Great Astronomers: Halley
Great Astronomers in Modern English
by Sir Robert S. Ball, 1895 (paraphrased by Leslie Noelani Laurio)
To view the table of contents for the rest of this book, click here.
Edmond Halley, 1656-1742
Discovered that periodic appearances of a comet were actually the same comet; also persuaded Isaac Newton to publish his Principia.
Isaac Newton was only fourteen years old when Edmund Halley was born, but in later years, they became dear friends and famous scientific contemporaries. If it hadn't been for Isaac Newton's renown greatness that overshadowed everyone else in the field, Halley would be even more well known today than he is.
Edmund Halley was born at Haggerstown on the eastern side of London, on October 29, 1656. His father, Edmund Sr., was a soap-boiler in Winchester Street and had been so successful at his business that he was wealthy. Not many details are known of Halley's childhood. He had a natural aptitude for learning, and some ability for mechanical invention. He was educated by Dr. Thomas Gale at St. Paul's School in London.
At this school, the young scientist excelled in various subjects, especially mathematics. By the time he left school, he had also made good progress in astronomy. At age seventeen he attended Queen's College, Oxford, as a commoner [commoners, or pensioners, had no scholarship and paid their own tuition]. He already had a good reputation -- the "Athenae Oxonienses" [history of Oxford alumni] says that Halley came to Oxford 'with skill in Latin, Greek, and Hebrew, and enough knowledge of geometry to make a complete dial.' [sundial?] He studied many different subjects, but his favourite subject was astronomy. His earliest first-hand observations included an eclipse that he viewed from his father's house on Winchester Street. He also studied theoretical branches of astronomy well enough to be able to apply mathematics to some complicated astronomy calculations.
Up until the time of Kepler, scientists had assumed without question that heavenly bodies orbit in perfectly uniform circles. Kepler proved that planets orbit in slightly irregular ellipses, not circles. But Kepler couldn't get away from the belief that planets must orbit around some point uniformly. He had proved that orbital ellipses where the sun is one of the focus points don't orbit uniformly, and, in fact, at some points of its orbit, planets swing around the sun with greater angular velocity than at other points. Ellipses that are nearly circular (and most important planets do have nearly circular orbits) are almost uniform when they travel around the empty focus point of the ellipse. [Kepler's first law: the orbital paths of the planets are elliptical with the sun at one focus of the ellipse. The second focus point is called "empty" because its exact point is unknown.] It seemed natural to assume that the orbit was uniform around the two focus points, but when Halley was still fairly young, he showed that the orbit around the empty focus point isn't quite uniform. He published a paper about this at age 19 [called 'Direct and Geometrical Method of finding the Aphelia and Eccentricity of the Planet,' 1676; this treatise finally put an end to the notion of a "center of uniform motion" that had begun with Ptolemy.] and this gave him a reputation of a respected theoretical astronomer.
But Edmond Halley didn't want to just write about astronomical events; he wanted to observe them first-hand. He realized that, in order for astronomy to progress and become more exact, it was necessary to determine the specific positions of stars with closer accuracy. So he decided to do that himself, building on the work that Tycho Brahe had started.
These days [1895], astronomers in our major observatories spend most of their time determining the precise locations of the stars. Having this exact information is fundamental, not just to make accurate scientific records, but also for navigation and extensive surveying projects that need to be meticulous. [Surveying large areas, such as boundaries, or distances between towns, utilizes latitude and longitude, and that depends on using stars as reference points.] The fact that Halley determined to concentrate on furthering this data shows his scientific vision.
But he found that Johannes Hevelius in Danzig, Poland, and John Flamsteed in Greenwich, England were already working on this. So he decided to focus his energy somewhere else that would be more useful to science. He decided to let Hevelius and Flamsteed catalogue the stars in the northern hemisphere, and he would go to the southern hemisphere, where nobody else had worked. The stars there were almost invisible from Europe, so the stars he could survey there would supplement the astronomers working in the northern hemisphere. If they put their data together, they would have a complete catalogue of all the most important stars seen from the entire world.
In our own time, when so many enthusiastic naturalists have devoted themselves to observation, it's hard to imagine an undiscovered territory that no one has ever explored before. But Halley was fortunate enough to begin work in a magnificent region that was entirely unknown. If a star couldn't be seen from Tycho Brahe's observatory in Denmark, it had never been properly observed and recorded. It was rumored that a Dutchman had visited Sumatra and observed some southern stars, which he had added to a celestial globe of stars. But on closer examination, Halley saw that their positions were so inaccurate that the southern hemisphere could justifiably be considered undiscovered.
At the age of twenty, without waiting long enough to get his degree, Halley asked his father for permission [and funding, no doubt] to go to the southern hemisphere to observe the stars near the south pole. His father was wise enough to encourage his son's thirst for knowledge, and wealthy enough to finance his trip. In fact, he wanted to make everything as easy as possible for his son, who showed so much promise, so he gave him a very generous £300 annual allowance. Edmond Halley also had letters of recommendation from King Charles II [the son of the King Charles who had lost his life during Cromwell's time], and letters from the directors of the East India Company. He set sail with his astronomical equipment in 1676 in a ship belonging to the East India Company, and sailed towards St. Helena [an island in the middle of the Atlantic Ocean west of Africa] to begin his work.
[Note: St. Helen Island Info says, "Halley wanted to observe the stars of the Southern Hemisphere and chose St. Helena primarily because it was, at the time, the only secure English territory south of the equator, but also because it was northerly enough for some Northern Hemisphere stars to be seen, thus linking the two hemispheres."]
It took him three months [!] of smooth sailing to get to the island. He set up his sextant with its radius of five and a half feet, and a 24 foot long telescope, and prepared to get to work. But he was disappointed. He had been told that St. Helena was a good place for astronomical observation, but he found that the sky was usually overcast, and it rained frequently, so his observations were interrupted a lot. So he only stayed there for a year. In spite of the frustrating conditions, he accomplished enough to be called 'our southern Tycho.' He earned his fame as an astronomer on the same lonely rock that would become famous as Napoleon's place of exile 150 later.
