Why does the Moon change shape every night?
After you watchWhy does the Moon change shape every night?
The short answer
The Moon changes shape because the Sun always lights up exactly one half of it, and as the Moon orbits Earth over about a month we see that lit half from a different angle each night. When the lit half points mostly away from us we see a thin crescent; when it points toward us we see a full Moon. Nothing is taking bites out of the Moon, and the dark part of a crescent is not Earth's shadow.
Try this next
- What if you keep walking the Moon all the way around Earth without stopping? Walk the Moon past the full spot and keep going. Before you do, predict: does the lit sliver shrink back to a crescent on the other side, or stay full? Watch which side the sliver hides on now.
- What if you moved the Sun to a different spot instead of moving the Moon? Imagine dragging the Sun around to the side. Predict which way the lit half would point, then check tonight's real Moon: the bright edge always points toward where the Sun went down.
- What if Earth's shadow could actually reach the Moon? Flip on the shadow toggle at the full-Moon spot and watch where the cone points. Predict first: will it land on the Moon, or sail past? That rare hit is a lunar eclipse.
Now you — bend it
- What if Slide YOURSELF (the walk control) all the way around to the spot directly between the Sun and the Moon, and try to find a phase even thinner than the thinnest crescent.When the lit half points fully away from you it's a new Moon, 0% lit. Predict whether the porthole goes completely dark, or whether some sliver always survives.
- What if The model lights exactly half the Moon and lets you orbit a flat circle. The real Moon's orbit is tilted about 5 degrees to Earth's path around the Sun. Imagine flattening that tilt to 0 degrees.With zero tilt, Earth's shadow would line up with the Moon every single full Moon. Predict how often a lunar eclipse would then happen: once a year, or every month?
- What if Flip Earth's shadow ON, then walk to a thin crescent and to the full-Moon spot and compare where the cone points each time.The shadow always shoots straight away from the Sun. Predict at which single orbit position the Moon could ever sit inside that cone, and why it's never the crescent.
Can you prove it?The dark part of a crescent is the Moon's own unlit half, not Earth's shadow, and the lit fraction you see depends only on the Sun-Moon-you angle. — Walk to several spots and read the lit fraction from the porthole; it follows k = (1 - cos a)/2, where a is the angle between the Sun direction and the Moon. Now turn the shadow on at each spot: the cone always points opposite the Sun and only reaches the Moon's orbit at the full-Moon position. So at a crescent the shadow is provably nowhere near the Moon, yet the dark part is still there.
Design your own test:Before you slide it, predict which orbit position gives a half Moon, and whether the lit half should be exactly 50% there, or a bit more or less.
Explain it to a 6-year-old: The Sun always lights one whole side of the Moon, like a ball half in sunshine, and the shape you see is just how much of the bright side is turned toward you tonight.
The whole story
How it works
The Moon makes no light of its own; it only reflects sunlight, and the Sun can light only the half of the ball that faces it. So one half of the Moon is always bright and the other always dark. As the Moon travels around Earth once a month, our viewing angle onto that fixed half-lit ball keeps changing. When the Moon is roughly between Earth and the Sun, its lit half faces away from us and we see almost no Moon (new Moon). When the Moon is on the far side of Earth from the Sun, the lit half faces us and we see a full Moon. Every shape in between — crescent, half, and gibbous — is just how much of the always-lit half happens to point our way that night.
What people get wrong
Many people think Moon phases are caused by Earth's shadow falling on the Moon and taking bites out of it. They are not. The dark part of a crescent is simply the unlit half of the Moon seen at an angle. Earth does cast a shadow, but it points straight away from the Sun and almost always misses the Moon. Earth's shadow only lands on the Moon during a lunar eclipse, which is rare.
The catch
The angle idea explains every single phase — crescent, half, gibbous and full — with no shadow needed at all. But Earth's shadow is still real: a few times a year the full Moon slides into it and darkens to a coppery red, which is a lunar eclipse. Eclipses are uncommon because the Moon's orbit is tilted about 5 degrees, so most months the shadow passes just above or below the Moon and misses it.
Questions kids ask
Is the dark part of a crescent Moon caused by Earth's shadow?
No. The dark part is just the unlit half of the Moon, seen from the side. The Sun always lights one half of the Moon; when most of that lit half points away from us, we only see a thin lit sliver. Earth's shadow points away from the Sun and normally misses the Moon entirely.
Why does the Moon look full some nights and like a sliver other nights?
Because the Moon orbits Earth, so our angle onto its always-half-lit surface changes night by night. When the lit half faces us we see a full Moon; when it faces mostly away we see a crescent. A full cycle from new Moon to new Moon takes about 29.5 days.
When does Earth's shadow actually fall on the Moon?
Only during a lunar eclipse, when the full Moon passes through Earth's shadow and turns dark or coppery red. It is rare because the Moon's orbit is tilted about 5 degrees, so most full Moons pass just above or below Earth's shadow instead of through it.
Does the Moon make its own light?
No. The Moon has no light of its own. It only reflects sunlight, which is why exactly one half of it is lit at any time and the shape we see depends entirely on our viewing angle.
Talk about it
- Before we look outside — guess what shape the Moon is tonight, and what shape it will be in a week.
- If the Moon makes no light of its own, where do you think all its brightness comes from?
- When you see a thin crescent, what do you think the dark part of the Moon actually is?
For grown-ups
The Moon shines only by reflected sunlight, so exactly one hemisphere is sunlit at any instant. Phases come from the changing Sun–Moon–observer geometry as the Moon orbits Earth over its roughly 29.5-day synodic month: we see a varying fraction of the lit hemisphere. New Moon sits roughly between Earth and the Sun with its lit side facing away; full Moon is opposite the Sun with the lit side facing us. Earth's shadow has nothing to do with ordinary phases — it falls on the Moon only during a lunar eclipse, which is uncommon because the lunar orbit is inclined about 5 degrees to the ecliptic, so the shadow usually passes above or below the Moon.
Keep going
What else makes you wonder?
- If the Sun only ever lights one half of the Moon, is there a part of the Moon that never gets daytime?
- We always see the same face of the Moon — so what does the side we never see look like right now?
- The Sun and the Moon look almost the same size in our sky — is that just luck, or is there a reason?