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Black out: what happens during an eclipse?

The UK will see an annular solar eclipse where the Moon will partially cover the Sun. In the modern world, we may have an understanding of what’s going on, but the movement of the Moon across the Sun, right in front of our eyes, still captures our imaginations. What was it like then, when people had no understanding of astronomy, the sky darkened and the Sun seemed to disappear?

The fear of the unknown and a bit of politics drove the desire for understanding explains Dr James Poskett, Assistant Professor in the History of Science and Technology at the University of Warwick.

“Humans have always been fascinated by solar eclipses. In fact, some of the earliest surviving examples of human writing describe that eerie moment when the black disc of the Moon moves across the face of the Sun.

“A collection of ancient Chinese writings, known as the Book of Documents, records a solar eclipse that took place on the 22 October 2134 BCE, over 4000 years ago. We also have examples of clay tablets, carved in ancient Babylon—in what is now Syria and Iraq—which describe a total solar eclipse that took place on 3 May 1375 BCE.

“Solar eclipses tended to provoke both fear and wonder. Throughout history, an eclipse was often been seen as a bad omen, something that rulers needed to carefully plan for and respond to. It was this fear of the unknown, the potential that the Sun might be blocked out, which motivated the development of astronomy in the ancient world.

Predict the eclipse

“Rulers in ancient Babylon employed temple priests who also worked as astronomers. They measured the passage of the Sun and Moon, and by the seventh century BCE were able to predict the timing of an eclipse to within around eight hours.

“In China, the emperor—who was known as the ‘Son of Heaven’—also employed astronomers at the court. They too were expected to predict the occurrence of an eclipse. This was a risky business. If an astronomer failed to predict an eclipse, they could be dismissed, or even executed. No wonder then, by the beginning of the fourteenth century CE, Chinese astronomers could predict the timing of a solar eclipse to within around 15 minutes.”

Eclipses became predicted, understood and in then used in research. Dr Poskett continues:

“In more recent times, a solar eclipse provided evidence to support one of the most important theories in modern science. In 1919, in the aftermath of the First World War, the British physicist Arthur Eddington led an expedition to observe a total solar eclipse from the island of Príncipe, off the coast of West Africa. In doing so, Eddington was able to confirm Albert Einstein’s theory of general relativity, proving that the gravity of the Sun bends the light emitted from distant stars. Today, we may no longer believe that a solar eclipse is a bad omen, but they remain a source of both endless fascination and scientific advance.”

What exactly is going on?

So, we established a number of centuries ago that the sun was not been stolen and bad luck is not upon us. But what exactly is going on? Dr David Brown is a research fellow at Warwick’s Department of Physics working on extra-Solar planets. He explains:

“A Solar eclipse happens when the Earth, Moon, and Sun line up in such a way that the silhouette of the Moon blocks some or all of the Sun’s light. Effectively, the Moon casts a shadow on Earth’s surface.

“This is only possible at new moon, but it doesn’t happen every time - there are only between two and five solar eclipses each year. This is because the Moon’s orbit isn’t aligned with Earth’s orbit around the Sun, but is instead tilted over by more than five degrees so that during a new moon the Moon usually seems to pass by the Sun without covering it. What’s more, the combination of Earth’s orbit, Earth’s rotation, and the Moon’s orbit means that each eclipse is visible from different places on Earth.”

There are four types of solar eclipse:

  • A total eclipse happens when the Moon’s silhouette completely blocks the Sun’s light. These events are possible because of a quirk of the Solar system. Though the Sun is many times larger than the Moon it is also much further away, and the relative sizes and distances are such that the two objects look approximately the same size in the sky.
  • An annular eclipse happens when the Moon lines up exactly with the Sun but appears slightly smaller than it, so that there’s a ring of sunlight visible around the outside of the Moon’s silhouette.
  • A hybrid eclipse is one that is visible as a total eclipse from some places on Earth, but in other locations appears as an annular eclipse. These events are rare compared to the other types of eclipse.
  • A partial eclipse happens when the Moon and Sun aren’t lined up exactly, and the Moon’s silhouette only partially covers the Sun. These can be almost unnoticeable; more than 90% of the Sun’s surface needs to be obscured before the change in brightness becomes obvious.

“The difference between a total eclipse and an annular eclipse is due to how big the Moon appears,” continues Dr Brown. “The Moon’s orbit isn’t an exact circle, but is in fact oval shaped. This means that the Moon’s distance from Earth changes, so its visible size also changes depending on where the Moon is in its orbit. In general the Moon looks slightly smaller to us than the Sun, so annular eclipses are more common than total eclipses. It’s worth noting that the Moon is gradually moving away from the Earth, so there will come a time millions of years in the future when total solar eclipses no longer happen.

“For the moment though, we can enjoy the occasional experience of totality (the point when the Moon completely blocks the Sun). It’s very occasional though; a total eclipse happens somewhere on Earth roughly every 18 months, it’s in a different place each time and a particular location will only see a total eclipse about every 400 years. Totality is also fleeting, only lasting seven and a half minutes maximum, though in most cases it lasts for much less time.”

Safety first

A final tip from an experienced astronomy expert, Dr Brown adds: “You should never look directly at the Sun with unprotected eyes, even during an eclipse. There are many safe ways to view an eclipse. You can get specialist eclipse glasses, or project an image of the Sun onto a flat surface using binoculars, a pinhole camera, or even a colander.”


9 June 2021


James PoskettDr James Poskett is a historian whose research engages broadly with the global history of science and technology from 1750 to the present day. His second book, Horizons: A Global History of Science, will be published by Penguin in October 2021.

David BrownDr David Brown is a Research Fellow working in the field of extra-Solar planets. He is part of Warwick's Astrophysics Research Group, in the Department of Physics.

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