The phases of the Moon and why they happen
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The Sun always lights exactly half of the Moon. What changes over a month is how much of that lit half is turned towards us. Hold on to that one idea and the names of the phases, and even the times the Moon rises, fall into place.
Half lit, all the time
The Moon makes no light of its own. Moonlight is sunlight reflected off grey rock and dust.
Imagine the Sun far away to one side. As the Moon travels around the Earth, we view its sunlit half from different angles:
- When the Moon lies roughly between the Earth and the Sun, its lit side faces away from us and we see almost nothing. This is new moon.
- When the Moon is on the far side of the Earth from the Sun, the whole lit side faces us. This is full moon.
- In between, we see part of the lit half and part of the dark half.
One cycle, from new moon to new moon, takes about 29.5 days. The word "month" comes from the same root as "Moon".
The eight phases
The first four phases are waxing: the lit part grows. The last four are waning: it shrinks.
| Phase | Days after new moon (approx.) | What you see (Northern Hemisphere) |
|---|---|---|
| New moon | 0 | Nothing, or a faint dark disc |
| Waxing crescent | 1–6 | Thin curve, lit on the right |
| First quarter | about 7 | Right half lit |
| Waxing gibbous | 8–14 | More than half lit, growing |
| Full moon | about 15 | Whole disc lit |
| Waning gibbous | 16–21 | More than half lit, shrinking |
| Last quarter | about 22 | Left half lit |
| Waning crescent | 23–29 | Thin curve, lit on the left |
"Gibbous" comes from an old word meaning humped, which describes the bulging shape between half and full.
First quarter confuses many people because the Moon looks like a half. The name refers to the Moon being a quarter of the way round its orbit, not to how much of it we see.
Waxing or waning?
Anywhere in the Northern Hemisphere, including the UK, Europe and the USA, a waxing Moon is lit on the right and a waning Moon on the left. A common memory aid is D, O, C: a lit shape like a D is growing, O is full, and a shape like a C is shrinking.
In the Southern Hemisphere, in places such as Australia and South Africa, it is the other way round, because observers there see the Moon "upside down" compared with Europe. Near the Equator, the crescent often lies on its back, like a smile or a frown.
When each phase is in the sky
The phase depends on where the Moon is relative to the Sun, so each phase also keeps its own hours.
| Phase | Rises around | Best seen |
|---|---|---|
| New moon | sunrise | not visible, lost in the Sun's glare |
| First quarter | midday | afternoon and evening; high in the south at sunset in the UK |
| Full moon | sunset | all night, setting around sunrise |
| Last quarter | midnight | after midnight and into the morning |
So the Moon is not only a night-time object. A first quarter Moon in the afternoon sky is easy to spot.
Worked example: how late is the Moon?
On average, the Moon rises about 50 minutes later each day. A full moon rises at 18:00. Roughly when will the Moon rise three days later?
- 3 × 50 = 150 minutes.
- 150 minutes = 2 h 30 min.
- 18:00 + 2 h 30 min = about 20:30.
The real delay varies with the season, so treat this as an estimate. It explains why a Moon that lit up the whole evening at full moon is missing from the early evening sky a few days later.
Three things that are not true
- "The Earth's shadow causes the phases." If it did, every full moon would be an eclipse. The Earth's shadow reaches the Moon only during a lunar eclipse, described in seasons, eclipses and tides.
- "The far side is always dark." The Moon spins once in exactly the time it takes to orbit the Earth, so the same face always points at us. The side we never see gets just as much sunlight as ours; at new moon it is fully lit. "Far side" is the right name, not "dark side". The first photographs of it were taken by the Soviet probe Luna 3 in 1959.
- "Half Moon means halfway through the month." First quarter comes only about a week after new moon.
You can test yourself on these and other classic slips with ModernScholar's science misconceptions activity.
Activity: a Moon diary
This works best over four weeks. Go out with an adult, or look from a window.
- Each time you see the Moon, draw a circle, shade the dark part, and write the date and time.
- Note where it is: low in the east, high in the south, low in the west.
- After two weeks, decide whether it was waxing or waning and say how you could tell.
- Look up the date of the next full moon and check your prediction from the worked example.
Indoors model: in a darkened room, with an adult, put a lamp on one side as the Sun. Hold a ball at arm's length as the Moon and turn slowly on the spot. Find the position where the ball looks like a first quarter Moon. (The lamp is off to your side, at right angles to the line between you and the ball.)
The Apollo astronauts timed their landing so that the Sun was low over the site, giving long shadows that showed up the boulders. Read about the mission in how Apollo 11 got to the Moon and back.
Frequently asked questions
What is a blue moon?
In everyday use, the second full moon in one calendar month. It happens because the 29.5-day cycle is a little shorter than most months. The Moon does not look blue.
Why does the Moon look bigger near the horizon?
It is an illusion. Measure it against a coin held at arm's length and it is the same size low down as high up. Scientists still debate exactly why our brains are fooled.
What is a supermoon?
A full moon that falls when the Moon is near the closest point of its slightly oval orbit. It is a little larger and brighter than average, though the difference is hard to notice by eye.
Sources and further reading
- Moon phases – NASA Science – the eight phases, when to see them, and related effects such as earthshine.
- Earth's Moon – NASA Science – NASA's overview of the Moon, its orbit, tides and exploration.
