Two things line up, and one slides into another’s shadow. Such events need a diary, because they do not come round often.
Earth casts a shadow into space, a long dark cone pointing away from the Sun. Usually the Moon sails past above or below it. Occasionally it goes straight in.
When it does, the Moon does not vanish. It turns a deep, dusty red. The only light reaching it has grazed the edge of Earth. On the way it is bent through our air, which strips out the blue. So that red is every sunrise and every sunset happening on Earth at that moment, all at once, thrown onto the Moon.
Anyone on the night side of Earth sees the same thing at the same time. And it lasts for hours. This is the easiest big sight in the sky.
Find my next lunar eclipse See how it works — the total eclipse of the Moon on 31 December 2028The same idea the other way round. The Moon passes between us and the Sun and drops its shadow on us. But the Moon is small. Its shadow is a dark spot only a few hundred kilometres wide at most. It races across the ground. Stand in that spot and the Sun goes out completely. Stand outside it and you only see a bite taken out.
There is a lovely coincidence behind this. The Sun is about 400 times wider than the Moon. It is also about 400 times further away. So from here the two look almost exactly the same size. That is why the Moon can cover the Sun so neatly.
“Almost” is the word to watch. The Moon’s distance changes, so:
Total — the Moon is near enough to cover the Sun completely. The sky goes twilight-dark in the middle of the day. It gets colder. Birds go quiet. And the Sun’s faint outer atmosphere, the corona, comes out. Ring — the Moon is a little too far away, so a bright ring is left all around it. Partial — you are off to one side of the shadow, and only part of the Sun is covered.
A partial eclipse is far less exciting than the numbers make it sound. Even with 95% of the Sun hidden, the sliver that is left is so bright that the day just looks a bit odd. The step from 99% to 100% is not a small last step. It is the whole show.
Only through a filter made for looking at the Sun. Check it for damage every single time. Safer still, and no filter needed: make a small hole in a piece of card, hold it up in the sunlight, and look at the bright little disc it throws onto a second card. That disc is the Sun. Look at the card, never at the sky.
The Sun can burn the back of your eye for good. It does not hurt while it happens, so there is no warning to react to. If a child is looking, a grown-up must be in charge of this.
The Moon goes round us once a month. So you might expect an eclipse every month, one of each. We do not get them.
The Moon’s orbit is tilted by 5.145 degrees to the Earth’s. That does not sound like much. But the Sun and Moon are only half a degree wide. Missing by 5.145 degrees is missing by ten times their own width. A shadow only lands when the Moon crosses the plane of the Earth’s orbit close to the time it lines up with the Sun. That is why eclipses come in clusters.
Those clusters have a name: an eclipse season. There are two a year, about six months apart, because the Sun has to pass close to one of the two points where the Moon’s tilted path crosses ours. Counting both kinds together, a year holds four to seven eclipses — four is much the commonest, and seven happens only two or three times a century. Space marks both seasons on the Earth’s orbit, and they creep slowly backwards year by year.
The Moon is always moving eastwards against the stars — about its own width every hour. That is far too slow to notice by staring. But once in a while it drives straight over a bright star or a planet. That is the moment you can see it moving.
What surprises everyone is how it goes. There is no fading and no dimming. The star is there, and then between one blink and the next it is gone. That is a real measurement, not a trick of the eye. It means the Moon has no air. Anything with air would smear the star out over several seconds as it sank through the haze. That is how a star low over our own horizon dims and flickers before it sets.
The Moon is close enough that where you stand changes where it appears to sit, by up to a degree, which is about twice its own width. So an occultation is never a fact about the night. It is a fact about your town. It is the same effect, and the same maths, that decides who stands in the shadow during a solar eclipse.
Only four bright stars can ever be hidden: Aldebaran, Regulus, Spica and Antares. The Moon never wanders more than about six degrees from the ecliptic. Those are the only bright stars in that lane. The planets are all in the same lane, so any of them can be covered too. And a planet is a tiny disc, not a point. So it takes several seconds or more to go — over a minute for Jupiter.
Sometimes the star only clips the very edge of the Moon. Then it can wink out and back several times in a minute. It hides behind mountains on the Moon and shows again down the valleys between them. People with small telescopes time these to map the Moon’s edge. They have done it since long before anyone went there.
About
Space
Planetarium
How far is the horizon?
How big are galaxies
What happens when you zoom out
When a picture squashes the numbers
Measuring the sky with your hand
Why a day is longer than one spin — on most planets
Does the Moon go round the Earth?
Why the sea goes up and down
A catalogue of everything
Stars that change brightness
How a star is born, and the ways it can end
How the universe began, and how anyone could know
Where a space telescope parks · the Lagrange points
Where does the solar system end?
The cosmic distance ladder, in one page
1 · The size of the Earth
2 · The size and distance of the Moon
3 · How the solar system is arranged
4 · The distance to the Sun and planets
5 · The distance to nearby stars
6 · The distance to other galaxies
7 · The scale of the universe
What is left to explain
What this site does not cover
Everything in the sky