Halley's Comet

1705: Astronomiae cometicae synopsis

In 1705 Edmond Halley published Astronomiae cometicae synopsis, also issued in English as A Synopsis of the Astronomy of Comets.

Using Newtonian orbital methods, he compared historical comet observations and tabulated 24 cometary orbits. The elements reported for the comets of 1531, 1607 and 1682 came out strikingly similar.

One comet, three appearances

Halley argued that the three apparitions were returns of a single body with a period of roughly 75 to 76 years, and forecast publicly that it would come back in 1758.

The records he leaned on were other people's: Petrus Apianus had observed the 1531 apparition, and Johannes Kepler and Christian Sørensen Longomontanus contributed to the reconstruction of 1607. The 1682 sighting was his own.

What the forecast did not contain

Halley recognized that planetary gravity could change the interval between returns, but he published no precise future path and no detailed numerical perturbation calculation behind the 1758 date. That work came later, from Alexis Clairaut with Jérôme Lalande and Nicole-Reine Lepaute.

Nor had he discovered the comet; people had recorded it for centuries, and the name Halley's Comet was applied only after the return succeeded.

Nothing was confirmed observationally in 1705. The prediction could not be tested until 1758–1759.