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Dublin: 7°C Monday 21 September 2020

What is a solar eclipse and why is one happening tomorrow?

It will be our last for 11 years, so savour it.

Image: Matthew Fearn

UP TO 90% of sunlight will be covered across Ireland between 8.20am and 10.30am tomorrow.

People across Europe, North Africa and some of the Middle East will be able to see a solar eclipse for the first time since 1999.

Solar eclipses occur somewhere on earth every 18 months or so, but because of the earth’s axis, it can only ever be viewed by a small number of people.

Tomorrow, it will be our turn.

But, what is a solar eclipse?

Put simply, a solar eclipse happens when the moon’s shadow falls somewhere on the surface of the earth. A lunar eclipse is the opposite, when the earth’s shadow falls across the moon.

A total eclipse happens within the darkest part of that shadow.


Ireland will get a partial eclipse tomorrow because it is in the penumbra of the eclipse, but the Faroe Islands will be in total blackout because it’s in the umbra.

895px-Geometry_of_a_Total_Solar_Eclipse.svg Source: Wikimedia

What does it look like?

This was London in 1999:

Solar Eclipse Source: Matthew Fearn

Why don’t we get one every month?

So, if eclipses are caused by objects (the earth and moon) crossing paths, why don’t we get them more often?

There’s a couple of reasons for this. First, the earth’s orbit around the sun and the moon’s orbit around the earth don’t follow the same path. The moon’s orbit is tilted by five degrees.

While that sounds minimal, it is what keeps the shadows off each other most of the time.

During the 21st century, there will be 224 eclipses, but very few total eclipses. The next total eclipse will be seen in parts of Malaysia and Indonesia next March.

Why do we get them at all?

Because there are two points, sweet spots if you will, where the moon’s orbit crosses the sun’s plane. These are called nodes. As the earth moves along its orbit, the moon lines up with the nodes about twice a year each.

If the moon comes between the sun and the earth, we get a solar eclipse. If the moon is behind earth, we get a lunar eclipse.

But the moon is tiny and the sun is massive, no?

Well… yes, but.

This is one of those crazy coincidences about the solar system. Even those the sun is around 400 times larger than the moon, it is 400 times further away from earth, which means that from our perspective, they look the same size.

Think of it like putting your hand six inches in front of your face while looking at a building. It’s not bigger than the building, but it can block your view of it.

However, the moon’s orbit around the earth is elliptical, meaning our view of its size varies by around 12% some months. When it’s closer to earth, we get total eclipses.

Should I look directly at it?

If you want to hurt and damage your eyes, work away. If you value being able to, you know, see things, don’t do that.

You can make your own projector, though. NASA has details here.

Read: Could the solar eclipse cause blackouts in Ireland?

Read: These five graphs dig into the figures behind wind energy in Ireland

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