The Moon's eight shapes
The Moon has eight named shapes, called phases. It doesn't really change: the Sun always lights half of it, and as it goes around Earth once a month we see more or less of that sunny half. Right side lit means it is growing; left side lit means it is shrinking.
The Sun shines on the Moon like a flashlight on a ball. As the Moon moves around Earth, we see more or less of the bright side.
Half of the Moon is always in sunlight. Over about 29 and a half days the Moon goes once around Earth, so the bright half turns towards us (full Moon) and away from us (new Moon) and back. Waxing means growing; waning means shrinking.
The phase is set by the angle between the Sun and the Moon as seen from Earth, which grows about 12° a day. At 0° the Moon is new, at 90° first quarter, at 180° full and at 270° last quarter. The cycle from one new Moon to the next, about 29.5 days, is longer than the Moon's 27.3-day orbit because Earth moves along its own orbit meanwhile.
- New Moon
The Moon is between us and the Sun, so its lit side faces away. You can't see it at all.
- Waxing crescent
A thin smile of light on the right. It grows a little every night.
- First quarter
Half lit, the right half. It is a quarter of the way around its trip.
- Waxing gibbous
More than half lit and still growing. It is up most of the evening.
- Full Moon
All lit up. It rises when the Sun sets and stays up all night.
- Waning gibbous
More than half lit, shrinking now from the right. It rises after dark.
- Last quarter
Half lit, the left half. It rises around midnight.
- Waning crescent
A thin smile on the left, up before sunrise. It is nearly gone.
How to tell growing from shrinking
In the Northern Hemisphere, a Moon lit on the right is waxing (growing) and is up in the evening; a Moon lit on the left is waning (shrinking) and is best seen late at night or in the morning. In Australia it is the other way round, because people there see the Moon upside down compared with us.
Why can you see the Moon in the daytime?
Because it is often up while the Sun is. Around first quarter it rises at about noon, and around last quarter it is still up in the morning sky after sunrise. It is bright enough to show through the blue.
“Right side lit, it's growing. Left side lit, it's shrinking. That's true in the Northern Hemisphere, anyway.”