When he returned to England, Edmond Halley drew a map of the stars he had observed and presented it to the king [Charles II] in 1677. Halley flattered the king by naming a constellation near the southern pole Robur Carolinum [Latin for "Charles' Oak," referring to the royal oak tree where Charles II hid from Cromwell's troops in 1651. The constellation never really caught on because it 'borrowed' stars from the well-known constellation named after Jason's argonauts.]. Halley described the constellation as being 'in perpetual memory' of the king's escape, as if that event were important enough to be preserved in the heavens forever.
King Charles II. must have appreciated the gesture, because Halley was promoted to Master of Arts at Oxford in November, 1678, and special notice was mentioned of his accomplishments in math and science through his observations at St. Helena. This was quite an honour for a young man who had left the university before his graduation.
On November 30, 1678, Edmond Halley was elected a Fellow of the Royal Society. He appreciated the distinction and was very active in the Society. He wrote some very important articles for the Philosophical Transactions, the Society's publication, which became the world's first science journal. By 1713, he was the Society's secretary until he succeeded Flamsteed as Astronomer Royal and moved to Greenwich in 1721.
["Fellowship of the Royal Society is an award granted to individuals that the UK-based Royal Society judges to have made a substantial contribution to the improvement of natural knowledge." -- Wikipedia]
Less than a year after being elected into the Royal Society, Edmond Halley was chosen as the Society's representative in a debate with Johannes Hevelius. We will find it hard to understand why the issue they discussed was debatable, but we have to remember that astronomy was in its infancy then. Things that seem so obvious to us were matters of serious thought and consideration for them. This was the issue: When Tycho Brahe mapped out the positions of the stars, he hadn't had access to a telescope. His instruments at Uraniborg were not unlike the sight on a hunting rifle -- and not much more accurate. Shortly after Tycho, Galileo invented a telescope, and no one could deny that it made the stars looks more clear. But did a telescope actually provide any advantage for measuring the position of a star? If a star can be seen with an unaided eye, its position can be determined with the kinds of instruments that Tycho used. Could a telescope help to determine its position with more accuracy? Today this seems obvious -- anyone with any experience using a telescope knows that it's much more precise than any mere sighting instrument! Observing only with the naked eye actually leads to errors sixty times more often than when using a telescope for marking the position of stars.
But this wasn't obvious in the 1600's. So Edmond Halley was sent to discuss this with the astronomers on the continent of Europe. Johannes Hevelius represented the older method -- the method Tycho had used with so much success. Hevelius argued vehemently that sights could help point an instrument at a star more precisely than a telescope could. Halley left for Gdansk [in Poland] in May, 1679. With his energetic disposition, he set to work star gazing on the very night he arrived in Poland! The telescopes available in those days weren't nearly as efficient as the ones we have today, so it isn't surprising that the results of the discussion weren't conclusive. Halley stayed in Poland over a year investigating and making observations. When he returned to England, he spoke highly of the skill Johannes Hevelius displayed with his antiquated methods, but Halley was too wise not to see that the use of a telescope was critical for accurate observations.
The following year, Halley went on a tour of Europe with Robert Nelson, a friend from school. We complain if it takes more than an hour to cross the English Channel, but Halley wrote this to Robert Hooke in June, 1680: 'We had stormy weather, so it took us forty hours to get from Dover [England] to Calais [across the English Channel in France].' He was well-known by this time, so he attracted some attention when he was in Paris. He spent a lot of his time in the Paris Observatory with Giovanni Domenico Cassini, the head astronomer, who made him very welcome. Together they made observations of the place of the comet that was attracting so much attention at the time [undoubtedly the Great Comet of 1680, also called Kirch's Comet, and Newton's Comet; see accounts and paintings of it here]. Their observations were useful later when Halley later investigated the path the comet had taken. Halley took advantage of his time in Paris to learn all he could from the distinguished scientists who lived there. Then he spent almost a year in Italy. He was described as friendly and extremely intelligent.
He returned to England in 1682 and married Mary Tooke. They had a happy marriage until she died 55 years later. After their marriage, they lived in Islington. He set up his astronomy equipment and started his observations there.
A few years later [1692], he published a paper about the variation of the magnetic compass explaining why the needle doesn't always point exactly to 'true north.' The difference between geographic North and magnetic north was something he had always been curious about, and he remained interested in it all his life. Later, Sir John Herschel wrote, 'we owe our first idea of the complexity of magnetism to Edmond Halley. His conclusions prove how wise and profound he was.' In 1692, Halley explained his theory of terrestrial magnetism: the earth, being hollow, is composed of layers, or shells, that rotate at different speeds. Each shell is separated by a layer of atmosphere [he wondered if the Aurora Borealis came from these atmospheric layers], and each has a slightly different magnetic pole. Thus, 'true north' is not actually static. He hoped to convince ship captains to record compass readings from various parts of the world and bring their readings back to the Royal Society for further study in the future. [Edmond Halley is credited with inventing the isogonic map to show the difference between magnetic north and true north from a given place. Read more here.]
This concept was so ahead of its time that the study of terrestrial magnetism was largely overlooked until 1811. He wasn't content to just speculate about it; he wanted to put his theory to the test. So he decided to observe the magnetic variation for himself. He asked for help from King William III. and was given command of a ship called the 'Paramour Pink,' and he sailed in the direction of the South Seas in 1694. Unfortunately, some of his men fell sick and one of his lieutenants mutinied, so he had to return home without accomplishing his mission. The government fired the lieutenant, and Halley found another smaller ship to accompany the 'Paramour Pink.' The two ships started out in September, 1699. He sailed as far as the 52nd degree of southern latitude, but was hindered by giant islands of ice.
