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TOTAL SOLAR eclipses have awed people since time out of mind. The first record of one, preserved on a clay tablet found at Ugarit, now in Syria, is believed from its age and location to describe either an eclipse that happened in 1375 BC or one in 1223 BC. Legendary explanations for eclipses include the Sun being eaten by dogs, frogs or dragons. The reality is more prosaic. Eclipses are caused by the Moon moving between the Sun and Earth in a way that casts a shadow on part of the Earth’s surface. Why are total ones, like the one crossing part of North America on April 8th, so rare?
Solar eclipses are a special case of phenomena called transits and occultations, in which an intervening heavenly body stops light from a star reaching an observer. If the obscuring orb appears smaller in the sky than the star, the result is called a transit and looks like a dark spot crossing the star’s surface (in the solar system Mercury and Venus transit the Sun, from Earth’s point of view, from time to time). If the obscuring object appears larger than the star, the star disappears completely—an occultation.
Solar eclipses may be either of these things, since the apparent sizes in the sky of Sun and Moon, viewed from Earth, are almost identical. Which appears bigger depends on how far each of the three bodies involved is from the other two at the moment in question. If the result is a transit, an eclipse is known as an annular (from the Latin anulus, meaning “ring”) because even at its maximum, a ring of the Sun’s luminous photosphere is still visible around the Moon. A total eclipse, though, is an occultation. The photosphere is covered completely, leaving visible only the corona, an envelope of thin plasma superheated by the Sun’s magnetic field.
If the Moon orbits Earth in the same plane as Earth orbits the Sun, eclipses would happen every month but would be total only in the tropics. In reality, the average interval between total eclipses is 18 months, and they may be seen from time to time all over the world. The path of totality across Earth’s surface is narrow and the period short (a maximum of just over seven and a half minutes). Outside these boundaries the Sun will appear partially eclipsed, looking like a pie that something has taken a bite from.
Besides being beautiful (the effect is like a black hole punched through the sky) total eclipses are useful. Past eclipses can be dated and located accurately, which allows the correlation with modern calendars of ancient records that mention them. The blocking of the photosphere, meanwhile, enables the inner part of the corona to be studied (telescopes that mimic eclipses by blocking the photosphere also occlude this region). And one particular eclipse, in 1919, was used to test Einstein’s idea that gravity affects the passage of light. Measuring the positions in the sky of stars near the eclipse’s limb showed that they were, indeed, out of normal alignment.
On April 8th observers in North America will witness this awesome phenomenon. At 11.07am local time, the Moon’s shadow will make landfall near Mazatlán, on Mexico’s Pacific coast. It will cross the American border near Radar Base in Texas. The eclipse will initially last four minutes and 27 seconds, and shrink to two minutes 53.5 seconds by the time it reaches the coast of Newfoundland, at 5.13pm.
The Great North American eclipse as it has been called, will be a sight to behold. But it should also be cherished because total eclipses of the Sun will not happen forever. Tidal friction causes the Moon to recede from Earth at 3.8cm a year, making it appear smaller and smaller in the sky. In 600m years or so the last, fleeting totality will occur. If any humans are still around after that, they will have to make do with annular.
© 2023, The Economist Newspaper Limited. All rights reserved. From The Economist, published under licence. The original content can be found on www.economist.com
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