17 totally awesome facts about an eclipse of the Sun

A total eclipse of the Sun captured in August 2008, showing delicate detail in the solar atmosphere, or corona. Image credit: Anthony Ayiomamitis

1. What causes a total eclipse of the Sun?

A total eclipse occurs when the Moon completely hides the Sun from view. By an amazing coincidence, though the Sun is 400 times bigger than the Moon, it is also 400 times farther away. That means that both appear the same size in the sky and so the Moon can fully mask the Sun when it passes in front of it.

2. Why don’t we see a total eclipse every month?

Eclipses can only occur at New Moon, and there is a New Moon every month. However, the plane of the Moon’s orbit is slightly tilted, by just over 5°, to that of the Earth’s orbit around the Sun. This means that, at most New Moon phases, the Moon actually passes above or below the Sun in the sky. Occasionally, New Moon occurs at the point where the Moon’s orbital plane intersects with the Earth’s, called a node, and an eclipse occurs.

3. Why can’t a total eclipse be seen everywhere?

During a total eclipse of the Sun, the shadow of the Moon is typically only around 100 miles wide (160 km) when it hits the Earth’s surface. Sometimes it is narrower and sometimes wider, depending on circumstances. As the Earth rotates and the Moon travels in its own orbit, this small shadow sweeps a path thousands of miles long across our planet. Only along that track will you see a total eclipse of the Sun.

4. What happens outside the shadow track?

If you are on either side of the eclipse track, you will not see a total eclipse, but a partial eclipse, and the Moon will not completely hide the Sun. The farther you are away from the track of totality, the smaller the partial eclipse that you will see. And beyond a certain point, the Moon does not hide any of the Sun at all. Also, of course, parts of the planet can’t see the eclipse because it is night and the Sun is below the horizon!

5. Is it safe to look at the Sun during a total eclipse?

Usually, you must never look directly at the Sun without an approved filter, such as welder’s glass, or the special eclipse glasses that are supplied for these events. These must be used throughout the partial phases of a solar eclipse. Only when the Sun is completely covered by the Moon, when the eclipse is total, is it safe to view with the unaided eye. The moment the Moon begins to move off the Sun, and a brilliant flash of sunlight appears, marking the end of totality, you must use your solar filter/eclipse specs again.

A young woman viewing the partial phases of a solar eclipse with specially filtered eclipse glasses. Image credit: Paul Sutherland

6. What will I see during a total eclipse?

You will be able to see nothing but the Sun until the moment that it is completely covered by the Moon. At that moment, when you are suddenly plunged into darkness, and you are at last able to view without a filter, you will see two special features of the Sun. One is the ghostly corona of its atmosphere, called the corona, the shape of which depends on whether the Sun is near maximum or minimum in its 11-year cycle of activity. And around the edge of the Moon’s silhouette, you may see fiery prominences, which are jets of hot plasma lifting off from the Sun’s edge. When totality ends, you will see what are called Baily’s Beads – brilliant flashes of sunlight shining through valleys on the Moon’s limb as it begins to move off the Sun. Totality is over! During the partial phases, and using your special eclipse glasses, you may see sunspots on the Sun’s visible surface if you are lucky and they are large enough.

7. Will it get dark during a total eclipse?

When totality begins, it will suddenly be dark, though there will be a bright glow like a sunset all around the horizon in all directions, from beyond the Moon’s shadow. At this moment, if you can bear to take your eyes away from the eclipse itself, you will be able to see the brighter planets and stars. At some historic eclipses, previously unknown bright sungrazing comets have been discovered during totality!

A wide-angle view of the total solar eclipse of July 2 over the La Silla observatory in Chile. Image credit: ESO/R. Lucchesi

8. Will it get cold during a total eclipse?

Yes, it can do! The drop in temperature as more and more of the Moon covers the Sun can be very evident, particularly at higher locations. So even if you eclipse is happening on a summer’s day, you will probably need a jumper or fleece to slip on to keep you warm!