Back in England in 1700, he published a general chart that shows the variations of the compass from the different places he visited. [View Edmond Halley's 'New and Correct Chart Shewing the Variations of the Compass' on Wikipedia.] On these charts he made lines connecting the various places where magnetic variation was identical. He gave the first example of how to represent large masses of complex facts in a way that appealed to the eye, and we still use this method today.
But the greatest service he did for the world was to bring Newton's 'Principia' before the world. Dr. James Glaisher wrote in 1888, 'If it wasn't for Edmond Halley, the Principia would not have existed.'
It wasn't until Edmond Halley visited Isaac Newton in 1684 that Newton had the idea of publishing his findings on gravitation. Halley and some other scientists of the time probably had a vague glimmering of the principles of gravity, but it would take Newton's genius to lay it out fully with the correct mathematical equations. In fact, Halley had already shown that if planets move in perfect circles around the sun, and if the squares of their periodic times are proportional to the cubes of their mean distances, then the attraction force of each planet must vary inversely as the square of its distance from the sun. But if planets travel in ellipses, then their distance from the sun isn't always the same, so it's more complicated to calculate the planet's movement mathematically. It's not as simple as assuming that the attraction force varies inversely as the square of the distance. Halley was puzzled with this question, but his mathematical abilities weren't adequate to solve it. Robert Hooke and Sir Christopher Wren were also interested in the same question. Hooke claimed to have found the solution, but he didn't reveal his solution. He said he hoped that others would try and fail first, and that would make them appreciate his brilliance in finding the solution all the more. Halley confessed that he couldn't solve it, and Wren offered to award a book worth 40 s. to either of them if they solved the puzzle and presented their proof within two months. That's how much Christopher Wren valued the Law of Gravitation upon which modern astronomy is based.
Halley couldn't figure it out, so he went to Cambridge to visit Isaac Newton -- and found that Newton had already solved the puzzle! Newton showed Halley how the motions of all the planets could be mathematically calculated by relying on a hypothesis that there's an attraction force directed towards the sun, and that force varies inversely as the square of the distance from that body.
Halley was smart enough to recognize how important Isaac Newton's research was, and he urged the reclusive genius to publish his findings for the benefit of science. Halley went back to Cambridge to learn more about the mathematics that helped Newton to make his sublime discoveries, and again he encouraged Newton to publish his findings and share them with the world. Finally, in December 1684, Halley was able to announce to the Royal Society that Newton was going to send them a paper about his research on gravitation that they could publish.
In 1686, the Royal Society was struggling with finances. They had recently published De Historia Piscium [History of Fishes] by Francis Willoughby, and it had cost them a lot of money. Their budget was left so sparse that they could barely pay the salaries of their permanent officials. Apparently the public didn't care about fishes, or maybe the book was disappointing, but they didn't recoup their expenses. Money was so tight that when the Royal Society sent Halley to measure the length of a degree of the earth's surface, they ordered that his expenses be paid either in £50 -- or in fifty copies of De Historia Piscium! [It was at this time that Halley published the world's first meteorological chart, using data from the time he spent at St. Helena.] The Council discussed Isaac Newton's Principia and determined that 'Halley should take on the business of getting the book printed at his own cost' -- so he did.
Isaac Newton was the kind of man who hated controversy. In fact, he would have preferred not to publish the third volume of Principia rather than have any conflict with Robert Hooke over the findings in it. He thought of changing the name of the book to De Motu Corporum Libri Duo ['On the movement of Two Books' -- Principia is a three-volume set], but after thinking about it, he decided to keep the original title. As he wrote to Halley, 'It will help the sale of the book, and I should do what I can to increase sales now that it's yours.'
Halley worked so hard to facilitate his friend's work that he was able to present a finished copy of Principia to King James that very year with his own explanatory paper. He even wrote a Latin poem in the style of a classic hexameter praising Newton's genius to preface his own paper. The last line, translated into English, says, 'No mortal can approach the gods any closer.'
Halley and Newton remained close friends until Newton's death in 1727. It has been rumoured that they had a falling out, but there's no real proof of this. In fact, to the contrary, Halley publicly defended Newton in 1727 in two published papers supporting Newton's system of chronology after it had been attacked by a certain person in the church. Anyone who has read these papers will be convinced of Halley's esteem for Isaac Newton.
[Note: Isaac Newton wrote a paper in 1718 called "A Short Chronicle, from the First Memory of Things in Europe, to the Conquest of Persia, by Alexander the Great;" it details the history of various ancient kingdoms and identifies some historical figures with mythology. It was criticized by M. Freret and Father Soueiet. An expanded edition was published posthumously as "The Chronology of Ancient Kingdoms."]
The generosity and zeal Halley showed for Isaac Newton's doctrines related to the orbits of celestial bodies resulted in a discovery of his own -- the discovery that made Halley a household name. After Newton explained how planets moved, he turned his attention to comets. He saw that the path of a comet was related to the attraction force of the sun, and he explained the principles that could help determine the path of a comet by using positions of the comet from three different dates as reference points. Edmond Halley recognized how significant this was at once. He realized that this provided a logical order that could allow astronomers to predict the path of a comet. The concept of gravitation meant that not only planets, but comets, too, revolve in ellipses. But the orbit of a comet is extremely elongated so that a certain part of its path is close enough and bright enough to be seen from earth. In fact, its ellipse is so elongated that's indistinguishable from a parabola. Using this knowledge, Halley began studying specific comets whose brightness had made them obvious enough to be recorded in past astronomical observations. With hard work and diligence, he identified the paths of 24 comets that had appeared between 1337 and 1698. Among these 24, he noticed three whose paths were similar, and he speculated that these three comets were actually the same comet seen three times. The first time was 1531, the second time was recorded by Kepler in 1607, and the third was recorded by Halley himself in 1682. From these dates, it seemed that the comet was returning every 75 or 76 years. Halley dug further and found a recorded observation of a comet that had appeared in 1456 -- exactly 75 years prior to 1531! And there was another recorded observation 1380, 75 years earlier, and another in 1305, another 75 years earlier!
