What are moon phases? The 8 phases of the Moon, explained simply
By the WhyCosmos team · Last updated:
Moon phases are the changing shapes of the Moon as seen from Earth. The Moon does not change shape and makes no light of its own: the Sun always lights half of it, and as the Moon orbits Earth we see a different share of that sunlit half. One full cycle, from new moon to new moon, takes about 29.5 days.
Many Indian families already use two moon phases by name, because festivals and the traditional calendar run on them: Amavasya, the new moon, and Purnima, the full moon. This page explains what happens in between, why the Moon seems to change shape, and the misconception many adults still carry from school: that Earth's shadow causes the phases. It does not.
How to explain it to your child
Explain it to an 8-year-old
In a dark room with one lamp, hold a ball at arm's length and turn slowly in a circle. Your head is Earth, the ball is the Moon and the lamp is the Sun. The lamp always lights half of the ball, but as you turn you see a full bright ball, then half, then a thin slice, then nothing. That is exactly what happens to the Moon every month.
Explain it to a 13-year-old
The Sun lights one half of the Moon at all times. As the Moon orbits Earth, the angle between the Sun, Earth and Moon changes, so the part of the lit half that faces us grows (waxing) and shrinks (waning). The Moon completes one orbit in about 27.3 days, but the phases repeat every 29.5 days, because Earth has also moved along its orbit around the Sun and the Moon needs about two more days to line up with the Sun again.
Key facts
| What causes the phases | Our changing view of the Moon's sunlit half as the Moon orbits Earth |
|---|---|
| Number of named phases | Eight: four main phases and four in-between phases |
| New moon to new moon | About 29.5 days, called a synodic month |
| One orbit of Earth | About 27.3 days |
| Does Earth's shadow cause phases? | No. Earth's shadow falls on the Moon only during a lunar eclipse |
| In the Indian calendar | Amavasya (new moon) and Purnima (full moon); Shukla paksha is the waxing fortnight, Krishna paksha the waning one |
| Visible in daytime? | Yes, for part of most days, except for a few days around new moon and around full moon |
What are the 8 phases of the Moon, in order?
Astronomers divide the cycle into eight phases. Four are main phases that mark exact points in the Moon's orbit; the four in between each last several days.
| # | Phase | What you see | When to look |
|---|---|---|---|
| 1 | New moon | Nothing: the sunlit half faces away from Earth | Not visible; it sits close to the Sun in the sky |
| 2 | Waxing crescent | A thin sliver that grows each evening | In the west, just after sunset |
| 3 | First quarter | Half of the Moon's face lit | Afternoon and evening; high in the sky at sunset |
| 4 | Waxing gibbous | More than half lit, still growing | Evening into the night |
| 5 | Full moon | The whole face lit | Rises around sunset and is up all night |
| 6 | Waning gibbous | More than half lit, now shrinking | Late night and early morning |
| 7 | Last quarter (also called third quarter) | The other half of the face lit | Rises around midnight; visible in the morning sky |
| 8 | Waning crescent | A thin sliver that shrinks each morning | In the east, just before sunrise |
"Waxing" means growing and "waning" means shrinking. From India, a waxing Moon is lit on its right-hand side and a waning Moon on its left. Because India lies fairly close to the equator, a crescent here often looks tilted, closer to a boat or a smile than to the letter C drawn in many European picture books.
Why does the Moon appear to change shape?
The Moon is a ball of rock that makes no light of its own. It shines only because it reflects sunlight, and at every moment the Sun lights exactly half of it: the half facing the Sun.
What changes is where the Moon is in its orbit around Earth. When the Moon is roughly between Earth and the Sun, its lit half faces away from us and we see a new moon. When it is on the far side of Earth from the Sun, its lit half faces us and we see a full moon. Everywhere in between, we see part of the lit half and part of the dark half, and that mix is the phase.
Are moon phases caused by Earth's shadow?
No. This is the most common wrong answer, and many adults learned it at school. Here is a quick way to see why it cannot be true: at full moon the Moon sits on the opposite side of Earth from the Sun, exactly where Earth's shadow points, yet that is when the Moon is brightest. If our shadow caused the phases, the full moon would be the darkest phase, not the brightest.
Earth's shadow does reach the Moon occasionally, only at full moon and only when the Sun, Earth and Moon line up closely. That event is a lunar eclipse, and it looks quite different from a phase: the Moon darkens and often turns coppery red.
How long is the Moon's cycle, and why is it longer than one orbit?
From one new moon to the next takes about 29.5 days, a period called the synodic month. The English word "month" comes from the Moon, and the traditional Indian lunar calendar counts its months by the same cycle, split into two fortnights: Shukla paksha, the bright or waxing fortnight that ends at Purnima, and Krishna paksha, the dark or waning fortnight that ends at Amavasya.
The Moon actually completes one orbit of Earth, measured against the distant stars, in about 27.3 days. The phases take about two days longer because Earth has moved along its own orbit around the Sun in the meantime, so the Moon has to travel a little further before the Sun, Earth and Moon return to the same arrangement.
Can we see the Moon in the daytime?
Yes. The Moon is above the horizon during daylight for part of most days. The easiest times are around first quarter, when it rises around midday and is high in the afternoon sky, and around last quarter, when it is still up in the morning after sunrise. You will not find it in daylight for a few days around new moon, when it sits near the Sun in the sky with its lit side facing away from us, or around full moon, when it rises at sunset and sets at sunrise.
A one-month moon diary: the best first astronomy project
You do not need a telescope to learn the moon phases. Ask your child to step outside at the same time each evening for a month, sketch the Moon's shape in a notebook, and note the date and where it was in the sky. Within two weeks most children spot the pattern themselves: the lit part grows night by night, the Moon rises about 50 minutes later each day, and the full moon comes up in the east just as the Sun sets in the west. Noticing a pattern before anyone explains it is how astronomy actually works.
The Sun, Moon and Earth are the first lesson of Astronomy 101, the WhyCosmos live course for ages 8 to 16, and the Moon's motion and eclipses follow in lesson 4.
Questions parents and kids ask
New moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last (or third) quarter and waning crescent. Then the cycle starts again with the next new moon, about 29.5 days after the last one.
At new moon the Moon is roughly between Earth and the Sun, so its sunlit half faces away from us. It also sits so close to the Sun in the sky that the Sun's glare hides it.
Yes. On any given night the phase is the same for everyone, because it depends on where the Moon is in its orbit, not where you stand. What changes with location is the Moon's tilt: from India a crescent often looks like a boat, and from the southern hemisphere the Moon appears upside down compared with the view from the north.
No. The far side gets just as much sunlight as the near side; at new moon it is fully lit. We never see it from Earth because the Moon turns once on its axis for every orbit, so it always keeps the same face towards us.
A solar eclipse can happen only at new moon, and a lunar eclipse only at full moon. Eclipses do not happen every month because the Moon's orbit is tilted by about 5 degrees, so the Sun, Earth and Moon usually do not line up exactly.
Amavasya is the new moon, when the Moon cannot be seen, and Purnima is the full moon. Many Indian festivals are timed by these phases: Diwali falls on an Amavasya, and Guru Purnima, as the name says, on a full moon.
Your child asks questions like this every week.WhyCosmos runs live online astronomy classes for ages 8–16, taught by real astrophysicists — where questions like these are the curriculum.
Related terms in this glossary
Sources and further reading
NASA: Moon phases · NASA: Moon facts · NASA Space Place: Moon phases