Not long after Halley returned from his third voyage, he picked up the searing question that seemed to always reappear in his life. His attention returned to that comet he witnessed a few times in 1682. The memory of its cascade of seemingly burning light held his intrigue, its ephemeral passage replaying in his mind as if on its own peculiar orbit within Halley’s skull. He hypothesized the comet not only orbited the Sun but that it was the same comet that had been seen earlier in the century.

What had mesmerized and often terrified Earth-bound audiences over the ages now—if Newton was right—had to follow the laws of the universe. Under the corrected vision of this age, comets were not errant anomalies on haphazard voyages but merely another class of orbital body governed by the cosmic clockwork.

The reason cometary paths were so hard to track is that they were visible so briefly compared with such celestial bodies as planets or stars. Measurement was tricky. At the time, determining whether their paths formed an ellipse, a hyperbola, or a parabola was next to im-

possible visually because comets could be seen only when they were close to the Sun.

With Newton’s calculus and his conception of gravity in hand, Halley had the tools to change this. Compiling every available bit of data, he took a fresh look at the past 200 years of comet observations by the world’s leading stargazers, including the tenacious Tycho Brahe, Kepler, Hevelius, Cassini, Flamsteed, and more. Computing the paths of comets with their bright heads and glowing tails was no longer a futile exercise, he reckoned. “I think there can be nothing plainer than that comets do move in orbs about the Sun [that approach the] parabolic.”

But the actual computations, which factor in the gravitational pull of not only Earth but also distant planets as the comet approaches, were highly complex. Not only was Halley concerned with the exact path the comets traveled but with the future path of their next trip around the Sun. He had to account for the perturbations caused by solar system giants Jupiter and Saturn and the precise position of Earth when the comets had been sighted or were to return.

For Halley it was the perfect astronomical puzzle. Over the next few years he completed the calculations of the orbits of some 24 comets. He described the endeavor as an “immense labour.” He assumed the orbits were parabolic. (One in particular would prove an exception.) Among other comets, he looked at the comet of 1680, which he had seen in Paris on his European tour. He calculated its orbital period to be 575 years, similar to those being talked about in Paris. Given, in part, that the estimate was unverifiable in human timescales, he devoted most of his attention to the comet of 1682.

Halley had started calculating that particular comet’s period even before his adventures aboard the Paramore. At a June 3, 1696, meeting of the Royal Society, according to its recording secretary:

But this was hardly fleshed-out. At the heart of the comet debate was the question of whether the comets were primarily repelled from the Sun or attracted by it by a seeming magnetic force. To prove Kepler’s inverse square law for planets, it didn’t matter whether the force that influenced their orbits was gravitational attraction or magnetic action, for both operated on the so-called inverse square principle. The planets were attracted toward the Sun by a force proportional to the inverse square of the distance between the planet and the Sun. The chief difference between gravity and magnetism, we have since learned, is that while gravity only attracts, magnetism both attracts and repels.

It was Halley who had initially persuaded Newton himself that the inverse square law of gravitation held true throughout the solar system. After accepting this notion, it was simple to make the leap that comets might be influenced by gravity to the extent that their orbits were elongated. Kepler had rejected the notion that comets could follow elliptical paths. He argued that only eternal figures like planets could chase such orbits. Just how stretched out the shape of a comet’s orbital circuit might be remained a key question.

After much ado, Newton developed the first theory of cometary motion as part of his Principia. His synthesis of universal gravitation would then forever link comets with Earth.

On his return to London and reunion with his family after his channel survey, Halley had analyzed the existing data further. Like his world map, it relied on numbers from many intrepid observers, this time across centuries. As he tackled one tedious calculation after another, Halley persuaded himself that the orbits were elliptical. From the outset even Newton had favored the more open shape of a parabola, which could extend infinitely in theory, never to return along the same path.

Increasingly, Halley became convinced the comets of 1531, 1607, and 1682 were the same celestial body that returned every 76 years. He was now prepared to predict that the comet would next visit in 1758 at Christmas time. The forecast was the first of its sort.

