Edmond Halley

• 1. Introduction

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  Edmond Halley is remembered for a comet he never saw. He predicted its return in 1758, sixteen years after his death at the age of 85, and the comet was named after him when it appeared right on schedule. But the comet is only a fraction of who Halley was. He mapped the southern sky from a remote Atlantic island, funded one of the most important books in the history of science out of his own pocket, dove to 60 feet beneath the River Thames, pushed the Czar of Russia around in a wheelbarrow, and spent his evenings building a theory of hollow Earth. How does a soap-maker's son from Haggerston become the second Astronomer Royal of Britain? And what does it mean that the man who helped give the world Isaac Newton's Principia Mathematica had no lasting scientific post of his own for the first half of his career?

• 2. The Southern Sky Project

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  Flamsteed helped Halley publish his very first paper in 1676, in the Philosophical Transactions of the Royal Society, on the subject of planetary orbits. At that moment, Halley was still an undergraduate at The Queen's College, Oxford. He had already written to Flamsteed the previous year to point out errors in the leading published tables on the positions of Jupiter and Saturn, and in some of Tycho Brahe's star positions. Flamsteed was building a catalogue of the northern celestial hemisphere. Halley proposed to map the south, and he dropped out of school to do it.

  Halley chose the south Atlantic island of Saint Helena, located west of Africa, because it offered a vantage point on the southern stars while still allowing cross-referencing with some northern ones. King Charles II backed the expedition. Halley sailed in late 1676 and set up an observatory equipped with a large sextant with telescopic sights. Over the course of more than a year there, he produced the first telescopic catalogue of the southern sky and recorded a transit of Mercury across the face of the Sun.

  That Mercury transit gave Halley a bigger idea. He reasoned that observing the solar parallax of a planet during a transit could be used to calculate the exact distances between Earth, that planet, and the Sun. The transit of Venus would give even more precise results, but Halley noted that it would not occur within his own lifetime. He filed the observation away and returned to England in May 1678 with his data.

  Oxford refused to let him re-enroll, because he had broken his residency requirements by leaving for the island. Halley appealed to Charles II directly. The king signed a letter demanding that Oxford grant him a Master of Arts degree without conditions, which the college did on the 3rd of December 1678. A few days before that, at the age of 22, Halley had been elected a fellow of the Royal Society. His 1679 catalogue, the Catalogus Stellarum Australium, described 341 southern stars. Robert Hooke personally presented it to the Royal Society.

• 3. Newton's Principia and the Comet

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  In August 1684, Halley traveled to Cambridge with a specific question about gravity. He wanted to understand the proof of Kepler's laws of planetary motion. Newton told him the problem was already solved, then said he could not locate his own calculations. He promised to redo them and send them along, which he eventually did, in a short treatise titled On the motion of bodies in an orbit.

  Halley read the treatise and grasped immediately that it was extraordinary. He returned to Cambridge specifically to arrange its publication. Newton, instead of simply publishing the short paper, expanded it into an entirely different work: the Philosophiæ Naturalis Principia Mathematica. Halley paid for it himself, and it appeared in 1687. It is difficult to overstate how unusual this was. Halley was not a wealthy patron; he was a working scientist who had taken the position of secretary of the Royal Society in early 1686, a role that required him to give up his fellowship and manage the society's correspondence and meetings. He also edited the Philosophical Transactions. He was spending his own money on someone else's book while doing an administrative job.

  By 1681, the Italian astronomer Giovanni Domenico Cassini had told Halley that comets moved in orbits. In September 1682, Halley made a series of observations of a bright comet. Years later, using Newton's law of universal gravitation, he compared the paths of comets recorded across many decades and concluded in his 1705 paper, Astronomiae cometicae synopsis, that the comets seen in 1456, 1531, 1607, and 1682 were all the same object. He predicted it would return in 1758. When it did, astronomers named it Halley's Comet.

• 4. Magnetism, Mapping, and the Sea

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  At a Royal Society meeting in 1691, Halley introduced a working model of a magnetic compass that used liquid inside the housing to dampen the wobble of the magnetised needle. It was a rudimentary device, but it pointed toward a much larger obsession. Halley wanted to understand why compass readings varied from place to place, and why those variations shifted over time.

