides, though they have nothing to do with the season.
Eclipses also follow the phases. A solar eclipse can only happen at new moon, when the Moon passes between Earth and the Sun. A lunar eclipse can only happen at full moon, when Earth sits between them.
People have used the phases to keep time for thousands of years. Many calendars, including the Islamic and Chinese ones, are built on the Moon's cycle.