Halley was 49 years old in 1705 when he boldly presented his prediction and budding proof to the Royal Society. His Synopsis of Cometary Astronomy, touting it, also was published at Oxford that year. He knew the chances were slim to none that he’d live to see whether his prophecy would come true in 1758. But he confidently asked the younger generation of scientists who would likely live to see the day “to acknowledge that this was first observed by an Englishman.”

Many doubted Halley’s hypothesis. Over the years even Halley would hedge his prediction. In 1717 he wrote that the differences between the orbits of 1531, 1607, and 1682 “seemed to me a little too large.” He noted that the variations in the successive periods were “much larger than those which we observe in the revolutions of any single planet, since one of these periods exceeds the other by more than a year.” He also noted that “the inclination of the comet of 1682 is 22′ larger than that of the comet of 1607.”

In 1717, while plumbing medieval accounts, Halley found evidence of comets arriving before the Renaissance that might well be additional early sightings of the comet that reappeared in 1682. They had occurred on Easter 1305, an unspecified month in 1380, and in June 1456.

At this time he also expanded the window of the comet’s return to late 1758 or early 1759. “All is nothing but a light trial, and we leave the effort of making this matter deeper to those who survive until the event justifies our predictions.”

ONCE ENSCONCED COMFORTABLY as Savilian Professor of Geometry in Oxford’s storied halls, Halley seemed to be more candid once again about his beliefs. For one thing, publication of the second edition of Newton’s Principia in 1713 likely burnished Halley’s reputation and

he probably felt more secure in his career. It was not long before he returned to the controversial quarry, the question of whether Earth’s life span is limited. This quest would summon forth both of Halley’s longtime passions—comets and geomagnetism. And once again the story features Isaac Newton and Robert Hooke.

At the time that Newton was crafting the Principia, Hooke was developing an “excellent System of Nature” of his own (which he presented in talks known as his Lectures on Earthquakes delivered to the Royal Society in 1686 through 1688). The persnickety Hooke was extremely gifted—even the unique genius of Newton could feel challenged.

Hooke’s general theory put forward Earth as a dynamic, living body, one whose fertility changed over time. At his scheme’s center were his ideas about the “Cause and Reason of the present Figure, Shape and Constitution of the Surface of the Body of the Earth, whether Sea or Land, as we now find it presented unto us under various and very irregular Forms and Fashions and constituted of very differing substances.”

Embracing his own theories on earthquakes, volcanoes, continent formation, magnetic variation, and even catastrophic changes in species, Hooke’s new history of the natural world, in brief, linked celestial mechanics to Earth’s dynamics. Everything from the formation of continents to changes in species were tied to his new treatise on Earth’s spatial orientation in relation to other planets and stars, as governed by the inverse square law, and were linked to Earth’s shape.

In this way, both Hooke and Newton’s philosophical systems were based on the inverse square law of gravitation, for which Hooke claimed priority but for which Newton provided the mathematical proof for the orbits of planets in his Principia. Hooke also wanted credit for all that was derived from the law, which was no small presumption. While their competing derivations basically offered an explanation of the same universe, the visions differed greatly in scope and substance. To Newton, Hooke’s demands were excessive and offensive. To Hooke, Newton’s Principia had stolen the thunder of his

grand synthesis of the universe and even borrowed some of his hypothesis’s core tenets, in particular his ideas about the inverse square law and planetary forms.

Hooke’s system hinged on the idea that the axis of Earth’s rotation underwent “certain slow progressive motion[s].” As a result, he asserted that Earth was an oblate spheroid. In his mind, his revelation connected the planet’s spatial orientation to the rest of the solar system and illustrated his dynamic history of Earth.

Initially, some key figures like Royal Society fellow John Aubrey, a noted diarist, bought Hooke’s system wholeheartedly: “This is the greatest discovery in nature, that ever was since the world’s creation: it never was so much as hinted by any man before,” he exclaimed in a letter. Halley and Newton apparently took Hooke’s ideas seriously enough to race him to press.

