As a bonus contribution to his Hump-Day History piece, Thony Christie also sent us along the following reflection on Halley’s Comet, Newton’s theory of gravitation, and December 25th.
Exactly two hundred and fifty years ago, on 25th December 1758, Johann Georg Palitzsch, a German farmer and amateur astronomer from the village of Prohlis near Dresden, observed a comet. Now astronomers have been observing comets for thousands of years, but this cometary observation was very special because it was a spectacular confirmation of one of the most significant theories in the entire history of science, Newton’s Theory of Gravity. The comet that Palitzsch had observed on Newton’s birthday was 1 P/Halley, as it is now officially known, or, more commonly, Halley’s Comet. And this observation was the first ever recovery of a comet, that is the observation of the predicted return of a comet.
Newton's birthplace. Click to see other favorite photos from the American Institute of Physics' Emilio Segrè Visual Archives
In order to fully understand the significance of this moment, we have to go back to the 1680s and Newton’s attempts to grapple with the universal theory of gravity. Comets had been a hot topic of discussion amongst English astronomers since a series a spectacular ones had lit up the night skies of Europe in the 1660s. The main topic of discussion was the form taken by the flight path of comets, about which there was no general agreement. The basis for the discussion was an exhaustive pamphlet on the subject that had been published by Johannes Kepler in 1618 in which he summarised all of the previous astronomical knowledge on comets. Some held with Kepler that comets flew through the solar system in straight lines, others were convinced that their trajectories were curved, and some adventurous souls even believed that the paths of the comets were elliptical like the Keplerian planetary orbits, as had first been suggested by William Lower, amateur astronomer and friend of Thomas Harriot as early as 1610.
In 1680 two comets were observed over London, the first in early November and the second in mid December, that were observed by all of the local astronomers and one, the Astronomer Royal John Flamsteed, became convinced that they were not two comets but one that had reversed its direction in the vicinity of the sun. Flamsteed sought support for his theory and took up contact, for the first time, with Isaac Newton in Cambridge. At first Newton rejected Flamsteed’s theory and explained away his observations in an off hand way, but something in the matter irritated him and he started to calculate the possible orbit of the comet based on Flamsteed’s figures, at the same time obtaining more data from Edmund Halley who had also observed the comet/comets.
Upon reconsideration Newton became convinced that Flamsteed was right and proceeded to devote himself to the study of the available cometary literature and the thought that maybe comets also obeyed the laws of planetary motion. He took particular interest in the comet of 1682, which we now know as Halley’s Comet. By the time he wrote his Principia, comets played a very central role in Newton’s scheme of things. One third of book three of Principia, “The System of the World”, (which is that part of the book that most people think of when they talk about Newton i.e. the theory of gravity applied to the Solar System and everything in it) is devoted to a discussion of comets and their flight paths. Newton shows that not only the planets and their satellites obey his law of gravity and Kepler’s laws of planetary motion, but that the comets too must bend to their authority thus proving that gravity is truly universal.
Following the publication of the Principia, which Halley nursed through the press and even paid for out of his own pocket, Halley continued Newton’s comet researches and dug up all of the historical records of cometary observations that he could find. He analysed all of these observations under consideration of Newton’s theory and published the results in 1705 both in the Philosophical Transactions of the Royal Society and as a separate pamphlet. His researches led him to the conclusion that the comets observed by Peter Apian in 1531, Kepler in 1607, and by Newton and himself in 1682 were actually one and the same comet with an orbital period of approximately 76 years. Refining his calculations and allowing for perturbations predicted by the Newtonian theory Halley predicted that the comet would reappear in the skies over Europe in 1758. Based as it was on the edifice of Newton’s theory, this prediction became an “experimentum crucis” for Newton’s theory of gravity. On 25th of December 1758, Newton’s theory passed this test with flying colours as a farmer in the German state of Saxony gave Isaac the perfect birthday present by witnessing the return of 1 P/Comet Halley.
*Unfortunately the title of this short piece suffers from a historical inaccuracy. It is true that Isaac Newton was born on the 25th December 1642, it is also true that Georg Palitzsch observed the return of Halley’s comet on 25th of December 1758. However, the first date is “Old Style”, that is before the introduction of the calendar reform in England (1752), whereas the second date is “New Style”, that is after the introduction of the calendar reform in Saxony (1700), so Palitzsch did not in reality observe the return of Halley’s comet on the anniversary of Newton’s birthday, but 11 days before! However the coincidence of dates is too nice to spoil with boring historical accuracy!