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 shadows.
Slide the Moon across the Sun. Watch the meadow go dark. When do you see the corona?
Drag the Moon disc across the Sun. The sky darkens mostly in the last minutes before totality, and only totality reveals the corona.
Slide the Moon over the Sun. Watch the sky get dark!
Drag the Moon around its orbit, or tap New moon and Full moon. Where do you get a solar eclipse? A lunar one?
A solar eclipse needs new moon (Sun–Moon–Earth); a lunar eclipse needs full moon (Sun–Earth–Moon). The window shows the view from Earth.
Move the Moon around the Earth. Can you make the Moon’s shadow land on Earth?
Seen from far above. Not to scale: the real Moon is about 30 Earths away.
Move the Moon nearer or farther along its oval orbit. When is it a total eclipse, and when a ring of fire?
The Moon’s apparent diameter varies by up to about 14% between perigee and apogee. Too far, and it can’t cover the Sun: an annular eclipse.
Move the Moon near and far. When does it hide the whole Sun?
The oval of the orbit is exaggerated so you can see it.
Pick a month, then drag the Moon round its tilted orbit. In which months can the shadow hit?
The Moon’s orbit is inclined about 5°. Only new or full moons near the nodes (the crossing points) line up. In this toy year the nodes face the Sun in March and September.
The tilt is drawn bigger than 5° so you can see it; the months are a simplified model.
Slide the Moon up and down through Earth’s shadow. When does it turn red?
The penumbra barely dims the Moon; inside the umbra it is lit only by sunlight refracted through Earth’s atmosphere, which is why it turns red.
Slide the Moon into Earth’s shadow. What colour does it turn?
Pick a month and move the Moon. Can you find a solar AND a lunar eclipse?
Change the month (the Sun’s direction) and the Moon’s position. Eclipses only occur near the nodes, at new or full moon.
Sometimes, on a sunny day, the sky slowly gets dark. The air feels cool. Some flowers close up, and chickens think it's bedtime! This is a solar eclipse. It's just a shadow.
When you stand in the sunshine, you make a shadow. The Moon and the Earth make shadows too.
The Moon goes round and round the Earth. Once in a while, it goes right in front of the Sun. Its shadow falls on the Earth, and the Sun gets covered up. That's a solar eclipse.
The Sun is much, much bigger than the Moon. But the Sun is very far away, so it looks small, just like the Moon. It's like hiding a faraway house behind your thumb!
Sometimes the Earth is in the middle, and the Earth's shadow falls on the Moon. The Moon does not go black. It turns a dim, rusty red. It's safe to look at with just your eyes.
A solar eclipse happens when the Moon passes directly between the Sun and Earth. The Moon blocks some or all of the Sun's light, and its shadow falls on part of Earth.17 Anyone standing in that shadow sees the Sun partly or completely covered.
A lunar eclipse is the same trick the other way round: Earth passes between the Sun and the Moon, and Earth's shadow falls on the Moon.3 So every eclipse is really just a shadow. As the OpenStax astronomy textbook puts it, an eclipse happens whenever Earth or the Moon enters the shadow of the other.10
For the Moon to get between us and the Sun, it has to be on the same side of Earth as the Sun. That position is the new moon phase, when the Moon's sunlit side faces away from us.1 A lunar eclipse needs the opposite line-up, with the Moon on the far side of Earth from the Sun. That's the full moon.3 (Curious about phases themselves? See why the Moon has phases.)
| Solar eclipse | Lunar eclipse | |
|---|---|---|
| Line-up | Sun, Moon, Earth | Sun, Earth, Moon |
| Whose shadow? | The Moon's, falling on Earth | Earth's, falling on the Moon |
| Moon phase | New moon | Full moon |
| Who can see it? | Only people in a narrow path or nearby | Everyone on the night side of Earth |
| How long? | Totality lasts seconds to about 7.5 minutes | Totality can last up to 1 hour 40 minutes |
| Safe to look at? | Only through eclipse glasses or a pinhole projector | Yes, with bare eyes |
Sources for the table: NASA and OpenStax.135610
A shadow in space has two parts. The dark inner cone is the umbra, where the Sun is completely blocked. Around it is the fainter penumbra, where only part of the Sun is blocked.1
The Moon's umbra is small by the time it reaches Earth. A typical path of totality may be less than 50 miles wide, while the partial eclipse can be seen over a thousand miles or more.1 Because Earth spins and the Moon moves along its orbit, the shadow races across the planet, tracing a path in about four to five hours.1 NOAA's weather satellites have even photographed the Moon's shadow crossing North America.9
A solar eclipse starts when the Moon first touches the edge of the Sun. For about an hour, more and more of the Sun is covered.10 In the last minutes before totality the sky darkens, some flowers close up and chickens may head off to roost.10 Then the bright disc disappears and the Sun's corona, its faint outer atmosphere, flashes into view around the black Moon.107 It can get as dark as dawn or dusk.2
Totality lasts anywhere from about 10 seconds to about 7.5 minutes.5 Then the Moon uncovers the Sun and the partial phases repeat in reverse.10