Halley even publicly accepted some of Hooke’s premises and credited him with suggesting that Earth was an oblate spheroid. He also wrote to John Wallis, then head of the Oxford Philosophical Society, that Newton “falls in with Mr. Hooke and makes the Earth of the shape of a compressed spheroid, whose shortest diameter is the axis, and determines the excess of the radius of the equator.”

Hooke’s theory kindled Halley’s interests in other ways. Halley accepted Hooke’s notion that Earth’s axis of rotation changes. He doubted, however, whether the type of gradual flooding that would have occurred as a result of changes in Earth’s rotational axis would have been extreme enough to wipe out entire species. Instead, Halley suggested in a 1686 paper that Earth’s axis might undergo rapid shifts caused by “the powers that first impressed this whirling motion on the ball [Earth]” or, perhaps naturally, “by the causal shock of some transient body, such as a comet.”

Still in the midst of editing the third book of Newton’s Principia, which covered cometary motion, Halley in his 1686 paper became the first from the Newtonian school of thought to put forth, publicly at least, the idea that comets played important roles not only cosmologically but also on Earth.

After the debut of the Principia, it would be Newton’s theories that were generally accepted and Hooke’s that remained at least couched as but “conjecture, of what may be, without any evidence from observations,” in the words of Oxford’s Wallis, “too extravagant for us to admit.” Perhaps once again, artful behind-the-scenes campaigning by both Halley and Newton may very well have contributed to the different reception of these rivals’ works.

In a later paper published in 1692, Halley expanded this collision idea and his 1683 hypothesis that Earth’s magnetic variation may be explained by four magnetic poles in its interior—the one Flamsteed had rudely alleged Halley borrowed from the unsung mathematician Peter Perkins. In a novel synthesis, Halley asserted that a cometary impact on Earth might convey different velocities on the inner and outer shells, which each held a pair of poles.

He suggested that some sort of fluid medium might separate the two shells or that there may even have been a significant air cavity between them—some 500 miles wide—perhaps as thick as the outer shell itself. He reckoned gravity could hold a concentric sphere within Earth much like the rings of Saturn are captured by gravity. (It was not then known that Saturn’s rings rotate; Newton hadn’t considered the phenomenon in his Principia.) Halley’s speculation was based on Newton’s erroneous designation in Book III that the Moon is denser than the Earth by a ratio of 9:5. (The actual mass ratio has since been found to be 1:81.) “I have adventured to make these subterranean orbs capable of being inhabited,” Halley added to the eternal pleasure of future science fiction writers.

Perhaps amazingly, this notion of a hollow Earth still chimes with possibility today. While some scholars contend Halley borrowed from Hooke’s earthquake lectures to develop his idea that Earth was comprised of shells, there is a stronger case to be made that his work was original and more directly stemmed from Newton’s incorrect estimate of lunar mass. Moreover, in Hooke’s view Earth’s interior was comprised of layers like an onion. His model never mentioned a series of magnetic poles, air gaps, or subterranean globes.

Yet it was with prompting by Hooke and Newton that Halley brought his ideas on Earth’s magnetism and comets in space full circle. He built on both Hooke’s changeable axis theory and Newton’s ideas about the role of comets in the origin of the universe, detailed in his Principia.

In 1694 before the Royal Society, Halley had expanded his thinking about Earth’s internal structure further. He hypothesized that the impact of a comet might not only explain Earth’s magnetic variation but might also have caused the flood behind the story of Noah’s Ark. In this unpublished work, he also commented that a comet might have wrought the annihilation of a “former world,” creating the “chaos out of whose ruins the present [world] might be formed.” The assertion had cost him the chance of winning the Savilian Chair of Astronomy at Oxford only a few years before.

Halley apparently attempted to clear his name in the scientific community of the charge that he had denied Earth had a finite age. Strategically, he intentionally put a more sophisticated spin on this question in his subsequent writings. In a prominent 1715 paper, for example, he refuted “the notion of the Eternity of all Things, though perhaps the World may be found much older than many have hitherto imagined.” His assertion was based on his observations of the saltiness of the seas and lakes, which placed a limit on the age of the Earth. He proposed that the age of the Earth is limited by the rate at which rivers carried salt from the sea into lakes without outlets. If a given lake had an original salt content, its age would have been over-estimated. If proven correct, his idea was evidence that Earth was older than the ecclesiastics estimated.

