Your turn
Each lab is one part of the video. Drag, slide and see what changes.
Each lab isolates one part of the geometry. The drawings are not to scale.
Slide the sliders and watch the Moon.
The lamp always lights half the ball. Walk around it (or drag the red dot). Where do you see it all lit?
The ball's lit hemisphere never changes. Only the observer's angle does, and with it the visible lit fraction.
The lamp lights up half the ball. Walk around it. When does the ball look all bright?
Drag the Moon around its orbit, or slide through the month. Where is it at full Moon?
Phase follows the Moon's angle from the Sun as seen from Earth: 0° new, 90° first quarter, 180° full.
Drag the Moon around the Earth. Where does it look all round and bright?
Seen from above the North Pole. Not to scale: the real Moon is about 30 Earths away.
Slide through two years of full Moons. Most miss Earth's shadow. Which ones hit it?
The Moon's orbit is inclined about 5°. Only full Moons near the crossing points of the two orbits enter Earth's shadow.
The tilt is drawn bigger than 5° so you can see it, and the timing is a simplified model.
The real Moon spins once per trip around Earth. Switch the spin off, then move the Moon. What would we see?
Unlock the rotation and move the Moon: without synchronous rotation, the far side would turn toward Earth every orbit.
Move the Moon through the month and spin the clock. Can you find the Moon in a daytime sky?
The Moon rises later each day as it moves along its orbit. Find which phases share the sky with the Sun, and when.
Move the Moon and the time of day. Can you find the Moon in the sky with the Sun?
Rough times for mid-northern latitudes; real times depend on where you live and the season.
Some nights the Moon is a thin slice. Some nights it is big and round. But the Moon never really changes shape!
The Moon does not make its own light. The Sun shines on it, and the light bounces off. The Sun always lights up one half of the Moon. The other half is dark.
The Moon goes around and around the Earth. It takes about a month to go all the way around.
When the bright half faces away, we can't see the Moon at all. Then a thin bright slice peeks out. It grows, until the whole bright half faces us. That's a full Moon! Then it shrinks again.
Earth's shadow does not make the Moon's shapes. Once in a while, Earth's shadow touches a full Moon, and the Moon turns dim and red.
The Moon doesn't make its own light. It shines because it reflects sunlight.2 Sunlight comes from one direction, so at every moment exactly half of the Moon is lit (its day side) and the other half is dark (its night side).1 As the Moon travels around Earth, we see different amounts of that lit half. That changing view is what we call the phases of the Moon.17
So the Moon never really changes shape. Only our angle on its sunlit half changes.2
You can prove this in two minutes. In a dark room, put a lamp with no shade on one side of the room. Hold a ball (or an orange on a pencil) at arm's length, a little above your head. The lamp is the Sun, the ball is the Moon, and your head is Earth.
The ball was half lit the whole time. Only your view changed.
Astronomers name eight phases.18 "Waxing" means growing and "waning" means shrinking.2 The positions below are seen from high above Earth's North Pole, with the Moon moving counterclockwise.7
| Phase | Where the Moon is | What we see (Northern Hemisphere) | Rises / sets (roughly) |
|---|---|---|---|
| New Moon | Between Earth and the Sun | Nothing: the lit side faces away | Rises and sets with the Sun |
| Waxing crescent | A little past the Sun | A thin sliver on the right | Morning / after sunset |
| First quarter | A quarter of the way around | Right half lit | Around midday / around midnight |
| Waxing gibbous | Between quarter and full | More than half lit, growing | Afternoon / before sunrise |
| Full Moon | Opposite the Sun | The whole lit half | Around sunset / around sunrise |
| Waning gibbous | Heading back toward the Sun | More than half lit, shrinking | After sunset / after sunrise |
| Third (last) quarter | Three quarters of the way around | Left half lit | Around midnight / around midday |
| Waning crescent | Nearly back to the Sun | A thin sliver on the left | Before sunrise / afternoon |
Rise and set times from NASA; the exact times depend on where you live and the time of year.1 In the Southern Hemisphere, everything is flipped left to right: the waxing Moon is lit on the left.7 But no matter where you are, everyone on Earth sees the same phase at the same time.7