This is one of the luckiest coincidences in the solar system. The Sun is about 400 times wider than the Moon, but it is also about 400 times farther away.110 So both look almost exactly the same size in our sky: about half a degree across, roughly half the width of your little finger held at arm's length.11
400× wider ÷ 400× farther = the same size in the sky
The Moon's orbit is not a perfect circle, so its distance from us changes.1 At its closest it can look up to 14% bigger than at its farthest.11 That gives four kinds of solar eclipse:2
There's a new moon and a full moon every month, so why not an eclipse each time? Because the Moon's orbit is tilted about 5 degrees compared with Earth's path around the Sun.13 Picture two hula hoops with the same centre, one tipped a little.10 At most new moons the Moon passes just above or below the Sun, and its shadow misses Earth. At most full moons it slips above or below Earth's shadow.3
The two paths cross at only two points. An eclipse can only happen when a new or full moon falls near one of those crossings. That happens in two short eclipse seasons a year, roughly every six months.111 Altogether, the Sun, Moon and Earth line up for an eclipse of some kind about four to seven times a year.3
A lunar eclipse happens at full moon, when Earth sits exactly between the Sun and the Moon and Earth's shadow falls on the Moon.3 Earth's dark shadow is about 1.4 million kilometers long. At the Moon's distance it is wide enough to cover about four full moons, which is why lunar eclipses can last so long.10 There are three kinds:3
About 20 minutes before the Moon reaches the dark shadow it dims a little. Then the curved edge of Earth's shadow creeps across it.10 In a total lunar eclipse the Moon doesn't vanish. It usually glows a dull coppery red, sometimes called a "blood moon".106
That red light is sunlight that has passed through Earth's atmosphere and been bent into the shadow.10 On the way, blue light scatters away (the same reason the sky is blue), while red and orange light gets through.3 NASA describes it as if "all the world's sunrises and sunsets are projected onto the Moon." The more dust or clouds in Earth's air, the redder the Moon looks.6
Totality can last up to about 1 hour 40 minutes.10 You don't need any special equipment to watch; binoculars just make the colour richer.6
Solar eclipses actually happen about as often as lunar eclipses. But a solar eclipse can only be seen from a small area each time, while a lunar eclipse can be seen from the whole night side of Earth.310 That's why most people have seen more lunar eclipses than solar ones. A total solar eclipse happens somewhere on Earth roughly once every year or two,11 but NASA estimates it would take about a thousand years for every spot in the lower 48 US states to get one.5
Eclipses also repeat in a pattern. After about 18 years, 11 days and 8 hours (the Saros cycle), the Sun, Earth and Moon return to almost the same arrangement and a nearly identical eclipse happens.5 Coming up next: an annular solar eclipse on February 6, 2027 over South America and Africa, and a total solar eclipse on August 2, 2027 across southern Spain, North Africa, Saudi Arabia and Yemen.12
Not forever. The Moon is slowly drifting away from Earth, about 1.5 inches (3.8 cm) a year. In another 600 million years or more, it will look too small to cover the Sun completely, and Earth will only get annular and partial eclipses.311
The Moon passing directly between the Sun and Earth. The Moon blocks some or all of the Sun's light and its shadow falls on part of Earth. People inside the dark centre of the shadow see a total eclipse; people in the paler outer part see a partial one.
Earth passing directly between the Sun and the Moon, so Earth's shadow falls on the Moon. That can only happen at full moon, when the Moon is on the far side of Earth from the Sun.
The Moon slowly covers the Sun over about an hour. In a total eclipse, the sky darkens like dusk, the air cools, some flowers close and animals act as if night is falling. For a few minutes the Sun's faint outer atmosphere, the corona, appears around the black Moon. Then the Moon slides off again.
The full Moon first dims slightly in Earth's faint outer shadow, then a dark 'bite' creeps across it as it enters the main shadow. In a total lunar eclipse the whole Moon turns a dull coppery red, for up to about 1 hour 40 minutes, then the shadow slides off.
The Moon's orbit is tilted about 5 degrees compared with Earth's path around the Sun. At most new and full moons the Moon passes a little above or below the Sun or Earth's shadow. Eclipses only happen when a new or full moon falls near one of the two points where the paths cross, in two 'eclipse seasons' each year.
Somewhere on Earth one happens roughly once every year or two, but the path of totality is narrow, often less than 50 miles wide. Any one spot waits a long time: NASA estimates it takes about a thousand years for every location in the lower 48 US states to see one.
A lunar eclipse is completely safe to watch with your bare eyes. A solar eclipse is not: for the whole eclipse, always use proper eclipse glasses (not sunglasses) or a pinhole projector with the Sun behind you, and ask a grown-up to help.