  In 1698, King William III gave Halley command of the Paramour, a vessel described as a 52-foot pink, to investigate the laws governing compass variation across the South Atlantic and to refine the coordinates of the English colonies in the Americas. On the 19th of August 1698, Halley took command, and in November of that year he sailed on what was the first purely scientific voyage ever undertaken by an English naval vessel. The expedition ran into trouble almost immediately. Officers challenged his authority and questioned his competence to command a ship. He returned to England in July 1699 to bring charges. The court issued what Halley considered a mild rebuke, and he was left dissatisfied.

  Halley was recommissioned on the 24th of August 1699 and sailed again in September. This second Atlantic voyage lasted until the 6th of September 1700, ranging from 52 degrees north to 52 degrees south. The data he collected became the General Chart of the Variation of the Compass, published in 1701. It was the first chart of its kind ever printed, and the first to use what became known as isogonic lines, or Halleyan lines, to show equal compass variation across a surface. Alexander von Humboldt later drew on the same technique when he drew isotherms on his maps.

  In 1701, Halley took the Paramour out one final time to study tidal patterns in the English Channel. The following year, Queen Anne sent him on diplomatic missions to European courts.

• 5. The Diving Bell and Hollow Earth

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  In 1691, Halley built a diving bell designed to extend how long a person could stay underwater. The problem with earlier bells was that the air inside was consumed quickly. Halley's solution was to replenish it by sending down weighted barrels of fresh air from the surface. He dived to 60 feet in the River Thames alongside five companions, and the group stayed submerged for over an hour and a half. The bell was too heavy to be useful for salvage work, but Halley kept refining it over time and eventually extended his own underwater exposure to over four hours. In the process, he suffered one of the earliest recorded cases of middle ear barotrauma.

  That same year, Halley was putting forward a theory of a very different kind about what lay beneath the surface. He proposed that Earth was hollow: a shell roughly 500 miles thick, enclosing two inner concentric shells and an innermost solid core. Each shell, in his model, was separated from the next by its own atmosphere, rotated at its own speed, and had its own magnetic poles. He suggested this arrangement could explain why compass readings shifted in ways that a single-core Earth could not account for. He envisaged the inner regions as luminous, and possibly inhabited, and speculated that gas escaping from the interior caused the aurora borealis.

  Neither the diving bell nor the hollow Earth theory made Halley popular with the Church. In 1694, the Royal Society censured him for suggesting that the biblical story of Noah's flood might be an account of a cometary impact. In 1691, he had sought the Savilian Professorship of Astronomy at Oxford, only to be blocked by the Astronomer Royal John Flamsteed and opposed by both the archbishop of Canterbury, John Tillotson, and Bishop Stillingfleet. The post went to David Gregory instead. Halley would not hold an academic position until 1703, when both Tillotson and Stillingfleet had died.

• 6. Later Years and Legacy

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  In November 1703, Halley was appointed Savilian Professor of Geometry at Oxford. Two years later, he published his work on comets. By 1706, he had learned Arabic to finish a translation begun by Edward Bernard of Books V through VII of Apollonius's Conics, using copies held at Leiden and the Bodleian Library at Oxford. He also completed a new translation of the first four books from the original Greek, a project started by the late David Gregory. In 1710, he published all of this together with his own reconstruction of the missing Book VIII, in the first complete Latin edition. Oxford awarded him an honorary doctorate of laws that same year.

  In 1717-18, Halley discovered that the stars were not fixed in place at all. By comparing his own measurements against those recorded in Ptolemy's Almagest, he showed that Arcturus and Sirius had both shifted measurably. Sirius had moved 30 arc minutes southward over 1800 years, a distance equivalent to the apparent diameter of the Moon.

  In 1720, Halley collaborated with the antiquarian William Stukeley on what was the first scientific attempt to date Stonehenge. They assumed the monument had been aligned using a magnetic compass and worked backwards through existing magnetic records to arrive at three candidate dates: 460 BC, AD 220, and AD 920. The dates were wrong by thousands of years, as later archaeology would confirm, but the underlying idea that scientific methods could be applied to ancient monuments was genuinely new.

  Halley succeeded Flamsteed as Astronomer Royal in 1720 and held the post until his death in 1742. He was buried in the graveyard of the old church of St Margaret's, Lee, in the same vault as the Astronomer Royal John Pond. His original tombstone was later moved by the Admiralty and can now be seen on the southern wall of the Camera Obscura at the Royal Observatory, Greenwich. Among the things named after him: a research station in Antarctica, a crater on the Moon, a crater on Mars, and a method for the numerical solution of equations. His 1693 analysis of mortality statistics drawn from the Breslau records of Caspar Neumann helped the British government price life annuities accurately, a contribution that shaped the foundation of actuarial science.