When Halley discovered that a comet had been recorded on several occasions at intervals of seventy-five or seventy-six years, he concluded that these must have been the same comet, obediently making its orbit around the sun every 75 or 76 years. Remember that, before this time, comets had either been considered signs of divine displeasure and bad omens, or as random visitors to our solar system that could happen any time without notice. Nobody knew where they came from, or where they went after they disappeared.
But Halley's theory needed to be tested. When would this comet appear again? This question was complicated because, on its orbit around the sun, the comet was influenced by attraction forces from several planets as it passed them. Its ellipse isn't as simple as it would be if the sun were the only celestial body it had to contend with. Each planet tries to pull the comet off track as it passes, and although their attraction force is small compared with the sun, it's enough to make significant irregularities in the comet's orbital journey. Halley had no way of calculating the precise effect that a planet might have on the comet. He figured that Jupiter would hinder the comet's return somewhat [because Jupiter is so big]. Without Jupiter's attraction force, he calculated that the comet would be due to appear in late in 1757 or early 1758. But Jupiter would add some delay, so Halley predicted that the comet would arrive either in late 1758 or early 1759 -- 53 years into the future. He knew he wouldn't live long enough to see whether he was correct, but he wrote, 'If it does return around 1758, objective people living at that time will have to acknowledge that this was discovered by an Englishman!' And his prediction was accurate! The comet was first spotted on Christmas Day, 1758, and traveled away towards the sun by March 13, 1759. Halley had died seventeen years earlier, but the fulfillment of his prediction proved he was right and made him a household name. The comet also appeared in 1835 and 1910 [and 1986; the comet actually appears every 74-79 years, not every 75-76 years].
Edmond Halley's next project was a series of investigations that would improve our understanding of how planets move. It was almost finished in 1719, although the results weren't published until 1749, after he had died [which was in 1742]. As he was closely investigating the travels of Venus, he realized that Venus was due to make a transit right across the face of the sun in 1761. This would provide an opportunity to determine the distance of the sun and reveal the size of our solar system. [He suggested that applying geometry to Venus's transit could be used to determine the 'astronomical unit,' which is the earth's distance from the sun; using his suggested method, it was calculated to be about 95 million miles. Read more here.] Considering what was known at the time, he predicted when Venus would cross the sun pretty accurately, and it's due to his efforts and sense of urgency that so many scientists took up an interest in investigating Venus's transit in 1761 so enthusiastically. Halley knew there was no chance of seeing it himself since he wouldn't live that long, but he was anxious that men who would be there wouldn't waste the opportunity, and he gathered as much data as he could to aid them when the time came. As it turned out, Halley put too much value on the transit of Venus in determining the solar distance from the earth because determining the exact time of contact between the edge of Venus and the edge of the sun can't be done as accurately as he had expected.
In 1691, Halley was nominated for the Savilian Professorship of Astronomy at Oxford. But John Flamsteed, who was the Astronomer Royal at the time, preferred someone else. However, with some influence from his friend Isaac Newton, he was appointed deputy Controller of the Mint at Chester in 1696. He didn't retain the position for long, though -- it was abolished two years later. And then in 1703, he finally got the Savilian Professorship appointment.
His observations of the 1715 solar eclipse added to his scientific reputation. This was the first total eclipse that had been visible in London since 1140. Halley did the necessary math and made much more precise calculations about when the eclipse would happen than even the official announcement. He watched it from a room in the Royal Observatory, and he described the outer atmosphere of the sun in detail. He didn't suggest whether it came from the sun or the moon, but today we know it's the sun's corona.
In February 1720, Edmond Halley succeeded John Flamsteed as Astronomer Royal -- which was no surprise, since nobody was more competent for the position. But when he assessed the state of things at the Observatory, he discovered that all the instruments were gone! They had been the personal property of Flamsteed, and his widow removed them after his death. Halley offered to buy them from her, but the personal differences that had prevented them from being friends and that had motivated Flamsteed to choose someone else as Savilian Professor of Astronomy made Flamsteed's widow unwilling to negotiate with him. The Greenwich Observatory looked very different then. Not only were there no instruments, but he had no assistants, so he had to make his observations and do all of his duties single-handedly.
But the next year, he was able to get a £500 grant from the Board of Ordnance, and a transit instrument [small telescope with a graduated mount for precisely tracking star positions] was installed immediately. Later, he added an eight-foot quadrant, and with these instruments, he began a series of observations of the moon. He hoped to continue this for a complete eighteen year lunar cycle, although he was already 64! He hoped to improve the theory of lunar motion so that the moon could be used more accurately to determine longitude at sea. And he lived long enough to make these observations and draw up tables from his data, although his charts weren't published until after his death. All astronomers except those in France adopted his tables.
Halley had been free from illness for his entire life, until he was paralyzed with a stroke in 1737. Even then, he continued to work at his telescope until 1739, when his health began to fail rapidly. He died on January 14, 1742 at the age of 85. His mind was sharp to the end of his life. He was buried in Kent in the same tomb as his wife, who had died five years earlier. John Pond, Astronomer Royal from 1811 to 1835, was laid in the same tomb years later.
Edmond Halley had a generous and sincere disposition, and he was free from jealousy and bitterness. He was a little taller than average and thin. His complexion was fair, and people said he had an unusually sprightly manner of acting and speaking. At a memorial service at the Paris Academie Des Sciences, where he had been elected a member in 1729, his eulogy said that 'he had all the qualities necessary to please princes who wanted to be instructed, along with vast knowledge and a constant presence of mind, yet he was also decent, careful, polite, and sincere.'
Peter the Great admired him greatly. He consulted Halley on issues connected with ship building, and even invited him to dine at his own table. But Halley's qualities were more noble than simply pleasing royalty. He was able to inspire the affection and admiration of his equals because of his unselfish devotion to his friends, and the friendliness and good-humor that marked his conversation.
by Sir Robert S. Ball, 1895 (paraphrased by Leslie Noelani Laurio)
To view the table of contents for the rest of this book, click here.
