American Scientist

The appearance of a total solar eclipse is so distinctive, its effect so dramatic, that history records several instances when eclipses shaped human events. A 6th-century eclipse persuaded factions battling south of the Black Sea to declare a truce. In 1806, Shawnee warrior Tecumseh used his foreknowledge of an eclipse to score a diplomatic coup. Today, solar eclipses are unlikely to affect diplomacy, but experiencing one is no less spectacular. In this passage adapted from their new book, The Sun, Smithsonian astrophysicist Leon Golub and Williams College astronomer Jay M. Pasachoff describe what it’s like to witness a solar eclipse and explain how researchers came to understand the origin of the corona that appears during the fleeting minutes of totality. (Those interested in learning how art influenced research into the solar corona's visual trickery can check out “The Art and Science of Solar Eclipses,” by Richard Woo, in the July– August 2016 issue of American Scientist.)

When we stand in the location of a total solar eclipse, we are in an ordinary-looking place and may have traveled halfway around the world. But we know—and trust—the scientists who have predicted that, at the anointed time, we will be in the shadow of the Moon. Though the Moon begins partly to cover the Sun an hour or more before totality, one wouldn’t notice for a long time that anything is happening. People around you may be going about their business, completely unaware of the treat that lies ahead. But about 15 minutes or so before totality, the light takes on an eerie quality, something you can’t put your finger on. Only with hindsight might you realize that shadows have changed in some way: They are no longer being projected by the full disc of the Sun, a half-degree across, but by a narrow crescent, so the shadows look sharper.

For the last few minutes before totality, the sky grows noticeably darker. But the Sun is about a million times brighter than the full Moon, so even when only 1 percent—one one-hundredth—of the everyday Sun is left, the residual is still 10,000 or so times what the full Moon’s brightness would be at night, and it is still light outside. The tiny crescent of the Sun is small but still at full brightness, so it is still unsafe to stare at the residual solar disc directly; you need the increasingly available “eclipse glasses” to see the crescent. (These would be better described as “partial-eclipse glasses,” as they need to be removed for you to see totality once the bright photosphere is completely covered. Occasionally, people aren’t given correct instructions and leave them on during totality—so they see nothing of totality because the solar corona is much too faint to be seen through these solar filters.)

Then, things begin to change more quickly. The Moon almost entirely covers the Sun, with only a few beads of sunlight visible as the last bits of the everyday Sun are visible in the lunar valleys aligned your way at the edge of the Moon. These “beads” are named after the English astronomer Francis Baily, who saw and wrote about them at the 1836 eclipse. The beads had actually been seen and commented upon earlier by Harvard’s Francis Williams at the 1780 eclipse seen from what we now call Maine, though it was then part of Massachusetts and behind enemy (that is, British) lines for the American astronomers.

The last bead of sunlight—and there is usually only one—is so beautiful and relatively bright compared to the darkening sky that it has been known, since the 1925 solar eclipse in New York City, as the diamond-ring effect. Only at this time can you safely take off your partial-eclipse glasses and look at the celestial phenomenon directly.

What exactly is the halo, or corona, that may be seen around the lunar silhouette during a total solar eclipse? Is it the atmosphere of the Moon? Even in the 21st century, some people (not astronomers!) mistakenly believe so. The eclipses of 1715 and 1724 were observed from widely separated locations in Europe, and the appearance of the corona was the same at both locations. Had the corona been located on the Moon, only 400,000 kilometers away from us on Earth, instead of on the Sun, 150,000,000 kilometers away (about 400 times farther), the corona would have appeared shifted over as viewed from one side compared to the other, an effect known as parallax. (See the parallax effect by looking at a distant object behind your thumb at the end of your outstretched arm, though this time first from one eye and then from the other.) Also, the Moon was seen to move across the corona, rather than having the corona move with the Moon. So the corona was apparently associated with the Sun itself rather than being part of the Moon. This point remained controversial for decades. The photographic proof at the 1860 eclipse that bright solar prominences at the edge of the lunar silhouette did not show parallax helped convince scientists that the corona was solar.

From The Sun, by Leon Golub and Jay M. Pasachoff, © 2017. Published by Reaktion Books in association with the Science Museum of London. Used with permission of the publisher.