While much of Halley’s radical thinking on magnetism would later fail to withstand scientific scrutiny, his ideas on comets would endure and he personally would remain devoted to the hypothesis on which he based his magnetic theory that Earth was comprised of shells. It was a premise that would help unlock the secrets of the aurora borealis—those luminous clouds that form on the horizon, usu-

ally only in the most northern latitudes, and seemingly extended almost to the zenith.

ON MARCH 6, 1716, dazzling streamers of colored lights danced across the night sky. Halley—along with most of northwestern Europe—watched this aurora, then an unexplained phenomenon. It was the first he’d ever witnessed. His eyes were trained on the atmosphere until 3 a.m., though it was so intense it was visible in the daylight. Above his London home the clouds exuded rays of yellow, red, and a shadowy green. The colors of the rays grew more vivid the farther north one traveled in England and Scotland. The rays, as he noted, were often portrayed in religious paintings as the glory emanating from God.

Contrary to Gilbert’s belief that the magnetic and geographic poles coincided, Halley had observed that they were skewed: Earth’s magnetic pole was displaced from its geographic pole. The auroral phenomenon followed the magnetic pole. He had reckoned there must be a connection between Earth’s magnetic field and the aurora.

Halley had reasoned that the rays correlated with the field lines of a uniformly magnetized sphere. He reckoned that the auroras were most intense at the Earth’s poles, where the field lines converged, and surmised that the circulation of matter in the Earth’s magnetic field caused the rays. (Essentially, he was right. Halley’s later observations would also shed light on other aspects of geomagnetism related to this puzzle. Later, Anders Celsius and his student Olaf Hjorter would also observe magnetic disturbances linked to the polar auroras or northern lights; in our time such events are associated with magnetic substorms. And in the past few decades the study of the solar wind or how charged particles move in the magnetic fields of celestial bodies, including Earth, has become a leading field of space research.)

In 1724, after at last obtaining his dream job as England’s second astronomer royal, Halley finally felt secure enough to publish his contentious paper written 30 years earlier. It discussed the deluge and the

likelihood of Earth arising from the fallout of a former world, destroyed by the impact of a comet that led to the creation of the present world. According to the minutes of a Royal Society meeting at which he first delivered his hypothesis 30 years earlier, the collision would generate a new axis, rotation period, and length of the day and year and “account for all these strange marine things [fossils] found on ye tops of hills and deep underground.”

Halley’s views, in fact, were not necessarily antithetical to those of the Church of England, according to current interpretations of his later paper. Halley believed the world was co-eternal with God and dependent on him. But most of his contemporaries did not appreciate the nuance. The only satisfactory view to them was that the world was eternal and independent of God. For example, in Newton disciple William Whiston’s 1696 New Theory of the Earth, an early treatise based on Newton’s Principia, he wrote that it was “now evident that gravity (the most mechanical affection of body) and which seems the most natural, depends entirely on the constant and efficacious and if you will the supernatural and miraculous influence of almighty God.”

But contemporary scholars contend that Newton was very much in support of Halley’s idea that the current Earth with “visible marks of ruin upon it” had evolved from a previous one. Moreover, Newton may have influenced Halley’s writings on the internal structure of Earth and its mutable orientation in space in his discourses with Hooke.

Yet it was his ongoing dialogue with Hooke that yielded much of Halley’s scientific legacy on such key astronomical and physical topics as Earth’s age, stellar cycles, comet cosmogony, and terrestrial magnetism.

WHEN NEWTON DIED in 1727, Halley may have been among the Royal Society members who escorted the body to its final resting place inside Westminster Abbey under a white and gray marble monument.

It supports a heavily decorated sarcophagus with a full-body sculpture of the knighted genius and a gilded relief panel detailing some of his key achievements.