Edmond Halley, 1656-1742
Discovered that periodic appearances of a comet were actually the same comet; also persuaded Isaac Newton to publish his Principia.
Isaac Newton was only fourteen years old when Edmund Halley was born, but in later years, they became dear friends and famous scientific contemporaries. If it hadn't been for Isaac Newton's renown greatness that overshadowed everyone else in the field, Halley would be even more well known today than he is.
Edmund Halley was born at Haggerstown on the eastern side of London, on October 29, 1656. His father, Edmund Sr., was a soap-boiler in Winchester Street and had been so successful at his business that he was wealthy. Not many details are known of Halley's childhood. He had a natural aptitude for learning, and some ability for mechanical invention. He was educated by Dr. Thomas Gale at St. Paul's School in London.
At this school, the young scientist excelled in various subjects, especially mathematics. By the time he left school, he had also made good progress in astronomy. At age seventeen he attended Queen's College, Oxford, as a commoner [commoners, or pensioners, had no scholarship and paid their own tuition]. He already had a good reputation -- the "Athenae Oxonienses" [history of Oxford alumni] says that Halley came to Oxford 'with skill in Latin, Greek, and Hebrew, and enough knowledge of geometry to make a complete dial.' [sundial?] He studied many different subjects, but his favourite subject was astronomy. His earliest first-hand observations included an eclipse that he viewed from his father's house on Winchester Street. He also studied theoretical branches of astronomy well enough to be able to apply mathematics to some complicated astronomy calculations.
Up until the time of Kepler, scientists had assumed without question that heavenly bodies orbit in perfectly uniform circles. Kepler proved that planets orbit in slightly irregular ellipses, not circles. But Kepler couldn't get away from the belief that planets must orbit around some point uniformly. He had proved that orbital ellipses where the sun is one of the focus points don't orbit uniformly, and, in fact, at some points of its orbit, planets swing around the sun with greater angular velocity than at other points. Ellipses that are nearly circular (and most important planets do have nearly circular orbits) are almost uniform when they travel around the empty focus point of the ellipse. [Kepler's first law: the orbital paths of the planets are elliptical with the sun at one focus of the ellipse. The second focus point is called "empty" because its exact point is unknown.] It seemed natural to assume that the orbit was uniform around the two focus points, but when Halley was still fairly young, he showed that the orbit around the empty focus point isn't quite uniform. He published a paper about this at age 19 [called 'Direct and Geometrical Method of finding the Aphelia and Eccentricity of the Planet,' 1676; this treatise finally put an end to the notion of a "center of uniform motion" that had begun with Ptolemy.] and this gave him a reputation of a respected theoretical astronomer.
But Edmond Halley didn't want to just write about astronomical events; he wanted to observe them first-hand. He realized that, in order for astronomy to progress and become more exact, it was necessary to determine the specific positions of stars with closer accuracy. So he decided to do that himself, building on the work that Tycho Brahe had started.
These days [1895], astronomers in our major observatories spend most of their time determining the precise locations of the stars. Having this exact information is fundamental, not just to make accurate scientific records, but also for navigation and extensive surveying projects that need to be meticulous. [Surveying large areas, such as boundaries, or distances between towns, utilizes latitude and longitude, and that depends on using stars as reference points.] The fact that Halley determined to concentrate on furthering this data shows his scientific vision.
But he found that Johannes Hevelius in Danzig, Poland, and John Flamsteed in Greenwich, England were already working on this. So he decided to focus his energy somewhere else that would be more useful to science. He decided to let Hevelius and Flamsteed catalogue the stars in the northern hemisphere, and he would go to the southern hemisphere, where nobody else had worked. The stars there were almost invisible from Europe, so the stars he could survey there would supplement the astronomers working in the northern hemisphere. If they put their data together, they would have a complete catalogue of all the most important stars seen from the entire world.
In our own time, when so many enthusiastic naturalists have devoted themselves to observation, it's hard to imagine an undiscovered territory that no one has ever explored before. But Halley was fortunate enough to begin work in a magnificent region that was entirely unknown. If a star couldn't be seen from Tycho Brahe's observatory in Denmark, it had never been properly observed and recorded. It was rumored that a Dutchman had visited Sumatra and observed some southern stars, which he had added to a celestial globe of stars. But on closer examination, Halley saw that their positions were so inaccurate that the southern hemisphere could justifiably be considered undiscovered.
At the age of twenty, without waiting long enough to get his degree, Halley asked his father for permission [and funding, no doubt] to go to the southern hemisphere to observe the stars near the south pole. His father was wise enough to encourage his son's thirst for knowledge, and wealthy enough to finance his trip. In fact, he wanted to make everything as easy as possible for his son, who showed so much promise, so he gave him a very generous £300 annual allowance. Edmond Halley also had letters of recommendation from King Charles II [the son of the King Charles who had lost his life during Cromwell's time], and letters from the directors of the East India Company. He set sail with his astronomical equipment in 1676 in a ship belonging to the East India Company, and sailed towards St. Helena [an island in the middle of the Atlantic Ocean west of Africa] to begin his work.
[Note: St. Helen Island Info says, "Halley wanted to observe the stars of the Southern Hemisphere and chose St. Helena primarily because it was, at the time, the only secure English territory south of the equator, but also because it was northerly enough for some Northern Hemisphere stars to be seen, thus linking the two hemispheres."]
It took him three months [!] of smooth sailing to get to the island. He set up his sextant with its radius of five and a half feet, and a 24 foot long telescope, and prepared to get to work. But he was disappointed. He had been told that St. Helena was a good place for astronomical observation, but he found that the sky was usually overcast, and it rained frequently, so his observations were interrupted a lot. So he only stayed there for a year. In spite of the frustrating conditions, he accomplished enough to be called 'our southern Tycho.' He earned his fame as an astronomer on the same lonely rock that would become famous as Napoleon's place of exile 150 later.
