When Computers Were Human

Computing When Halley’s Comet Comes Closest to the Sun

In 1682, astronomer Edmund Halley discovered the comet that eventually came to bear his name. In 1695, he combined his findings in astronomy with Isaac Newton’s new theories on gravitation and planetary motion to deduce that the comet had an elliptical orbit around the sun. He proved that it had appeared before, in 1531 and 1607, and predicted its return in 1758. Halley could not give a precise date because such an equation needed to include the influence of the sun, the Earth and other big celestial bodies, such as Jupiter. Computations involving more than two celestial bodies were beyond the scope of Newton’s early theories. Determining such an orbit would have taken a vast number of calculations.

Many years later, French mathematician Alexis-Claude Clairaut, who had developed new mathematical techniques, hired two assistants to compute and forecast an accurate date for the comet’s 1758 return. They began in June 1757, using only “goose quill pens and heavy linen paper.” Clairaut predicted that the comet would reach “perihelion” – its closest point to the sun – within a month of April 15, 1758. The actual date...