Eclipses

On 8 April 2024, in the middle of the day, the sky over parts of Mexico, the United States and Canada went dark. Street lights came on. Birds stopped singing. Stars came out in the daytime. A few minutes later the daylight came back.

Nothing was wrong with the Sun. Something got in its way.

Every ball in space has a shadow

Hold your hand in the sunshine and it throws a shadow on the ground. The Earth and the Moon do the same. Each one is lit by the Sun on one side, and each one throws a long shadow out into space on the other side, pointing away from the Sun.

Most of the time these shadows fall on nothing at all. An eclipse happens when the Sun, the Earth and the Moon line up, and one of the shadows falls on the Earth or on the Moon.

The Moon hides the Sun

Sometimes the Moon passes exactly between the Sun and the Earth. Its shadow falls on a small part of the Earth. For people standing there, the Moon covers the Sun. That is a solar eclipse ("solar" means "of the Sun").

The Moon passes between the Sun and the Earth, and its shadow falls on a small part of the Earth; from there, the Sun is covered by the dark Moon

When the Moon covers the whole Sun, it is a total eclipse: the sky goes dark for a few minutes, and a faint white glow appears around the black Moon. Outside the middle of the shadow, people see the Moon cover only part of the Sun.

The Moon is between the Earth and the Sun at new moon, which we met in the Phases of the Moon lesson. So a solar eclipse can only happen at new moon.

Careful

Even when most of the Sun is covered, the small part left can hurt your eyes. Only look at an eclipse through special eclipse glasses made for it. Sunglasses are not enough.

Just the right size

There is a lucky accident here. The Sun is about 400 times wider than the Moon, but it is also about 400 times farther away. So in the sky the two look almost exactly the same size, and the small Moon can cover the huge Sun just right.

The Moon is small and close, the Sun is 400 times wider and 400 times farther, so both look the same size from the Earth

The Earth's shadow on the Moon

Sometimes it is the other way round. The Earth passes exactly between the Sun and the Moon, and the Moon moves into the Earth's shadow. That is a lunar eclipse ("lunar" means "of the Moon").

The Earth is between the Sun and the Moon, and the Moon is inside the Earth's shadow, glowing a dark red

The Moon is on the far side of the Earth from the Sun at full moon. So a lunar eclipse can only happen at full moon. It lasts much longer than a solar eclipse, often more than an hour, and everyone on the night side of the Earth can see it. It is safe to watch with your eyes.

The Moon doesn't go black. It turns a dark red, like a copper coin. A little sunlight still reaches it: it passes through the Earth's air on the way, and the air lets red light through better than other colours. It is the same reason sunsets are red. During a lunar eclipse, the Moon is lit by all the sunrises and sunsets on the Earth at once.

Why not every month?

There is a new moon and a full moon every month. So why don't we get two eclipses every month?

Because the Moon's path around the Earth is tilted a little, about 5 degrees. Most months, at new moon the Moon passes a little above or below the Sun, and its shadow misses the Earth. At full moon it passes a little above or below the Earth's shadow. Only a few times a year do they line up exactly.

Seen from the side, the Moon's path around the Earth is tilted a little, so most months the Moon passes above or below the line from the Sun to the Earth

A total solar eclipse comes to some place on the Earth about every year and a half. But the Moon's shadow is small, so for any one town it is very rare: on average, about once in a few hundred years.

Try it at home

You need the lamp (the Sun), the orange (the Earth) and a grape or a small ball (the Moon).

In a dark room, hold the orange near the lamp.
Move the grape behind the orange, into its shadow: the grape goes dark. That's a lunar eclipse.
Now move the grape between the lamp and the orange. Look for its small shadow on the orange: anyone standing in that shadow would see a solar eclipse.
Move the grape a little higher or lower, and the shadows miss. That is what happens most months.

Words to know

eclipse - when the Sun, the Earth and the Moon line up and a shadow falls on the Earth or the Moon
solar eclipse - the Moon passes between the Sun and the Earth and hides the Sun
lunar eclipse - the Earth passes between the Sun and the Moon, and its shadow falls on the Moon

Big idea

An eclipse is a shadow. In a solar eclipse the Moon's shadow falls on the Earth, at new moon. In a lunar eclipse the Earth's shadow falls on the Moon, at full moon. The Moon's tilted path means they only line up a few times a year.