Summary

Edmond Halley (Latinized as Edmundus Halleius), born on November 8, 1656 in Haggerston (en) in the borough of Hackney in London and died on January 14, 1742 in Greenwich, was a British astronomer and engineer.

Engineer and multidisciplinary scientist, he is best known for being the first to have determined the periodicity of the comet of 1682, which he fixed by calculation at about 76 years. When this comet returned in 1758, it was named after him. It is one of the few comets that have a name other than that of its discoverer.

Childhood

The date of his birth is uncertain: Edmond Halley believed that it was November 8, 1656. He was born in Hackney, a small village in the vicinity of London, now absorbed by the capital. Of his mother we know only the name, Anne Halley, née Robinson. His father was also named Edmond, he was a rich soap maker and salt merchant who had built his fortune on the recent horrors of the bubonic plague, which had given Londoners a taste for personal hygiene. This man was never afraid to devote all the money necessary to the education of his son, who proved to be a curious boy full of interest in science.

Little is known about his childhood, except what Edmond Halley himself was willing to confess: "From my earliest years, I devoted myself to the study of astronomy," he wrote in his memoirs. "a pleasure so great that it is impossible to explain it to anyone who has not had this experience."

Without having any direct proof, it is however probable that Edmond Halley, barely 10 years old, was led to become interested in astronomy - and more particularly in comets - following the spectacular appearance in the London sky of the two great comets of 1664 and 1665, which popular belief held responsible, for the first, for the great plague of London, and for the second, for the great fire that devastated the capital.

A few years later, Edmond Halley, again with the help of his father's largesse and encouragement, entered St. Paul's School, one of the best in all of England, where the young man was noted for his brilliant abilities. In 1672, he entered Queen's College, Oxford, where he was similarly noted, but where he had the misfortune to learn of the death of his mother a few months later, on October 24, 1672, shortly before his sixteenth birthday.

On March 10, 1675, Edmond Halley had the audacity to write to the Astronomer Royal of England, John Flamsteed, to point out errors in the official tables of the positions of Jupiter and Saturn. Impressed by the abilities and especially the enthusiasm of the young man - whose calculations proved to be correct, but who knew in his letter how to show both the respect due to his elders and the enthusiasm of his youth -, John Flamsteed helped him, the following year, to publish, at the age of nineteen, his first scientific article in the Philosophical Transactions, a journal of the Royal Society of London.

Astronomer, scientist, engineer

Strengthened by the recognition thus obtained by the scientific community, Edmond Halley decided to leave Oxford without passing his degree, and this to embark for the island of St. Helena, in order to draw the first map of the southern sky. He is supported in this expedition by the Royal Society, which also managed to obtain the support of King Charles II. His departure took place on his 21st birthday, in November 1676. He stayed eighteen months and built an observatory (equipped with a 7.3 meter long telescope) located in the Longwood district.

From these long months of observation, Edmond Halley not only brought back to England the most accurate map of the southern sky that had ever been drawn, but also several observations rich in information, including the influence of latitude on the period of pendulum clocks (due to a minute difference in centrifugal force at the equator), and a census of nebulae never before observed by Europeans. After the observation of a transit of Mercury in front of the Sun, he published an exposé on the method to be used to determine the distance Earth-Sun during a transit of Venus, without unfortunately having the opportunity to proceed during his lifetime.

Sailor and oceanologist

Halley had already experienced long-distance travel: in 1676 he had gone to St. Helena to study southern cosmography, and in 1679 he had published his Catalogus stellarum australium.

In 1690, he made a diving bell, supplied with air by weighted barrels, and managed to stay underwater with five other companions, for more than an hour and a half.

"By this means I have left three men for an hour and three-quarters under ten fathoms of water, without the least inconvenience to them, and in as perfect a freedom of action as if they had been in the open air."

He perfected his bell, which he wanted to use to explore wrecks, and managed to extend the duration of the underwater stay up to four hours, but the heaviness and size of the device made it impractical, and he lacked modern techniques and materials allowing in particular to strongly compress the air.

