Moon phases explained: why does the Moon change shape? • The Science 360

By The Science 360 · Updated October 2026 · 8 min read

Why does the Moon change shape every night?

The Moon does not change shape. Half of it is always lit by the Sun, and as the Moon circles Earth we see a different share of that lit half. The cycle from one new Moon to the next takes 29.53 days, and the phase depends only on the angle between the Sun and the Moon as seen from Earth. Earth's shadow has nothing to do with it, except during a lunar eclipse.

In the simulation the slider is the day of that cycle. At day 7.4 the angle is 90 degrees and half the visible disc is lit (first quarter). At day 14.8 the Moon is on the far side of Earth from the Sun and the whole disc is lit. The lit fraction is (1 − cos θ) ÷ 2, where θ is the angle, so at 45 degrees it is only 15%. Two things follow that people often miss: the Moon rises about 49 minutes later each day, and the same face always points at Earth. The solar system facts post has more about our neighbourhood.

Key results from the simulation

One cycle, 29.53 days: the Moon gains 12.2 degrees per day on the Sun, so it takes 29.53 days to get back to the same phase. That is the synodic month.
Lit fraction = (1 − cos θ) ÷ 2: 0% at new, 15% at 45 degrees, 50% at first quarter, 85% at 135 degrees, 100% at full.
Rise and set follow the phase: a first quarter Moon rises near 12:00 and sets near 00:00, a full Moon rises near 18:00 and sets near 06:00. Each day the Moon rises about 49 minutes later.
The same side always faces Earth: the Moon spins once per orbit, so the gold arc on the orbit diagram always points at Earth.

Six moments in the lunar cycle

Each section gives the numbers the simulation shows for that day, says what you would see in the sky, and has a button that sets the slider. Rise and set times are a rule of thumb for mid-latitudes near an equinox, in local solar time.

New Moon: the lit half faces away

Day 0, 0 degrees

At new Moon the Moon sits almost exactly between Earth and the Sun, so the lit half faces away from us and we see the dark half. The lit fraction is 0%. The Moon rises with the Sun, near 06:00, and sets near 18:00, so it is above the horizon all day but lost in the glare.

It is not a perfect line-up. The Moon's orbit is tilted about 5 degrees, so most new Moons pass above or below the Sun and nothing is blocked. When the line-up is exact you get a solar eclipse, which the eclipse simulator shows. New Moon is the best night for faint objects, which is why telescope owners plan around it, see the Celestron telescope guide.

Lit 0% Rises 06:00 Sets 18:00 Best seen Not visible

Waxing crescent: a sliver after sunset

Day 3.7, 45 degrees

About 3.7 days after new Moon the angle is 45 degrees and 15% of the disc is lit, a crescent on the side facing the Sun. In the northern hemisphere that is the right-hand edge. It rises around 09:00, so you see it low in the west after sunset and it sets soon afterwards, near 21:00.

The dark part is not completely black. Sunlight bounces off Earth onto the Moon's night side, a glow called earthshine. The simulation keeps a faint trace of it on the dark side of the disc. "Waxing" just means growing: each night the lit part is a little bigger.

Lit 15% Rises 09:00 Sets 21:00 Best seen Evening

First quarter: half the disc, a quarter of the orbit

Day 7.4, 90 degrees

At first quarter the Moon has covered a quarter of its orbit. From Earth the Sun-Moon angle is 90 degrees, so exactly 50% of the visible disc is lit, and the terminator, the line between day and night on the Moon, is a straight line from pole to pole. The Moon rises near 12:00, is highest around 18:00 and sets near 00:00.

It is called a quarter because it is a quarter of the way round the cycle, not because a quarter is lit. This is the best phase to look at through a telescope: the long shadows along the terminator make craters and mountains stand out. Our cosmic objects post lists other things worth pointing a telescope at.

Lit 50% Rises 12:00 Highest 18:00 Sets 00:00

Full Moon: Earth between Sun and Moon

Day 14.8, 180 degrees

At full Moon the Sun, Earth and Moon are nearly in a line with Earth in the middle. The whole near side is lit (100%). The Moon rises near 18:00, just as the Sun sets, is highest around 00:00 and sets near 06:00, at sunrise.

Surprisingly, a full Moon is a poor time for sightseeing with a telescope. The light comes straight from behind us, so there are no shadows and the surface looks flat. A typical month has one full Moon, but because 29.53 days is shorter than most calendar months, now and then there are two. Because the Moon's orbit is tilted, it normally slips just above or below Earth's shadow, which is why there is no lunar eclipse every month.

Lit 100% Rises 18:00 Highest 00:00 Sets 06:00

Last quarter: the other half is lit

Day 22.1, 270 degrees

Three quarters of the way round, the angle is 270 degrees. The disc is again 50% lit, but now the lit side is the left-hand side as seen from the northern hemisphere. The Moon rises near 00:00, is highest around 06:00, in the morning, and sets near 12:00.

Waxing and waning are mirror images. If you can only remember one rule: a waxing Moon is lit on the side where the Sun set (right in the northern hemisphere), a waning Moon on the side where the Sun will rise. In the southern hemisphere the sky is upside down, so the lit sides swap.

Lit 50% Rises 00:00 Highest 06:00 Sets 12:00

Waning crescent: a sliver before dawn

Day 25.8, 315 degrees

With 3.7 days to go before the next new Moon the angle is 315 degrees and 15% of the disc is lit, on the left. The Moon rises near 03:00, a few hours before the Sun, so it is a thin crescent low in the east before dawn.

Then the cycle starts again. The 29.53 days is longer than the 27.32 days the Moon needs to circle the stars, because Earth has meanwhile moved along its own orbit and the Moon has to travel about 53 hours more to catch up with the Sun's direction. For more on our nearest neighbour, see the solar system facts post.