When he returned to England, Edmond Halley drew a map of the stars he had observed and presented it to the king [Charles II] in 1677. Halley flattered the king by naming a constellation near the southern pole Robur Carolinum [Latin for "Charles' Oak," referring to the royal oak tree where Charles II hid from Cromwell's troops in 1651. The constellation never really caught on because it 'borrowed' stars from the well-known constellation named after Jason's argonauts.]. Halley described the constellation as being 'in perpetual memory' of the king's escape, as if that event were important enough to be preserved in the heavens forever.
King Charles II. must have appreciated the gesture, because Halley was promoted to Master of Arts at Oxford in November, 1678, and special notice was mentioned of his accomplishments in math and science through his observations at St. Helena. This was quite an honour for a young man who had left the university before his graduation.
On November 30, 1678, Edmond Halley was elected a Fellow of the Royal Society. He appreciated the distinction and was very active in the Society. He wrote some very important articles for the Philosophical Transactions, the Society's publication, which became the world's first science journal. By 1713, he was the Society's secretary until he succeeded Flamsteed as Astronomer Royal and moved to Greenwich in 1721.
["Fellowship of the Royal Society is an award granted to individuals that the UK-based Royal Society judges to have made a substantial contribution to the improvement of natural knowledge." -- Wikipedia]
Less than a year after being elected into the Royal Society, Edmond Halley was chosen as the Society's representative in a debate with Johannes Hevelius. We will find it hard to understand why the issue they discussed was debatable, but we have to remember that astronomy was in its infancy then. Things that seem so obvious to us were matters of serious thought and consideration for them. This was the issue: When Tycho Brahe mapped out the positions of the stars, he hadn't had access to a telescope. His instruments at Uraniborg were not unlike the sight on a hunting rifle -- and not much more accurate. Shortly after Tycho, Galileo invented a telescope, and no one could deny that it made the stars looks more clear. But did a telescope actually provide any advantage for measuring the position of a star? If a star can be seen with an unaided eye, its position can be determined with the kinds of instruments that Tycho used. Could a telescope help to determine its position with more accuracy? Today this seems obvious -- anyone with any experience using a telescope knows that it's much more precise than any mere sighting instrument! Observing only with the naked eye actually leads to errors sixty times more often than when using a telescope for marking the position of stars.
But this wasn't obvious in the 1600's. So Edmond Halley was sent to discuss this with the astronomers on the continent of Europe. Johannes Hevelius represented the older method -- the method Tycho had used with so much success. Hevelius argued vehemently that sights could help point an instrument at a star more precisely than a telescope could. Halley left for Gdansk [in Poland] in May, 1679. With his energetic disposition, he set to work star gazing on the very night he arrived in Poland! The telescopes available in those days weren't nearly as efficient as the ones we have today, so it isn't surprising that the results of the discussion weren't conclusive. Halley stayed in Poland over a year investigating and making observations. When he returned to England, he spoke highly of the skill Johannes Hevelius displayed with his antiquated methods, but Halley was too wise not to see that the use of a telescope was critical for accurate observations.
The following year, Halley went on a tour of Europe with Robert Nelson, a friend from school. We complain if it takes more than an hour to cross the English Channel, but Halley wrote this to Robert Hooke in June, 1680: 'We had stormy weather, so it took us forty hours to get from Dover [England] to Calais [across the English Channel in France].' He was well-known by this time, so he attracted some attention when he was in Paris. He spent a lot of his time in the Paris Observatory with Giovanni Domenico Cassini, the head astronomer, who made him very welcome. Together they made observations of the place of the comet that was attracting so much attention at the time [undoubtedly the Great Comet of 1680, also called Kirch's Comet, and Newton's Comet; see accounts and paintings of it here]. Their observations were useful later when Halley later investigated the path the comet had taken. Halley took advantage of his time in Paris to learn all he could from the distinguished scientists who lived there. Then he spent almost a year in Italy. He was described as friendly and extremely intelligent.
He returned to England in 1682 and married Mary Tooke. They had a happy marriage until she died 55 years later. After their marriage, they lived in Islington. He set up his astronomy equipment and started his observations there.
A few years later [1692], he published a paper about the variation of the magnetic compass explaining why the needle doesn't always point exactly to 'true north.' The difference between geographic North and magnetic north was something he had always been curious about, and he remained interested in it all his life. Later, Sir John Herschel wrote, 'we owe our first idea of the complexity of magnetism to Edmond Halley. His conclusions prove how wise and profound he was.' In 1692, Halley explained his theory of terrestrial magnetism: the earth, being hollow, is composed of layers, or shells, that rotate at different speeds. Each shell is separated by a layer of atmosphere [he wondered if the Aurora Borealis came from these atmospheric layers], and each has a slightly different magnetic pole. Thus, 'true north' is not actually static. He hoped to convince ship captains to record compass readings from various parts of the world and bring their readings back to the Royal Society for further study in the future. [Edmond Halley is credited with inventing the isogonic map to show the difference between magnetic north and true north from a given place. Read more here.]
This concept was so ahead of its time that the study of terrestrial magnetism was largely overlooked until 1811. He wasn't content to just speculate about it; he wanted to put his theory to the test. So he decided to observe the magnetic variation for himself. He asked for help from King William III. and was given command of a ship called the 'Paramour Pink,' and he sailed in the direction of the South Seas in 1694. Unfortunately, some of his men fell sick and one of his lieutenants mutinied, so he had to return home without accomplishing his mission. The government fired the lieutenant, and Halley found another smaller ship to accompany the 'Paramour Pink.' The two ships started out in September, 1699. He sailed as far as the 52nd degree of southern latitude, but was hindered by giant islands of ice.
