Topic of the Moment – solar eclipses

The UK will see a partial solar eclipse on 20 March

Luc Viatour /

The Pomo people indigenous to northern California believed that eclipses were due to a bear taking a bite out of the Sun. To the Vietnamese, the hungry culprit was a giant frog; the Chinese blamed dragons, the Vikings wolves, and ancient Hindus claimed the Sun was swallowed by the disembodied head of the spirit Rahu, punished for stealing diving nectar.

Inuit mythology comes a little closer to the truth: the Moon god and Sun goddess have a celestial falling-out, the latter walking away but eventually being caught by the former.

The more mundane, although still visually impressive, reality is simply that a solar eclipse occurs when the view of the Sun from the Earth is obscured by the Moon passing between the two.

The Moon will cast a shadow about 100 miles wide in which the Sun is obscured completely, and it will be partly covered over a much more extensive area. This month’s event will be a total eclipse from the Faroe Islands and Svalbard, but only a partial eclipse in the UK. Total solar eclipses happen about once every 18 months, but extremely rarely reoccur in the same place.

Sagredo / Wikimedia Commons

Under observation

Records of solar eclipses go back thousands of years, but the first telescopic observation wasn’t made until 1709 in France. Six years later, astronomer Edmund Halley observed the eclipse of May 1715, predicting its timing to within four minutes and drawing a map of the path of the moon’s shadow – which turned out to be inaccurate by about 18 miles. The first eclipse photograph was taken in July 1851.

Looking directly at the Sun can cause retinal damage, so care must be taken when viewing an eclipse. One way to watch an eclipse safely is via pinhole projection: poke a very small hole through one sheet of white cardboard and let sunlight pass through it and form an image on another sheet. The image will be brighter when the second sheet is closer to the first and larger when further away – observers can strike a balance that works for them.

Alternatively, an eclipse can be viewed directly using solar filters, which can be bought in the form of glasses. Normal sunglasses are unsafe, however.

Telescopes and cameras will need specially designed, metal-coated solar filters.

Blinded by science

Solar eclipses are not merely spectacular events, though – they’re also an opportunity to do some valuable science, allowing observation of phenomena normally hidden or outshone by the Sun’s light.

The solar corona – the Sun’s plasma outer atmosphere – is one such area for study. We normally can’t see the corona (at visible-light wavelengths) since the Sun’s light is too bright, but eclipses provide a chance to get a better look, and astronomers are set to descend on the Faroe Islands and Svalbard in the hope of getting more insight into why the corona is so much hotter than the Sun’s surface.

In 1919, Arthur Eddington travelled to the island of Príncipe, off the west coast of Africa, to take advantage of a solar eclipse and test one of the predictions of Einstein’s general theory of relativity – that the path taken by light bends in a gravitational field, known as gravitational lensing.

One way of testing that theory is to compare the light from distant stars at one time and at another time when it has to pass by a massive body such as the Sun – which is only possible during an eclipse, as the stars aren’t visible during the daytime. Eddington’s expedition did discover a lensing effect, and this served as early confirmation of general relativity.

In turn, general relativity solved another puzzle: variations in Mercury’s orbit that were at odds with Newton’s theory of gravity. Until then it was thought that there may be another planet orbiting the Sun closer than Mercury does, which would also only have been visible during an eclipse. None was ever seen and Einstein’s theory made the idea redundant, but the name given to the hypothetical world, Vulcan, at least survived in science fiction.

Making history

Eclipses can also be used to study the past. Because their occurrence is easily predicted, or previous eclipses backtracked, they can be used to ‘fix’ the dates of historical events.

The Greek historian Herodotus wrote of an eclipse having occurred before the Persians under Xerxes I launched an invasion of Greece, traditionally believed to have been around 480 BC. Astronomer John Russell Hind found that there would have been an eclipse in the area on 17 February 478 BC – we can now work out more accurately when the events of the period happened.

Historical eclipses are incredibly useful in determining the chronology of ancient China and the near east, and attempts have even been made to put an accurate date on the crucifixion of Jesus based on a biblical mention of a darkening of the Sun, although they weren’t successful.

So whether you’re travelling to see the total eclipse or enjoying the partial one from home, you’ll be looking at an event that has grabbed the attention of scientists, stargazers and mythmakers for millennia. But if you miss it, the next one will be along on 10 June 2021.