Science · The Sun, the Earth and the Moon

The phases of the Moon

Why does the Moon sometimes look like a thin crescent and sometimes like a big full moon? Discover that the phases come from our viewing angle, not from the Earth's shadow, and that one cycle of phases takes about 29.5 days.

18 min
1 simulation
Mini quiz

Updated: 16 October 2026

Preview

The question

One evening you see a thin crescent in the sky. A week later half of the Moon is lit, and two weeks later a big, round full moon greets you. Is the Moon really changing shape? Or is something covering part of it? The answer is much simpler, and much more surprising, than you might think.

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Ordering

Put the Moon's phases in order

Swap the cards by dragging them or with the up and down buttons. When the order is finished, submit your answer.

Predict first

On the nights after the new moon, how do you think the lit part of the Moon changes?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

Phases of the Moon

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Fig. 1Phases of the Moon

Drag the Moon or use the slider

Seen from the EarthWaxing crescent

Lit: 10%
Day: 3.0 of 29.5
Sun–Moon angle: 37°

Since new moon

What if?

Move the Moon closer or farther: how big does it look, how much stronger are the tides, how long do the phases take?

The Sun and the Moon in the sky, to the same scale.Left: the SunRight: the Moon; dashed circle is today’s size

Apparent size: 0.52°; ×1 today’s
Tidal force: ×1
Full moon brightness: ×1
Cycle of phases: 29.5 days

Today the Moon is on average 384,400 km away and looks 0.52° wide; the Sun looks 0.53°. The Sun is 400 times as wide as the Moon and 389 times as far: by this coincidence the two look almost the same size.

In reality: The Sun’s tidal pull is 46% of the Moon’s: at new and full moon the two pull the same way and the tides grow. The Moon moves away by about 3.8 cm a year; in the very far future there will be no total solar eclipses.

Assumptions: Circular orbit; the Moon’s mass, the Earth’s spin and the distance to the Sun stay the same. For the tides only the ratio of the force is worked out (∝ 1/d³); how high the sea actually rises depends on the coast and the ocean basin. Brightness is taken as proportional to the Moon’s apparent area. Apparent size and eclipses are worked out as seen from the Earth’s centre (geocentric). Numbers come from the radii, masses and distances in NASA’s fact sheets.

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

From the Earth, half of the Moon’s disc looks lit (first quarter). How much of the Moon is the Sun lighting at that moment?

Gold: the half lit by the Sun (always half of the Moon)Purple: edge of the half facing the Earth, and the line of sightThe numbers come from the model in this article: one cycle of phases (a synodic month) lasts 29.53 days on average, the Sun–Moon angle ψ grows steadily with time, and the visible lit fraction is k = (1 − cos ψ) / 2. The Moon is fixed to its arm, so it always turns the same face (the same dark plains) to the Earth. Sunlight is taken as parallel; sizes and distances are not to scale, and the orbit’s tilt of about 5° and its slight ellipse are left out, so the figure shows no eclipses. A main phase is named within 1 day of its exact moment. The window shows the view from the Northern Hemisphere.

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Frequently asked

4 questions

Are the Moon's phases caused by Earth's shadow?

No. The Sun always lights up half of the Moon; as the Moon travels around Earth, we see different parts of that lit half. Earth's shadow falls on the Moon only during a lunar eclipse, and that can happen only at full moon.

Why isn't there a lunar eclipse at every full moon?

The Moon's orbit around Earth is tilted by about 5° compared with Earth's orbit around the Sun. So at full moon the Moon usually passes a little above or below Earth's shadow. An eclipse happens only when the Sun, Earth and Moon line up exactly.

Can I see the Moon during the day?

Yes. The Moon is not only in the night sky. Especially on days close to first quarter and last quarter, it is also visible in the daytime sky; because it reflects sunlight, it shines faintly but noticeably against the blue sky.

Is the far side of the Moon, the side we never see, always dark?

No. Although we always see the same face from Earth, the far side gets sunlight too. In fact, at new moon, when we can hardly see the Moon at all, the whole far side is lit up.