Halley’s Eclipse

“They will see that there is in it nothing more than Natural, and nomore than the necessary result of the Motions of the Sun and Moon.” — so wrote astronomer Edmond Halley about the solar eclipse that he predicted for 1715. For millennia, eclipses – especially solar eclipses — had been considered frightening events that portended bad news. You get a sense of the forboding that witnesses felt in the second image, a fifteenth-century illuminated manuscript showing an solar eclipse that happened during the reign of Alexander the Great.

But educated people had been able to predict eclipses as early as the Babylonian era, and by the time of the Ancient Greeks, astronomers had worked out the causes of lunar and solar eclipses. One of their tools has the most Tolkien name ever: “the Saros Cycle”. Basically, the Saros Cycle determines that every 18 years and 11 1/3 days, the moon and the nodes of its orbit return to the same place with respect to the sun. That has given astronomers since Ancient Greek times a rough estimate of when an eclipse would reoccur — but because it is off by a third of a day, it is imperfect: the eclipses appear in a different spot on our planet’s surface.

By the 18th century, the heliocentric model of the solar system had long been worked out. Britain experienced five total solar eclipses in that era, so there was a lot of ability to test predictive models. Britain also had the young astronomer superstar Edmond Halley, who was elected to the Royal Society when he was only 22. The image you see here was the most detailed eclipse map designed for public understanding. It showed with the darkest oval where the shadow of the moon would most obfuscate the sun in the totality of a solar eclipse.

Halley’s map was the start of a broad-based public understanding of the scientific nature of eclipses. We of course now understand that they are not portents of doom, but the results of the churning of the beautiful and awesome natural world.