The first and last quarter phases are named for how far the Moon is through its cycle, a quarter and three quarters of the way around. They also happen to show a quarter of the whole Moon: half of the lit half.1
One full cycle of phases, from new Moon to new Moon, takes about 29.5 days. This is called the synodic month.17 Oddly, the Moon only takes about 27.3 days (27 days, 7 hours and 43 minutes) to go once around Earth measured against the distant stars. That's the sidereal month.7
Why the difference? While the Moon goes around Earth, Earth is also moving along its path around the Sun. So the Moon has to travel a little farther, about two more days, before it lines up with the Sun again and the next new Moon arrives.7
1 lunar cycle ≈ 29.5 days vs 1 orbit ≈ 27.3 days
No, and this is the most common mix-up. Earth's shadow always points straight away from the Sun. The only time the Moon can be on that side of Earth is at full Moon, which is the phase where the Moon looks brightest. When the Moon does pass through Earth's shadow, that's a lunar eclipse: the Moon dims and often turns a coppery red over a few hours.4 The red comes from sunlight bent through Earth's atmosphere. The blues scatter away, and the reds get through.4
The Moon's orbit is tilted by about 5 degrees compared with Earth's path around the Sun. Most full Moons pass a little above or below Earth's shadow. Only when a full Moon happens near the place where the two paths cross do we get a lunar eclipse, about two to four times a year.7 The same tilt explains why a new Moon usually doesn't cause a solar eclipse: it passes just above or below the Sun from our point of view.19
| Phases | Lunar eclipse | |
|---|---|---|
| Cause | Our changing view of the Moon's sunlit half | Earth's shadow falling on the Moon |
| When | All the time, in a 29.5-day cycle | Only at some full Moons |
| Looks like | A crisp curved edge between light and dark | A dim, often reddish Moon |
The Moon does spin, but it takes exactly as long to turn once on its axis as it does to go once around Earth. So it never turns its back on us, like a dancer circling a partner while always facing them.5 This is called tidal locking or synchronous rotation. Earth's gravity slowed the young Moon's spin until the two matched, and all the solar system's large moons are tidally locked to their planets.5
Because it spins so slowly, a day on the Moon (and a night) each last about two Earth weeks.3 As lunar day and night sweep across the side facing us, we see the phases. NASA puts it neatly: Moon phases are "the overlap between lunar daylight and the Moon's Earth-facing side."3
Not really. The side we never see is better called the far side. It gets just as much sunlight as the side facing us, only at different times.67 In fact, at new Moon, when our side of the Moon looks black, the far side is in full sunlight.6
The Moon isn't only a night-time object. When and where it's up depends on its phase. A first quarter Moon rises around midday and is often easy to spot on a sunny afternoon. A waning gibbous Moon can still be up after sunrise.1 The only phase you can't see is the new Moon. It's up during the day, but its dark side faces us and it's lost in the Sun's glare.1
Sometimes, during a crescent phase, you can faintly see the "dark" part of the Moon too. That's earthshine: sunlight bouncing off Earth, lighting the Moon's night side.1
The Sun always lights up half of the Moon. As the Moon travels around Earth, we see different amounts of that lit half: none at new Moon, all of it at full Moon, and slices in between.
No. Earth's shadow points straight away from the Sun, so it can only reach the Moon at full Moon. When it does, that's a lunar eclipse. Phases come from our changing view of the Moon's sunlit half.
New Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third (last) quarter and waning crescent. Then the cycle starts again.
About 29.5 days from one new Moon to the next. The Moon takes about 27.3 days to orbit Earth, but because Earth is moving around the Sun at the same time, it needs about two more days to line up with the Sun again.
In a dark room, hold a ball at arm's length near a lamp. Turn slowly in a circle. The ball is always half lit, but you'll see it go from dark, to a crescent, to half, to full and back, just like the Moon.
The Moon is tidally locked: it spins on its axis exactly once for every trip around Earth, so the same side always faces us. The far side isn't dark, though. It gets just as much sunlight.
The Moon is up for about half of every day, and when it's in the sky it's often in daylight. A first quarter Moon, for example, rises around midday and is easy to spot in the afternoon.