In 1698, he was given command of a 52-foot long pinnace, the Paramore, by the British sovereigns, Mary II and William III of Orange, and was given the direction of what was most likely the first oceanographic mission. The pinnace (a sturdy, bulging Dutch ship) set sail in November 1698, but Halley had to interrupt his voyage and return to shore seven months later because the officers on board refused to obey a civilian.

Halley, granted by the sovereigns a patent as "temporary captain" of the Royal Navy, set sail again in September 1699. He sailed across the ocean from 50° North to 52° South, and gathered a wealth of scientific observations, in particular on the variations of the compass. He reached the South at the latitude of the South Georgia Islands, which he recognized but did not take possession of: it was James Cook who took charge of this in January 1775.

An outstanding sailor, he studied, during his voyages aboard the Paramore, the atmospheric circulation (and in particular the trade winds, which he attributed to the heating of the air under the equator), the ocean currents, and established a detailed map of the magnetic declination, the first precise map of isogons. He also designed the first weather map, the forerunner of those presented every evening on television. Passionate about the sea, he studied the habits of many aquatic creatures, including cuttlefish and sturgeon. He even devised a method to keep the plaice alive, in order to sell them in the middle of winter.

Universal genius

In contact with Isaac Newton, Halley wondered whether the attraction of a comet passing too close to the Earth could move the oceans to the point of flooding continental regions: with this hypothesis, he was also one of the first to try to rationally explain the biblical Flood. He was also the first to question the consequences of a comet colliding with the Earth.

He tried to measure the size of the atom, but without success. A curious mind, he also studied Roman history and clockwork. By his own admission, Halley tasted opium quite regularly, but without falling into a physical or psychic addiction.

Translator

Halley translated from Arabic the seventh book of the Mathematical Collection of Pappus of Alexandria and the two books of Apollonius of Perge's treatise On the Section of Ratio, which was thought to be lost. On the basis of the seventh book of Pappus, he also proposed a reconstruction of the lost book of Apollonius entitled On the section of area.

Comets at the time of Edmond Halley

The idea that comets are identical in nature to planets (i.e. a celestial body in motion) and even that they have their own orbit was not unknown to the ancients:

"Some Italic philosophers and some of those called Pythagoreans assure that the comet is one of the planets, which appears only at a very long interval and whose ascension is very small.

- Aristotle, Meteorology, Book I, chap. VI, § 3

Although it is not clearly expressed, this opinion contains the idea of the periodic return of comets, celestial stars by nature.

Aristotle, for his part, saw comets as purely atmospheric ("sublunar") phenomena, because the sky - consisting of the "sphere of fixed stars" - was declared to be "fixed and unchanging" in his world system. This Aristotelian conception of the universe lasted for several centuries, until Tycho Brahe challenged it with the observation of the supernova of 1572: clearly, contrary to the assertion of Aristotle and the Church (then Aristotelian since Thomas Aquinas), the heavens were not immutable. The fatal blow was dealt to this conception of the world five years later, with the appearance of the great comet of 1577, which remained visible for many months, allowing Brahe to develop with his colleagues the most diverse hypotheses on these new celestial bodies.

However, if the Church was forced to recognize the "planetary" nature of comets, their raison d'être was not questioned: comets were always considered as divine signs, most often announcing the wrath of the Creator. As such, comets had to be unpredictable phenomena, as could be any divine message in response to any human action.

For this reason, it was also considered that, "by nature", the orbit of comets must be parabolic - a hypothesis formulated by Johannes Hevelius in the 17th century -, since each comet makes only one and only one passage around the Sun. This theory suited the observations of the time: indeed, in the vicinity of the inner solar system (thus from the point of view of a terrestrial observer), it is very difficult to differentiate a very elongated ellipse from the end of a parabola. The precision of the instruments of the time was insufficient to be able to differentiate two orbits so close. The parabolic orbit of comets was therefore the norm when the young Edmond Halley began to observe the sky...

