Halley's Comet Prediction (1705): The Return of 1758

Halley’s comet prediction was the first successful forecast of a comet’s return. In 1705 the English astronomer Edmond Halley used Isaac Newton’s theory of gravitation to argue that the bright comets of 1531, 1607 and 1682 were one and the same object travelling on a long elliptical orbit, and that it would come back around 1758. When it reappeared on Christmas night 1758, sixteen years after Halley’s death, it became a celebrated proof of Newtonian physics and has carried his name ever since.

Background: what were comets?

For most of history comets were regarded as unpredictable and often as omens of disaster. Aristotle had placed them in the upper atmosphere, and even after Tycho Brahe showed in 1577 that comets lay far beyond the Moon, astronomers disagreed about their paths. Some, including Johannes Kepler, thought they moved in straight lines.

Edmond Halley (1656–1742) was well placed to solve the problem. He had observed the bright comet of 1682 himself, and he had persuaded Isaac Newton to publish the Principia in 1687, paying for its printing. In Book III of that work, Newton treated comets as bodies moving under the Sun’s gravity on conic-section orbits, and showed how to compute such an orbit from observations.

Halley’s calculations

Halley spent years gathering historical observations of comets and laboriously calculating their orbits using Newton’s method. In 1705 he published his results, first in Latin as Astronomiae Cometicae Synopsis in the Royal Society’s Philosophical Transactions and in the same year in English as A Synopsis of the Astronomy of Comets.

The work listed the orbital elements of 24 comets observed between 1337 and 1698. Halley treated their paths as parabolas, which is a practical approximation for very elongated orbits. He then compared the orbits and noticed that three comets, those of 1531 (observed by Peter Apian), 1607 (observed by Kepler) and 1682, had remarkably similar paths. Their appearances were also spaced about 75 to 76 years apart.

Halley concluded that they were a single comet on a closed elliptical orbit around the Sun. He explained the small differences in the intervals between returns as the effect of the gravitational pull of the giant planets, especially Jupiter. On that basis he predicted that the comet would return about the year 1758. He also suggested, more tentatively, that earlier bright comets such as that of 1456 might have been earlier appearances of the same object.

Key people

Edmond Halley was one of the most versatile scientists of his age. As a young man he catalogued the stars of the southern sky from St Helena (1676–1678). He later produced a magnetic chart of the Atlantic, published early mortality tables used in life insurance, studied the trade winds and proposed the method of observing transits of Venus to measure the distance to the Sun.

Isaac Newton supplied the theory. Without his demonstration that gravity bends a comet’s path into a conic section, Halley would have had no way to compare orbits from different centuries.

Alexis Clairaut, Joseph-Jérôme Lalande and Nicole-Reine Lepaute carried out the perturbation calculations of 1757–1758. Lepaute’s contribution was substantial, although Clairaut did not acknowledge it in later editions of his work; Lalande later credited her role.

Johann Georg Palitzsch was a self-taught astronomer who kept a private observatory on his farm. His sighting made him briefly famous across Europe.

Myths and facts

Halley discovered the comet. He did not. It had been seen many times before, at least since ancient China. His achievement was recognising that several sightings were the same object and predicting its return.

Halley predicted the exact date. He gave only an approximate year, around 1758. The precise perihelion forecast was Clairaut’s work, and even that was about a month out.

Comets cause disasters. The fear of comets as omens persisted for centuries, and in 1910 some people bought “comet pills” fearing gases in the tail. Halley’s work helped show that comets are natural bodies following predictable laws.

Refining the forecast

Halley continued to work on the problem and became Astronomer Royal in 1720, but he died in January 1742 without seeing the outcome. As 1758 approached, the French mathematician Alexis Clairaut undertook a much more precise calculation, working with the astronomer Joseph-Jérôme Lalande and the mathematician Nicole-Reine Lepaute.

Together they computed, by hand, the delaying effect of Jupiter and Saturn on the comet over its previous two orbits. The task occupied months of intense arithmetic, carried out with tables and pen and paper, and it was one of the largest calculations attempted up to that time. In November 1758 Clairaut announced to the Paris Academy of Sciences that the comet would reach perihelion, its closest point to the Sun, around 13 April 1759, with a possible error of about a month.

The return of 1758–1759

On the night of 25 December 1758, Johann Georg Palitzsch, a farmer and amateur astronomer at Prohlis near Dresden, saw the comet with a telescope. Charles Messier in Paris had been searching for it and observed it in January 1759. The comet passed perihelion on 13 March 1759, within the margin Clairaut had given.

The French astronomer Nicolas-Louis de Lacaille was among the first to call it Halley’s Comet. Today it carries the formal designation 1P/Halley, the “1P” marking it as the first comet recognised as periodic.

By the numbers

Legacy

The comet’s return was a turning point in the acceptance of Newton’s theory, which still faced resistance on the Continent. It showed that comets were ordinary members of the solar system, bound by the same law as the planets, and helped to dispel their reputation as supernatural omens.

Historians later used the orbit to identify earlier sightings, including a Chinese record of 240 BC and the comet shown on the Bayeux Tapestry in 1066. In 1986 a fleet of spacecraft, led by the European Space Agency’s Giotto probe, flew past the comet and photographed its nucleus, a dark, irregular body roughly 15 kilometres long. Later missions to other comets, such as Rosetta and Philae, built on that experience.

Halley’s work is also a reminder of the role of the Royal Society as a forum where observations were shared and published, making such long-term predictions possible.

Timeline

• 1531: Peter Apian observes a bright comet, later shown to be Halley’s.
• 1607: Johannes Kepler observes the comet’s next return.
• 1682: Halley observes the comet in England.
• 1687: Newton’s Principia shows how to compute cometary orbits.
• 1705: Halley publishes his Synopsis of the Astronomy of Comets, predicting a return about 1758.
• 1742, January: Halley dies at Greenwich.
• 1758, November: Clairaut announces his refined prediction in Paris.
• 1758, 25 December: Johann Georg Palitzsch sights the returning comet.
• 1759, 13 March: The comet passes perihelion.

Why it matters

Halley’s prediction turned comets from frightening portents into objects whose movements could be calculated years in advance. Its fulfilment in 1758 was one of the first great public demonstrations that science could predict the future behaviour of the heavens.