Back in England in 1700, he published a general chart that shows the variations of the compass from the different places he visited. [View Edmond Halley's 'New and Correct Chart Shewing the Variations of the Compass' on Wikipedia.] On these charts he made lines connecting the various places where magnetic variation was identical. He gave the first example of how to represent large masses of complex facts in a way that appealed to the eye, and we still use this method today.
But the greatest service he did for the world was to bring Newton's 'Principia' before the world. Dr. James Glaisher wrote in 1888, 'If it wasn't for Edmond Halley, the Principia would not have existed.'
It wasn't until Edmond Halley visited Isaac Newton in 1684 that Newton had the idea of publishing his findings on gravitation. Halley and some other scientists of the time probably had a vague glimmering of the principles of gravity, but it would take Newton's genius to lay it out fully with the correct mathematical equations. In fact, Halley had already shown that if planets move in perfect circles around the sun, and if the squares of their periodic times are proportional to the cubes of their mean distances, then the attraction force of each planet must vary inversely as the square of its distance from the sun. But if planets travel in ellipses, then their distance from the sun isn't always the same, so it's more complicated to calculate the planet's movement mathematically. It's not as simple as assuming that the attraction force varies inversely as the square of the distance. Halley was puzzled with this question, but his mathematical abilities weren't adequate to solve it. Robert Hooke and Sir Christopher Wren were also interested in the same question. Hooke claimed to have found the solution, but he didn't reveal his solution. He said he hoped that others would try and fail first, and that would make them appreciate his brilliance in finding the solution all the more. Halley confessed that he couldn't solve it, and Wren offered to award a book worth 40 s. to either of them if they solved the puzzle and presented their proof within two months. That's how much Christopher Wren valued the Law of Gravitation upon which modern astronomy is based.
Halley couldn't figure it out, so he went to Cambridge to visit Isaac Newton -- and found that Newton had already solved the puzzle! Newton showed Halley how the motions of all the planets could be mathematically calculated by relying on a hypothesis that there's an attraction force directed towards the sun, and that force varies inversely as the square of the distance from that body.
Halley was smart enough to recognize how important Isaac Newton's research was, and he urged the reclusive genius to publish his findings for the benefit of science. Halley went back to Cambridge to learn more about the mathematics that helped Newton to make his sublime discoveries, and again he encouraged Newton to publish his findings and share them with the world. Finally, in December 1684, Halley was able to announce to the Royal Society that Newton was going to send them a paper about his research on gravitation that they could publish.
In 1686, the Royal Society was struggling with finances. They had recently published De Historia Piscium [History of Fishes] by Francis Willoughby, and it had cost them a lot of money. Their budget was left so sparse that they could barely pay the salaries of their permanent officials. Apparently the public didn't care about fishes, or maybe the book was disappointing, but they didn't recoup their expenses. Money was so tight that when the Royal Society sent Halley to measure the length of a degree of the earth's surface, they ordered that his expenses be paid either in £50 -- or in fifty copies of De Historia Piscium! [It was at this time that Halley published the world's first meteorological chart, using data from the time he spent at St. Helena.] The Council discussed Isaac Newton's Principia and determined that 'Halley should take on the business of getting the book printed at his own cost' -- so he did.
Isaac Newton was the kind of man who hated controversy. In fact, he would have preferred not to publish the third volume of Principia rather than have any conflict with Robert Hooke over the findings in it. He thought of changing the name of the book to De Motu Corporum Libri Duo ['On the movement of Two Books' -- Principia is a three-volume set], but after thinking about it, he decided to keep the original title. As he wrote to Halley, 'It will help the sale of the book, and I should do what I can to increase sales now that it's yours.'
Halley worked so hard to facilitate his friend's work that he was able to present a finished copy of Principia to King James that very year with his own explanatory paper. He even wrote a Latin poem in the style of a classic hexameter praising Newton's genius to preface his own paper. The last line, translated into English, says, 'No mortal can approach the gods any closer.'
Halley and Newton remained close friends until Newton's death in 1727. It has been rumoured that they had a falling out, but there's no real proof of this. In fact, to the contrary, Halley publicly defended Newton in 1727 in two published papers supporting Newton's system of chronology after it had been attacked by a certain person in the church. Anyone who has read these papers will be convinced of Halley's esteem for Isaac Newton.
[Note: Isaac Newton wrote a paper in 1718 called "A Short Chronicle, from the First Memory of Things in Europe, to the Conquest of Persia, by Alexander the Great;" it details the history of various ancient kingdoms and identifies some historical figures with mythology. It was criticized by M. Freret and Father Soueiet. An expanded edition was published posthumously as "The Chronology of Ancient Kingdoms."]
The generosity and zeal Halley showed for Isaac Newton's doctrines related to the orbits of celestial bodies resulted in a discovery of his own -- the discovery that made Halley a household name. After Newton explained how planets moved, he turned his attention to comets. He saw that the path of a comet was related to the attraction force of the sun, and he explained the principles that could help determine the path of a comet by using positions of the comet from three different dates as reference points. Edmond Halley recognized how significant this was at once. He realized that this provided a logical order that could allow astronomers to predict the path of a comet. The concept of gravitation meant that not only planets, but comets, too, revolve in ellipses. But the orbit of a comet is extremely elongated so that a certain part of its path is close enough and bright enough to be seen from earth. In fact, its ellipse is so elongated that's indistinguishable from a parabola. Using this knowledge, Halley began studying specific comets whose brightness had made them obvious enough to be recorded in past astronomical observations. With hard work and diligence, he identified the paths of 24 comets that had appeared between 1337 and 1698. Among these 24, he noticed three whose paths were similar, and he speculated that these three comets were actually the same comet seen three times. The first time was 1531, the second time was recorded by Kepler in 1607, and the third was recorded by Halley himself in 1682. From these dates, it seemed that the comet was returning every 75 or 76 years. Halley dug further and found a recorded observation of a comet that had appeared in 1456 -- exactly 75 years prior to 1531! And there was another recorded observation 1380, 75 years earlier, and another in 1305, another 75 years earlier!
