Three hundred years ago today, on May 3, 1715, the skies began darkening over England at about 9:30 in the morning. Clouds were not passing in front of the sun–the moon was. A rare total solar eclipse occurred, blocking out the sun’s disc completely. The period of darkness lasted from three to four minutes in most places. In London, where thousands of people looked skyward to see the sight (warnings about not looking directly at an eclipse weren’t common then as they are today), the period of darkness lasted 3 minutes, 33 seconds. Most of northern Europe, including Holland, the Scandinavian countries and Russia, fell into the shadow of the eclipse. Given that the weather was good across Europe that day, the 1715 eclipse was one of the most spectacular astronomical events of the early modern period.
One of the many people who observed the eclipse that day in London was Edmond Halley, the noted British astronomer, inventor, explorer and later the Astronomer Royal of Great Britain. The eclipse held special significance for Halley because he had predicted it, using the same sort of mathematical analysis and astronomical observation with which he correctly predicted the return of the comet that bears his name, which was last seen in 1986. Halley understood the motions of stars and planets better than almost anyone in Europe at the time, and almost anyone in the world since the age of the great astronomers of the medieval Arabian civilizations. Halley’s computations were so precise that he came within four minutes of predicting the exact time of day the total darkness period would occur.
This is Halley’s prediction of the 1715 eclipse. The date given is April 22 instead of May 3 because Britain, late in adopting the Gregorian calendar, was still using “old style” dates.
Although you could say it’s “just” an eclipse–they’re rare, but they do happen regularly, the most recent one visible from Europe occurring on March 20 of this year, 2015–Halley’s prediction of the darkness was actually pretty significant because of the time he lived and what was going on in intellectual thought and the history of science at the time. The year 1715 fell during the period known as the Enlightenment, which was a flowering of thought, particularly in the sciences and philosophy, that historians mark as one of the key prerequisites for our modern world. Indeed, virtually everything around us in our daily lives today, from the technology in our pockets to the Constitutional rights we debate with such ferocity, is rooted to one degree or another in the Enlightenment. Although the Enlightenment is usually associated with France, in fact it was a conversation occurring trans-nationally and across many disciplines. Halley and his discoveries were certainly part of that conversation, though Halley is not thought of as an Enlightenment thinker as often as people like Rousseau or Voltaire.
Yet Halley’s discoveries were arguably more “enlightening” than the contributions of many other Enlightenment thinkers, for one very obvious reason: ordinary people could see that he was right. Unlike philosophers like Immanuel Kant or others whose lofty ideas operated in the realm of theory–useful and influential though it was–Halley was a mathematician whose calculations were aimed at describing the physical world. His prediction about the eclipse could be easily verified, and when it actually happened, there was no doubt about the efficacy of the scientific tools he used. He was right, and everybody could see it, right up there in the sky. It’s hard to imagine a more powerful validation of the philosophy of reason and science.
Halley died in 1742 and is remembered today as one of the most brilliant scientific thinkers Britain has ever produced. The comet named after him–and yet another visible manifestation of the power of his intellect–will make its next scheduled appearance in the summer of 2061.