9. How long does an eclipse of the Sun last?

It takes around two hours from the moment the Moon encroaches on to the Sun’s disk until the time it leaves it completely. However the length of totality is just a fraction of that. It varies according to factors such as how close the Moon is to us in its orbit. The longest that a total eclipse can last is just under 7 minutes 30 seconds. The shortest is less than 1 minute 30 seconds.

10. What are the various stages of a total eclipse?

The moment that the Moon starts to move in front of the Sun is called First Contact. The point where it completely covers the Sun is Second Contact. The moment totality ends, as the Moon begins to move off the Sun, is Third Contact, and the time when the last bit of the Moon moves off the Sun’s disk is called . . . you guessed it . . . Fourth Contact.

11. How do animals react to a total eclipse?

Scientists are still investigating how the animal world behaves during an eclipse. But it is said that they can become confused, with birds becoming quiet as if it is evening and dusk is approaching, and mammals beginning to settle down for “the night”.

12. What other types of eclipse are there?

Not every solar eclipse is a total eclipse. Often the Moon does not pass directly in front, so we only get a partial eclipse of the Sun. Also, the Moon’s orbit around the Earth is not circular but elliptical. At times when the Moon is at a far point in its elliptical orbit, it is not large enough in the sky to cover the whole Sun. Instead a ring of bright sunlight is left around the Moon at mid-eclipse and we have what is called an annular eclipse. There is another, rare type of eclipse, called a hybrid eclipse, where different observers along the eclipse track see either a total or an annular spectacle depending on their location on the Earth’s curved surface.

Three phases of a previous annular eclipse, photographed by the writer from Spain in 2005. Image credit: Paul Sutherland/Skymania

13. How frequent are solar eclipses?

Total eclipses of the Sun are rare from any particular part of the world because the spectacle is confined to such a narrow track. However, across the Earth as a whole, there occur an average of 2.4 solar eclipses a year. Sometimes tracks for different eclipses cross, so that lucky people at that intersection can witness both events (clouds willing). Parts of Missouri, Illinois, and Kentucky, in the USA, can see two total solar eclipses in under seven years, because they are on the track of the August 21, 2017, event and that of April 8, 2024.

14. Is there any pattern to eclipses?

Yes! When ancient humans first began to predict eclipses, they discovered that particular eclipses are linked in a pattern where they occur about 18 years and 11 days apart. This period is called a saros, and is the time it takes for the Sun, Moon and Earth to return to the same relative geometry. For example, the widely observed Great American Eclipse of August 21, 2017, occurs exactly one saros after the total eclipse of August 11, 1999, which crossed the UK and Europe.

15. Are eclipses useful to scientists and historians?

Total eclipses of the Sun first showed us the Sun’s faint atmosphere or corona,and today they still allow solar physicists to study its structure in detail, as well as other solar phenomena. A famous eclipse in 1919 allowed the British scientist Arthur Eddington to confirm distortions in space-time predicted by Einstein’s General Theory of Relativity.

Ancient eclipses are useful to historians because they can help pinpoint when wars and other events occurred. Also, knowing from where these eclipses were seen, they can tell how the speed of the Earth’s rotation and the length of the day has changed over time.

16. Is a total eclipse of the Sun really an eclipse?

Strictly speaking, the word eclipse is a misnomer. That’s because a body is in eclipse when it passes into shadow. An eclipse of the Moon IS an eclipse because the Moon passes into the Earth’s shadow cast into space. An eclipse of the Sun is more like an occultation, where the Moon physically hides a distant star.

17. Do other planets experience eclipses of the Sun?

Yes, the moons of the outer planets can pass in front of the Sun as seen from their surfaces. Probes sent to Mars have photographed its larger moon Phobos pass in front of the Sun, for example, though it is too small to cover up more than a small part of the disk. Nowhere else enjoys the remarkable geometry that makes a total eclipse from Earth so special.

Related: What is an eclipse of the Sun?

Related: What is an eclipse of the Moon?


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An eclipse of the Sun by Phobos as seen from the surface of Mars by NASA’s Curiosity rover on August 20, 2013. Image credit: NASA/JPL-Caltech/Malin Space Science Systems/Texas A&M Univ.

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