The scholar's challenge

Halley's first "official" observation of a comet was in 1680 - the same comet that inspired Pierre Bayle's Thoughts on Comets - on a ship that crossed the English Channel to France. It was Jean-Dominique Cassini, discoverer of the division of Saturn's rings, who welcomed him warmly at the Royal Observatory in Paris, and who encouraged the young man to consider the hypothesis of a periodic return of comets:

"Mr. Cassini did me the favor of entrusting me with his readings of the comet as I was preparing to leave the city; in addition to the observations he made on March 18 (1681), he submitted to me a theory on its movement, namely that the comet is the same one that appeared to Tycho (Brahe) in the year 1577, that its revolution describes a great circle in which the Earth is included."

- Alan H. Cook, Edmond Halley : Charting the Heavens and the Seas, Clarendon Press, 1998, p. 115.

Cassini had indeed noticed that three comets came from the same part of the sky with similar speeds: if the paternity of the hypothesis of a periodic return of comets belongs to Cassini, it is Halley who will take the subject sufficiently to heart to try to validate it scientifically. But, although the subject seems to fascinate the young man, it is only ten years later that he will work on its demonstration.

In 1682, he observed the unspectacular comet that would later bear his name, but left only a few notes in his observation book. His meeting with Isaac Newton in August 1684 seems to have rekindled Halley's scientific ardor, which had fallen into a routine after his meeting and marriage to Mary Tooke, with whom - as all accounts agree - he lived a sincere and passionate romance for nearly fifty-five years.

He will mention many times in his correspondence the immense interest he had in this subject of study:

"The opinion of Aristotle (...) that comets were nothing but sublunar vapors or aerial meteors prevailed to such an extent among the Greeks, that this most sublime part of astronomy was totally neglected; since that time, no one finds it worthwhile to observe and relate the wanderings and uncertain trajectories of the vapors floating in the ether.

Halley and Newton became close friends, and together they developed their passion for comets. Thus, taking up point by point the observations already made, and relying on Newton's work on the law of gravitation, they demonstrated that comets should have the same orbits as the planets. When Newton's major work, the Principia, was published in 1687 - undoubtedly one of the most remarkable scientific works of the 17th century - Halley wrote a vibrant tribute to Newton's genius as a preface.

It is thus at the age of thirty-nine that Halley tackled the problem that would later assure him his greatest title of glory. To do this, he undertook to record all the cometary passages of the recent and distant past. He was helped in this by luck, as his century was, by a quirk of nature, richer in comets than previous centuries. His investigation made him go back to the testimonies of Pliny the Elder or Seneca. He recalculated the orbits of 24 comets that passed through the perihelion between 1337 and 1698. It was a titanic, meticulous and long work. He managed, after several years, to isolate three passages that took place in 1531, 1607 and 1682.

Although the correspondence between these data appeared to be perfect, Halley was concerned about the slight differences that could not be explained solely by measurement inaccuracies. Moreover, the interval varied by more than a year. Halley hypothesized that some as yet unexplained force was responsible for such discrepancies, but could not be persuaded of this because of the lack of a rigorous scientific explanation. He asked Newton to calculate the possible gravitational perturbations between his comet and other comets. A few calculations showed him the falsity of this hypothesis, but it was enough to warm his mind enough to calculate the perturbations caused by Jupiter and by Saturn (then the last known planet of the Solar System). The calculations showed an almost perfect correlation between his theory and the observed passages.

With these results, he published in 1705 the results of his work in a work entitled Synopsis of the Astronomy of Comets, in which he considered that the three comets observed respectively by Apien in 1531, by Kepler in 1607 and by himself in 1682 were one and the same comet, and made the prophecy - entirely scientific - of the return of this comet for Christmas 1758. Halley knew, when he wrote this study, that he would never see the confirmation of these calculations in his lifetime, the next passage having to be realized in the year of his hundred and two years.

It is in this work that he also mentions, in filigree, the possibility of a "reservoir of comets", which will be formally theorized two centuries later by Jan Oort:

"makes me suspect to have a much larger number, which moving in regions more distant from the Sun, become very obscure; and having no tail, pass towards us remaining invisible."

