Halley’s successful prediction of the return of the comet named for him was a stunning confirmation of the correctness of Newton’s law of gravity and laws of motion. Halley’s prediction also established that the comet Halley orbits the Sun, and that it therefore does not travel within interstellar space and pass close to the Sun only once.

In January, 1684, three members of the Royal Society of London met in a coffeehouse to discuss what planetary orbits Planetary orbits would be like if the attractive force of the Sun became weaker as the reciprocal of the square of a planet’s distance from the Sun. Robert Hooke claimed that he had already worked out that the orbits would be ellipses, but he offered no proof. He claimed that he would give his proof after others worked at the problem, but he, too, found the problem extremely difficult. Sir Christopher Wren must have had his doubts, because he then offered a reward of any book costing up to forty shillings to anyone who could come up with a proof within two months. No one did. The third member of the coffeehouse discussion was Edmond Halley, one of the great scientists of his day. [kw]Halley Predicts the Return of a Comet (1705)
[kw]Comet, Halley Predicts the Return of a (1705)
[kw]Predicts the Return of a Comet, Halley (1705)
Halley’s comet[Halleys comet]
Comets
[g]England;1705: Halley Predicts the Return of a Comet[0210]
[c]Astronomy;1705: Halley Predicts the Return of a Comet[0210]
[c]Science and technology;1705: Halley Predicts the Return of a Comet[0210]
[c]Mathematics;1705: Halley Predicts the Return of a Comet[0210]
[c]Physics;1705: Halley Predicts the Return of a Comet[0210]
Halley, Edmond
Newton, Sir Isaac
Newton, Sir Isaac;Halley’s comet[Halleys comet]
Wren, Sir Christopher
[p]Hooke, Robert
Palitzsch, Johann Georg

In August, Halley visited Sir Isaac Newton at Cambridge University to put to him the question about orbits. Newton immediately replied that the orbits would be ellipses; he had worked out the problem years before. Although Newton could not find his notes, he promised Halley that he would again work out the proof. When he received Newton’s proof, Halley was so impressed that he urged Newton to expand his ideas into a book.

With Halley’s frequent encouragement, Newton produced one of the greatest scientific books of all time, Philosophiae naturalis principia mathematica (1687; The Mathematical Principles of Natural Philosophy, Mathematical Principles of Natural Philosophy, The (Newton) 1729; best known as the Principia, 1848). Halley corrected and edited the Principia, and then paid for its printing. The masterpiece contained Newton’s law of gravitation Gravity and his three laws of motion. Motion;and matter[matter]
Matter;and motion[motion] The book was the key to understanding not only the great motions of planets in the heavens but also earthly and mundane things such as the falling of apples.

Book 3 of the Principia included Newton’s collected observations of Halley’s comet of 1680, along with a discussion that the comet might be in a parabolic orbit about the Sun. As book 3 neared publication, Hooke demanded that it include a preface acknowledging him as the first to formulate the law of gravitation. Hooke was one of several who had suggested that gravity becomes weaker with the reciprocal of the square of distance from the Sun, but he did not do anything with this idea. Newton was incensed and vowed to withhold publication of book 3, but by using great diplomacy, Halley persuaded Newton to proceed with publication.

Halley’s diplomacy would be required again during a conflict between Newton and another scientist, John Flamsteed, Flamsteed, John who made it his life’s work to produce a new map Star catalogs of the starry heavens. Appointed astronomer royal in 1675, he was expected to share his findings. However, he always insisted that his measurements needed to be further refined; also, he had published little. Newton became president of the Royal Society in 1703. As such, he visited Flamsteed, who promised that his work would soon be ready, but it was only after several more years that Flamsteed gave a copy of his observations and a draft of his catalog to the Royal Society with the instruction that the catalog was not ready for publication. More years passed with little progress, and Newton became quite abusive toward Flamsteed. Finally, at Newton’s request, Halley prepared Flamsteed’s catalog for publication and had an incomplete version published in 1712 as Historia coelestis Britannica
Historia coelestis Britannica (Flamsteed) (complete version, in three volumes, published in 1725; partial English translation, 1982). The catalog extended the map of the northern skies from one thousand to three thousand stars. Astronomers were delighted with the work, but even though Halley had kept Flamsteed thoroughly informed and praised his work, Flamsteed was so enraged at Newton and Halley that he publicly burned all of the copies he could get his hands on. As a final twist in the affair, upon Flamsteed’s death in 1719, Halley was appointed his successor as astronomer royal.