Halley died some 15 years later on January 14, 1742, rather peacefully after imbibing a glass of wine. Both men lived to approximately 85, far longer than most in their day. But it was not long enough for Halley to see whether his comet would reappear.

In his will, Halley requested to be buried next to his wife of 55 years in the churchyard of St. Margaret at Lee near Greenwich and not far from the Royal Observatory. Mary Tooke had predeceased him by six years. Shortly after her death, Halley endured a minor stroke that left his right hand partially paralyzed and contributed to his gradual physical decline, but his mind was sharp to the end. His only surviving son, Edmond the naval surgeon, born the year the Paramore set sail, also died before him—but by only one year.

In his own lifetime, Halley’s comet work was known by few. It held no more significance publicly than his papers on Palmyra or diving bells or the transit of Venus, for that matter. The comet was barely mentioned in his obituaries, which duly touted his work on enhancing navigation.

All that would change. Through 1758, as anticipation of the predicted comet’s return grew, such astronomers as J. P. Loys de Cheseaux and even a Barbados plantation owner named Thomas Stevenson speculated that the irregularities in the periods were large enough that in fact two comets might be involved, each returning every 151 years. Still others suggested the period was decreasing arithmetically.

Interest in the comet’s pending visit reached the New World. A 12-part series written by an anonymous author appeared in the Boston Gazette, a leading paper in Britain’s colony of Massachusetts:

Before a mixed chorus of naysayers and believers in the public and the scientific community at large, the comet was sighted on Christmas Day 1758—16 years after Halley’s death. A well-to-do German farmer and amateur astronomer, Johann George Palitzch, documented its return above a little village near Dresden. Meanwhile in Paris, astronomer Charles Messier independently sighted the comet on January 21, 1759.

Halley’s forecast was dead on. The comet was not only delayed in its return due to the gravitational pull of Jupiter and Saturn, but it also appeared in the part of the sky Halley had foretold some 55 years in advance. It was not long before the gleam returning above Earth would be referred to as “Halley’s comet” throughout Europe and eventually the rest of the world. Posterity would indeed not forget that the discovery was owed to an Englishman. For science, with regard to the public, the “second coming” of the comet would hardly be underestimated.

And at least for those who could understand the significance of its return, sighting a comet in the heavens would no longer be misinterpreted as an ominous sign of God’s impending vengeance. (Such notions were instead replaced by a palpable fear that the physical fallout from a cometary impact could wreak havoc on the planet.) Moreover, the return of Halley’s comet also affirmed Newtonian gravitational dynamics.

As Benjamin Martin wrote soon after the event in May 1759, “As it is the first comet which has been predicted, and has returned exactly according to that prediction, it cannot but excite the attention and admiration of the curious in general, and fill the minds of all

astronomers with a ravishing satisfaction, as it has, by this return, confirmed Sir Isaac Newton’s rational of the solar system, verified by the cometarian theory of Dr. Halley and is the first instance of astronomy brought to perfection.” The intervals between the subsequent appearances of Halley’s comet have ranged from under 75 to over 79 years, analyses reveal. Questions remain today as to all the forces, besides the pull of Jupiter and Saturn, that may affect the timing of its return near Earth.

In truth it turns out that only one of the 24 comets analyzed by Halley and published in the 1705 table was periodic. Later research would also reveal, for example, that Halley was wrong about his calculations concerning the comet of 1680. Its period was not 575 years. It likely won’t return to tour the planet from the heavens any time soon. Its orbit is parabolic—and on the order of thousands of years.

Yet one correct prediction was proof enough.

Without Newton’s work that he helped shape, Halley could not have forecast the periodic return of his comet or made many of his other key contributions to geophysics and astronomy. Halley would be among the first to apply Newton’s principle of universal gravitation and resulting laws to the physical realm. Perhaps a lot of that had to do with the reality that many natural philosophers of Halley’s day didn’t immediately buy into Newton’s theory since it entailed such phenomena as unexplained action at a distance.

Ironically, that comet of 1682 was the closest major comet preceding the Principia and would become a blazing symbol that forever connects the comet not only with the book that cracked its puzzle but also the seminal cooperation of Halley and Newton.