L4. Phases of the Moon

Unit 1 · Earth in Space

R-report

L4. Phases of the Moon

Some nights the Moon is a full silver disc, some nights a thin curved sliver, and some nights it is missing entirely. Nothing is taking a bite out of it — so what is really going on?

The Moon does not make light

Start with the fact that explains everything else: the Moon produces no light of its own. It is a dark ball of rock, and we see it only because sunlight bounces off it. Sunlight comes from one direction, so it lights exactly half the Moon at any moment — the half facing the Sun. That never changes. There is always a lit half and a dark half. What changes is our viewing angle. The Moon orbits Earth roughly once a month, so we see its lit half from a different direction each night. Sometimes we look at the lit side face-on, sometimes we look mostly at the dark side, and usually we see some mixture of the two. The shape you call a phase is simply how much of the sunlit half happens to be turned toward you. A useful demonstration: hold a ball at arm’s length in a dark room lit by a single lamp, and slowly turn on the spot. The ball runs through every Moon phase.

The eight phases and what they look like

One full cycle takes about 29.5 days and always runs in the same order. - New Moon: the Moon is roughly between us and the Sun, so its lit half faces away. It is invisible. - Waxing crescent: a thin lit sliver appears on the right-hand side. - First quarter: exactly half the visible disc is lit. It rises around midday and sets around midnight. - Waxing gibbous: more than half lit, still growing. - Full Moon: Earth is between the Sun and Moon, so we see the whole lit half. It rises at sunset and sets at sunrise. - Waning gibbous, last quarter, waning crescent: the same shapes in reverse, now lit on the left. "Waxing" means growing and "waning" means shrinking. In the northern hemisphere, a Moon lit on its right side is waxing; lit on the left, it is waning.

Two things people get wrong

First, phases are not caused by Earth’s shadow. Earth’s shadow only touches the Moon during a lunar eclipse, which happens a couple of times a year — not every night. Phases are about which side is lit and which way we are looking, not about shadows at all. Second, the Moon is often visible in daytime. Because it moves around the sky over the month, a first-quarter Moon is up all afternoon. Look for a pale Moon in a blue sky and you will start noticing it regularly. One more fact worth knowing: the Moon always shows Earth the same face. It rotates once for each orbit — an effect called tidal locking — so the far side is never turned toward us. That is not a "dark side": it gets exactly as much sunlight as the near side. We simply never see it from Earth, and nobody did until a Soviet spacecraft photographed it in 1959.

Putting the investigation together

The Moon is a dark rock lit from one side by the Sun. As it orbits Earth each month, our view of that lit half changes, and those changing views are the phases — new, crescent, quarter, gibbous, full, and back again over about 29.5 days.

A good way to test your understanding is to predict rather than recall. If you know tonight is a first quarter Moon, you can work out that it will be full in about a week, that it is high in the sky at sunset, and that it will be lit on its right side. When a model lets you predict, you genuinely understand it.