About 1695, Halley began to collect detailed information on comets, both ancient and modern. While many reports were too vague to be of great use, others linked a comet’s position in a constellation with a time. Using a method outlined in the Principia, in which the five parameters defining an orbit could be deduced from three well-spaced observations, and after what he called “an immense labor,” Halley published a list of orbital elements for twenty-four comets in 1705. It was published in Latin by Oxford University and again in the Royal Society’s Philosophical Transactions, and published in English as A Synopsis of the Astronomy of Comets, Synopsis of the Astronomy of Comets, A (Halley) also in 1705. The synopsis included bright comets sighted from 1337 to 1683. Halley pointed out that the orbits of the comets of August of 1531, October of 1607, and September of 1682 were so similar that these orbits were probably made by the same comet. Many had supposed that comets traveled in straight lines or in parabolic orbits (a parabola is a curve that does not cross itself). In either of those cases, a comet would pass close to the Sun once only and then vanish into interstellar space. According to Halley’s calculations, some comet orbits were highly elongated ellipses, which meant that the comets orbited the Sun and should again be observable as they continued to return.

Halley calculated that the comet of 1682 should again be visible near the end of 1758, and while Halley did not expect to live long enough to see the 1758 return, he hoped that if it did come back at the predicted time, the world would recall that this had first been predicted by an Englishman. As time passed, Halley tried to calculate the effects of Jupiter on the comet’s orbit, and to complicate matters, there was one effect unknown to Halley: The jetting of matter from a comet’s nucleus makes precise predictions of comet orbits impossible. (Modern calculations, and the effects of planets, show that the period of return for comet Halley has varied from sixty-eight to seventy-nine years over a period of three millennia.)

After a long and productive life, Halley died in 1742. Sixteen years later, in 1758, there was a flurry of astronomical activity, as astronomers and mathematicians tried to refine calculations for where and when Halley’s comet would reappear. It was first seen on Christmas evening by Johann Georg Palitzsch, a young German astronomer. Believing he saw a faint bit of fuzz about where the comet was expected, he set up his telescope and confirmed that it was most likely the comet Halley. The news of Halley’s successful prediction spread quickly. Within ten years the 1758 comet was known as Halley’s comet (modern convention calls it Comet Halley), the first comet named for a person.

Like swords hanging over the land, comets Comets had been commonly taken to be ill omens from the gods. The first scientific explanation for comets was ventured by the great Greek philosopher-scientist Aristotle, who believed that comets were gases in the Earth’s upper atmosphere that had caught fire and were therefore luminous. For nearly two millennia there were no known attempts to determine a comet’s orbit, for during this time in scientific history, to do so would make no more sense than attempting to find the orbit of a particular cloud in the sky.

When the brilliant comet of 1577 blazed in the night sky for weeks, the renowned Danish astronomer Tycho Brahe Brahe, Tycho carefully compared the background stars he saw near the comet with those stars seen by friends in other countries. The discovery that they were the same stars could only mean that the comet was not in Earth’s atmosphere after all but instead was far beyond Earth’s moon.

Believing that comets existed among the planets, it made sense for Halley to try to find their orbits, but calculating orbits required Newton’s Principia. Without Halley’s urging, ongoing encouragement, and diplomacy, the Principia likely would not have been written. Once written, it would not have been published without Halley’s corrections, editing, and funds. Furthermore, once published, its worldwide acceptance would have been slower without Halley’s prediction and its confirmation by the spectacular return of Comet Halley. Halley’s prediction (included in the second edition of the Principia, in 1713) stands as a remarkable prophecy of an event that would be witnessed fifty years in the future based on the mathematical analysis of a physical model.

• Cook, Alan. Edmond Halley. Charting the Heavens and the Seas. Oxford, England: Clarendon Press, 1998. A biography of Halley that includes a good description of astronomical instrumentation in the early eighteenth century and reasons for the insistence on accurate measurement.
• Sagan, Carl, and Ann Druyan. Comet. New York: Random House, 1985. A well-written and extensive description of Halley and his work.
• Schaaf, Fred. Comet of the Century: From Halley to Hale-Bopp. New York: Copernicus, 1997. Discusses comets in general, and comet Halley in particular. Examines Halley’s discoveries and the appearances of Comet Halley beginning in 1404 b.c.e. to the return predicted for 2134 c.e.
  
• Thrower, Norman J. W., ed. Standing on the Shoulders of Giants: A Longer View of Newton and Halley. Berkeley: University of California Press, 1990. Eighteen essays commemorate the three-hundredth anniversary of the Principia and the 1985-1986 return of comet Halley. Written especially for the serious student who wishes deeper analysis of Halley, Newton, and Comet Halley.
• Weissman, Paul. “Cruisin’ with Comet Halley.” In Our Worlds: The Magnetism and Thrill of Planetary Exploration, edited by S. Alan Stern. New York: Cambridge University Press, 1999. A good discussion of Halley’s work leading to his prediction of the comet’s return. Includes a description of Comet Halley and its orbit.

Astronomy Wars in England

Quest for Longitude

Flamsteed’s Star Catalog Marks the Transition to Modern Astronomy

Hadley Describes Atmospheric Circulation

Jean-Sylvain Bailly. Halley’s comet[Halleys comet]
Comets