When Halley discovered that a comet had been recorded on several occasions at intervals of seventy-five or seventy-six years, he concluded that these must have been the same comet, obediently making its orbit around the sun every 75 or 76 years. Remember that, before this time, comets had either been considered signs of divine displeasure and bad omens, or as random visitors to our solar system that could happen any time without notice. Nobody knew where they came from, or where they went after they disappeared.
But Halley's theory needed to be tested. When would this comet appear again? This question was complicated because, on its orbit around the sun, the comet was influenced by attraction forces from several planets as it passed them. Its ellipse isn't as simple as it would be if the sun were the only celestial body it had to contend with. Each planet tries to pull the comet off track as it passes, and although their attraction force is small compared with the sun, it's enough to make significant irregularities in the comet's orbital journey. Halley had no way of calculating the precise effect that a planet might have on the comet. He figured that Jupiter would hinder the comet's return somewhat [because Jupiter is so big]. Without Jupiter's attraction force, he calculated that the comet would be due to appear in late in 1757 or early 1758. But Jupiter would add some delay, so Halley predicted that the comet would arrive either in late 1758 or early 1759 -- 53 years into the future. He knew he wouldn't live long enough to see whether he was correct, but he wrote, 'If it does return around 1758, objective people living at that time will have to acknowledge that this was discovered by an Englishman!' And his prediction was accurate! The comet was first spotted on Christmas Day, 1758, and traveled away towards the sun by March 13, 1759. Halley had died seventeen years earlier, but the fulfillment of his prediction proved he was right and made him a household name. The comet also appeared in 1835 and 1910 [and 1986; the comet actually appears every 74-79 years, not every 75-76 years].
Edmond Halley's next project was a series of investigations that would improve our understanding of how planets move. It was almost finished in 1719, although the results weren't published until 1749, after he had died [which was in 1742]. As he was closely investigating the travels of Venus, he realized that Venus was due to make a transit right across the face of the sun in 1761. This would provide an opportunity to determine the distance of the sun and reveal the size of our solar system. [He suggested that applying geometry to Venus's transit could be used to determine the 'astronomical unit,' which is the earth's distance from the sun; using his suggested method, it was calculated to be about 95 million miles. Read more here.] Considering what was known at the time, he predicted when Venus would cross the sun pretty accurately, and it's due to his efforts and sense of urgency that so many scientists took up an interest in investigating Venus's transit in 1761 so enthusiastically. Halley knew there was no chance of seeing it himself since he wouldn't live that long, but he was anxious that men who would be there wouldn't waste the opportunity, and he gathered as much data as he could to aid them when the time came. As it turned out, Halley put too much value on the transit of Venus in determining the solar distance from the earth because determining the exact time of contact between the edge of Venus and the edge of the sun can't be done as accurately as he had expected.
In 1691, Halley was nominated for the Savilian Professorship of Astronomy at Oxford. But John Flamsteed, who was the Astronomer Royal at the time, preferred someone else. However, with some influence from his friend Isaac Newton, he was appointed deputy Controller of the Mint at Chester in 1696. He didn't retain the position for long, though -- it was abolished two years later. And then in 1703, he finally got the Savilian Professorship appointment.
His observations of the 1715 solar eclipse added to his scientific reputation. This was the first total eclipse that had been visible in London since 1140. Halley did the necessary math and made much more precise calculations about when the eclipse would happen than even the official announcement. He watched it from a room in the Royal Observatory, and he described the outer atmosphere of the sun in detail. He didn't suggest whether it came from the sun or the moon, but today we know it's the sun's corona.
In February 1720, Edmond Halley succeeded John Flamsteed as Astronomer Royal -- which was no surprise, since nobody was more competent for the position. But when he assessed the state of things at the Observatory, he discovered that all the instruments were gone! They had been the personal property of Flamsteed, and his widow removed them after his death. Halley offered to buy them from her, but the personal differences that had prevented them from being friends and that had motivated Flamsteed to choose someone else as Savilian Professor of Astronomy made Flamsteed's widow unwilling to negotiate with him. The Greenwich Observatory looked very different then. Not only were there no instruments, but he had no assistants, so he had to make his observations and do all of his duties single-handedly.
But the next year, he was able to get a £500 grant from the Board of Ordnance, and a transit instrument [small telescope with a graduated mount for precisely tracking star positions] was installed immediately. Later, he added an eight-foot quadrant, and with these instruments, he began a series of observations of the moon. He hoped to continue this for a complete eighteen year lunar cycle, although he was already 64! He hoped to improve the theory of lunar motion so that the moon could be used more accurately to determine longitude at sea. And he lived long enough to make these observations and draw up tables from his data, although his charts weren't published until after his death. All astronomers except those in France adopted his tables.
Halley had been free from illness for his entire life, until he was paralyzed with a stroke in 1737. Even then, he continued to work at his telescope until 1739, when his health began to fail rapidly. He died on January 14, 1742 at the age of 85. His mind was sharp to the end of his life. He was buried in Kent in the same tomb as his wife, who had died five years earlier. John Pond, Astronomer Royal from 1811 to 1835, was laid in the same tomb years later.
Edmond Halley had a generous and sincere disposition, and he was free from jealousy and bitterness. He was a little taller than average and thin. His complexion was fair, and people said he had an unusually sprightly manner of acting and speaking. At a memorial service at the Paris Academie Des Sciences, where he had been elected a member in 1729, his eulogy said that 'he had all the qualities necessary to please princes who wanted to be instructed, along with vast knowledge and a constant presence of mind, yet he was also decent, careful, polite, and sincere.'
Peter the Great admired him greatly. He consulted Halley on issues connected with ship building, and even invited him to dine at his own table. But Halley's qualities were more noble than simply pleasing royalty. He was able to inspire the affection and admiration of his equals because of his unselfish devotion to his friends, and the friendliness and good-humor that marked his conversation.
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