He died on January 14, 1742, having seen his wife die four years earlier, and his son in the same year.

Posthumous triumph

When Halley predicted the return of the comet in 1758, his prophecy did not arouse much enthusiasm: indeed, it was more than half a century in the future. And when Halley died in 1742, the obituaries insisted at length on his maritime expeditions, his discoveries and on the diving bell of which he was the inventor, and passed in silence his cometary prediction, which fell into oblivion.

However, in 1757, a French mathematician, Alexis Clairaut, took the decision to resume Halley's calculations in order to improve the accuracy of the calculations and to predict more accurately the date of the comet's return. The deadlines were short, the calculations had to be redone before the reappearance of the comet, to cut short any accusation of deception. The calculations of gravitational interaction of the comet with the Earth, Jupiter and Saturn were colossal for the short time he had, a little over a year. He thus called upon Joseph Jérôme Lefrançois de Lalande, and the mathematician Nicole-Reine Lepaute.

After months of calculations, the team of three "official" astronomers announced in November 1758 that the comet would make its perihelion passage on April 13, 1759. The world astronomical community - questioning Halley's prediction for a part of it, still seeing nothing coming for Christmas 1758 - started to feverishly scan the sky again. On December 25, 1758, the comet was observed at the exact spot where Halley had predicted it by an observer near Dresden, and reached its perihelion on March 13, 1759, exactly one month before the date fixed by Lepaute, Lalande and Clairaut.

Three years before his death, E. Halley stated: "If the return predicted by us for the year 1758 is realized, impartial posterity will not refuse to recognize that it was an Englishman who first announced it. This wish was largely fulfilled, since the astronomical community decided, following this posthumous success, to give the name of Halley to this comet.

In 1692, Edmond Halley put forward the idea that the Earth was made up of a hollow shell about 800 km thick, two concentric inner shells, and a central core, having respectively the approximate diameters of the planets Venus, Mars and Mercury. These shells would be separated by an atmospheric layer, each would have its own magnetic poles, and they would rotate at different speeds. Halley proposed this model to explain anomalies in compass readings. He hypothesized the existence of a luminous atmosphere inside the Earth, which produced the aurora borealis by escaping to the outside. He also put forward the hypothesis that the interior worlds could be inhabited. These hypotheses are related to the numerous theories of the hollow Earth.

Sources

1. Edmond Halley
2. Edmond Halley
3. ^ a b This date is by Halley's own account, but is otherwise unconfirmed.[6]
4. ^ a b This was perhaps the first astronomical mystery solved using Newton's laws by a scientist other than Newton.[55]
5. ^ He wrote as late as 1716 in hopes of a future expedition to make these observations.[19]
6. ^ This contribution caused Flamsteed to nickname Halley "the southern Tycho".[24] Tycho had catalogued the stars observed by Johannes Hevelius.[25]
7. Ces dates correspondent au calendrier julien, utilisé en Grande-Bretagne jusqu'en 1752, qui vit aussi le changement de début d'année du 25 mars au 1er janvier. Dans le calendrier grégorien, ces dates correspondent au 8 novembre 1656 et au 25 janvier 1743. (voir Michel Toulmonde, Les dates de Newton dans l'Astronomie, Février 2007).
8. Jean-Michel Faidit, La comète impériale de 1811 : son découvreur Flaugergues, son influence sur Napoléon, le vin de la comète, Toulon, Presses du Midi, 2012, 131 p. (ISBN 978-2-8127-0312-6), p. 7
9. G.J. Babu and E.D. Feigelson: Astrostatistics. 1996 Chapman and Hall.
10. Mathematics Genealogy Project (англ.) — 1997.
11. Астронет > Галлей (Хэлли) Эдмонд
12. 1 2 3 4 БРЭ.
13. Les membres du passé dont le nom commence par H Архивная копия от 26 сентября 2020 на Wayback Machine (фр.)
14. См. Random House Dictionary of the English Language.