Lit 15% Rises 03:00 Sets 15:00 Best seen Before dawn

All eight phases side by side

Phase	Day	Angle θ	Lit	Rises	Sets	Best seen
New Moon	0.0	0°	0%	06:00	18:00	Not visible
Waxing crescent	3.7	45°	15%	09:00	21:00	Evening
First quarter	7.4	90°	50%	12:00	00:00	Evening
Waxing gibbous	11.1	135°	85%	15:00	03:00	Evening, night
Full Moon	14.8	180°	100%	18:00	06:00	All night
Waning gibbous	18.5	225°	85%	21:00	09:00	Night, morning
Last quarter	22.1	270°	50%	00:00	12:00	Morning
Waning crescent	25.8	315°	15%	03:00	15:00	Before dawn

Values come from the formulas the simulation uses, for a Moon on a circular orbit with a uniform speed, seen from mid-latitudes near an equinox. Real timings differ by a few hours, see the limits below.

How the simulation works

The slider sets the day, from 0 to 29.53. The angle between the Sun and the Moon as seen from Earth, called the elongation, is θ = day ÷ 29.53 × 360°, which is 12.2 degrees per day. The Moon is drawn on a circle round Earth at that angle from the Sun's direction, counter-clockwise as seen from above the north pole, the way it really moves.

The lit fraction of the visible disc is (1 − cos θ) ÷ 2. The edge between light and dark on the Moon, the terminator, is always a half circle seen from the side, so from Earth it is an ellipse whose horizontal half-width is the Moon's radius times |cos θ|. At a quarter it has width zero (a straight line), at a crescent it bulges toward the lit limb, at a gibbous phase it bulges toward the dark limb. The simulation draws exactly that curve and softens it a little to mimic the gradual fall-off of sunlight.

For rise and set, the Moon is θ degrees east of the Sun, and Earth turns 15 degrees per hour, so the Moon crosses the sky θ ÷ 15 hours after the Sun. Assuming the Sun rises at 06:00 and sets at 18:00, the Moon rises at 06:00 + θ ÷ 15 and sets 12 hours later. Per day the Moon moves 12.2 degrees, which is 49 minutes of Earth's turn, so it rises about 49 minutes later each day.

Why the same face? Earth's gravity raised tidal bulges on the young Moon and the friction slowed its spin until its rotation period matched its orbital period (27.32 days). The gold arc on the orbit diagram shows this: it stays on the Earth-facing side all the way round. The Moon image is generated procedurally with the real near-side maria positions, not a photograph. The animation is a time-lapse of about one day per second.

What the model leaves out

Orbit shape and speed: the Moon moves faster at perigee and slower at apogee. Real phases arrive up to about half a day earlier or later than the uniform model says, and the time between new Moons varies from about 29.3 to 29.8 days.
The 5 degree tilt: the orbit diagram is flat. In reality the Moon can be a few degrees above or below Earth, which changes where the terminator tilts but not the lit fraction. See the eclipse simulator for the tilt.
Latitude and season: rise and set times depend on where you live and the time of year. The rule of thumb is for mid-latitudes near an equinox, and the real moonrise can be an hour or more earlier or later, or never happen near the poles.
Libration: the Moon rocks slightly, so over time we see about 59 percent of its surface. The picture shows the average face.
Distance: the simulation treats the Sun as very far away, so the sunlight is parallel. The error in the phase is less than a quarter of a degree.
Scale: Earth and the Moon are enlarged in the diagram. At true scale, with the orbit drawn this size, Earth would be only a few pixels wide.

Common misconceptions

“The phases are caused by Earth’s shadow.” No. Earth’s shadow only falls on the Moon during a lunar eclipse. At most phases the Moon is nowhere near the shadow, and at a quarter Moon it is far from it. The phase is just the view of the sunlit half from different angles.

“The far side is the dark side.” No. The far side gets just as much sunlight as the near side, over a month. At new Moon the far side is the lit one. "Dark" here really means "unseen", not "unlit".

Frequently asked questions

Why does the Moon have phases?

Because half of the Moon is always lit by the Sun, and as the Moon orbits Earth we see that lit half from changing angles. At new Moon we look at the dark half, at full Moon we look at the lit half, and in between we see a mixture. The full cycle takes 29.53 days.

How long is a lunar cycle?

29.53 days from one new Moon to the next, called the synodic month. The Moon circles Earth against the stars in 27.32 days, but by then Earth has moved on, so the Moon needs about 53 hours more to line up with the Sun again.

What is the difference between waxing and waning?

Waxing means the lit part is growing, from new Moon to full Moon (days 0 to 14.8). Waning means it is shrinking, from full Moon back to new Moon. In the northern hemisphere a waxing Moon is lit on the right and a waning Moon on the left.

Why do we always see the same side of the Moon?

The Moon rotates once for every orbit around Earth (27.32 days), a state called tidal locking, so the same face always points toward us. We do not see the far side from Earth, although it gets sunlight just like the near side.

What time does the Moon rise?

It depends on the phase. As a rule of thumb a new Moon rises with the Sun, a first quarter Moon rises near 12:00, a full Moon near 18:00 and a last quarter Moon near 00:00. Each day it rises about 49 minutes later. Check a local almanac for the real time at your location.

How do I tell if the Moon is waxing or waning?

Look at when you can see it. A Moon in the evening sky is waxing, a Moon in the morning sky is waning. Or check the lit side: right-hand side lit means waxing in the northern hemisphere and waning in the southern. For more things to look for in the sky, see 10 cosmic objects that will amaze you.