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A lamp, a globe and a Moon on real gravity: why does the Moon change shape?
Moon Phases lab · Last updated: 16 October 2026
You set a lamp, a globe and a Moon moving under real gravity and watch the Moon change from a thin crescent to a full disc and back. They find out that the Moon never changes shape at all. What changes is how much of its sunlit half we can see from Earth.
What you'll discover
- Half of the Moon is always lit by the Sun, just as half of Earth is always in daytime.
- As the Moon orbits Earth, we see different amounts of its sunlit half, which gives the phases.
- A full cycle of phases, from new Moon to new Moon, takes about 29.5 days.
The science in plain words
The Moon makes no light of its own. It shines because it reflects sunlight, and at any moment the Sun lights up exactly half of it. The Moon travels round Earth roughly once a month, and as it does, the angle between the Sun, Earth and Moon changes. From Earth we see different amounts of the lit half, and that is what we call the phases.
At new Moon the Moon lies roughly between Earth and the Sun, so its lit side faces away from us and it is almost invisible. A few days later a thin crescent appears. About a week after new Moon we see half the disc lit, called first quarter. The lit part keeps growing, or waxing, until full Moon, when the Moon is on the far side of Earth from the Sun and we see its whole sunlit face. Then it shrinks, or wanes, through last quarter back to new Moon.
One complete cycle takes about 29.5 days, which is where the idea of a month comes from. The phases are not caused by Earth's shadow, a common mistake. Earth's shadow only falls on the Moon during a lunar eclipse. The Moon also keeps the same face turned towards us, because it spins once for every orbit.
Where it fits
- India, NCERT: Class 8 science, Chapter 11: Keeping Time with the Skies
- United States: Grades 6 to 8
- Next Generation Science Standards: MS-ESS1-1
Common questions
Why does the Moon change shape?
The Moon does not really change shape. The Sun always lights half of it, and as the Moon orbits Earth we see different amounts of that lit half. When we see all of it, it is a full Moon; when we see none, it is a new Moon.
How long does it take for the Moon to go through its phases?
A full cycle of phases, from one new Moon to the next, takes about 29.5 days. Calendars based on the Moon, such as many festival calendars in India, use this cycle to count months, which is why lunar months alternate between 29 and 30 days.
Are the Moon's phases caused by Earth's shadow?
No. This is a very common mistake. The phases come from our changing view of the Moon's sunlit half as it orbits. Earth's shadow only falls on the Moon during a lunar eclipse, which happens at full Moon and only when the three bodies line up exactly.
Related labs
- Solar & Lunar Eclipses: Line up the lamp, the globe and the Moon: why is there not an eclipse every month?
- Ocean Tides: Why does the sea come in and go out twice a day?
- Earth's Seasons: Carry the globe round the lamp. Why is summer long and winter short?
See all earth and space labs or every Taz science lab.
Try it yourself
The Moon Phases lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.