Great Astronomers: J. Herschel
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.
Sir John Frederick William Herschel, 1792-1871
Studied binary stars, nebulae, and explored the skies in the southern hemisphere.
This famous son of a famous father was born at Slough, a large town in Berkshire [England] near Windsor on March 7, 1792. Sir William Herschel, who we read about in an earlier chapter, had married late in life [at age fifty], and John was his only child.
The environment young John was raised in was excellent training for his future career as an astronomer, which brought him almost as much fame as his father's. He had one great advantage that his father did not have: even from a very young age, he was allowed to pursue his own intellectual interests as much as he wanted. William Herschel had had to snatch what time he could during his busy life as a musician. But John enjoyed learning, and he had the time and the financial means to study. His early years have been described by Professor Charles Pritchard in his 1872 "Report of the Council of the Royal Astronomical Society:"
'A few characteristics of John Herschel's childhood that he himself mentioned later in his life were remembered fondly by those who were dear to him. His home was unique, and a perfect environment to nurture the gifts of a talented child like John Herschel. The head of the household was his old, observant, quiet father. His aunt Caroline, whose work and fame are still recognized as a brilliant counterpart to her famous brother, was always nearby. Young John passed his boyhood in the company of these two remarkable persons and under the shadow of his father's wonderful telescope. His father and aunt were always quietly at work, busy at research which didn't seem to be important outside the walls of the house, although, years later, William was able to eloquently teach his countrymen to appreciate the wonders of the natural world. We can catch a glimpse of the kinds of conversations the father and son had from a couple of incidents that were impressed on the boy's mind. He once asked his father what was the oldest thing in the world. The father, in the Socratic method of answering with a question, asked, 'What do you think is the oldest thing?' John didn't give a satisfactory answer, so William picked up a stone from the garden path. 'This stone, son, is the oldest thing I know of.' Another time William asked his son, 'What sort of things, do you think, are most alike?' The slight, blue-eyed boy thought for a moment, and then said, 'Leaves from the same tree are the most like each other.' The father replied, 'Then go and gather up a handful of leaves from that tree and pick the two that are most alike.' Young John wasn't able to find two leaves alike! He hid that lesson in his heart, and it influenced his later research in astronomy. These are trifling incidents, but John, who was naturally reserved, said later that these made a strong impression on his mind. We can trace his grasp and understanding of a few concepts in the incident with the stone, and the fine, subtle discrimination of an individual thing even when surrounded with similar things. These traits characterized John's philosophy.'
John Herschel went to St. John's College, Cambridge, when he was seventeen. He studied science, much to his father's satisfaction. He graduated as a Senior Wrangler [best grades in his mathematics class] in 1813. George Peacock, who later became the Dean of Ely Cathedral and remained life-long close friends with John Herschel, was in second place.
Soon after he got his degree, John Herschel showed evidence of a strong aptitude for scientific investigation. He sent an article about mathematics to the Philosophical Transactions of the Royal Society. His famous name might have helped him to get it published and brought him some attention so that his talents were recognized early in his career. He was made a Fellow [member] of the Royal Society at the unusually young age of twenty-one. But John Herschel still had not decided to devote his life to science. He continued to prepare himself for a career in law. He entered The Honourable Society of the Middle Temple [a barrister/lawyer's association] as a student and began reading with a practicing barrister [as part of his training, probably like an internship].
But it was not John Herschel's destiny to become a lawyer. He was associated with some of the leading scientific men of his era, and soon realized that he was more interested in science than law. He ended up abandoning his original plans and pursued science research exclusively. This is not so surprising, considering how skilled he was at mathematics. By the time he was twenty-nine, he had published so many articles about mathematics, and his research was so valued, that the Royal Society awarded him the Copley Medal, the highest honor they had.
His father, William Herschel, died in 1822, and John inherited his estate, including his father's telescopes and astronomical instruments. Between having these tools at his disposal and his honour for his father, he decided to become an astronomer and complete the great survey of the heavens that his father had contributed so much of his life to.
The first systematic task he worked on was measuring some "Double Stars." Double stars [also called binary stars] appear as one bright star to the unaided eye, but they are actually two stars that appear so close together that the eye sees them as a single point of light. Sometimes they can be seen as two separate stars with a telescope, but some are so close together that it takes an extremely powerful instrument to distinguish them.
Even though two stars are actually separated by vast distances, if they're in our same line of vision, they can seem like one really bright star. This is probably the case for most double stars. In the early days of astronomy when very few double stars were known and telescopes weren't strong enough to show both stars clearly, it was generally assumed that double stars were simply due to perspective with two stars being in the same line of vision. It was never even suspected that there might be a physical connection between the two stars. It was just a coincidental placement that made them look to us as if they were in the same line of vision.
Sir William Herschel [John's father] had accepted this explanation. Since they were in the same line of vision, he thought they might be useful as reference points for helping to determine how far away various stars are from the earth. He realized that as the earth made its annual orbit around the sun, the earth's displacement would make the closer of the two stars seem to shift a little closer to the farther star. If that shift could be measured, then the distance of the closer star could be measured somewhat precisely.
But the effect he was looking for wasn't the effect that actually happened. If the earth's movement made the closer star seem to move, then that shift should have been a yearly event, and then the stars should both appear back in their original positions. William Herschel waited for this to happen so he could measure the shift. And he did observe that type of shift in many of the double stars. But in some pairs, it looked like both the distance and the relative positions of the two stars were shifting. Even more surprising was that this wasn't happening on a yearly basis with the earth's orbit around the sun. In some cases, it looked like one star was revolving around the other one in an orbit that took more than a year! This was a remarkable discovery. This couldn't be caused by the earth's displacement as it orbited the sun. Something else was going on. But what? William Herschel had discovered that some of the double stars, or binary stars, had a relationship where one orbited around the other one in a similar way as the earth orbiting around the sun. So these stars didn't just fool the eye by looking like they were the same distance away from us -- they actually were relatively close to each other. The distance between them was vastly less than their distance from the earth. The heavens contained pairs of twin suns doing a celestial dance!
[YouTube has a video that zooms in on the R Aquarii binary star system with a super strong telescope. For further study, the YouTube channel Astronimate has "Five Minute Solar System" videos, including one on Supernovas that explains some different types of stars and the physics of how stars burn.]
After this discovery, astronomers were eager to focus their research on binary stars. Naturally, young John wanted to focus on binary stars, too, out of admiration for his father's discovery. His mathematical abilities made him an ideal candidate to pursue this kind of research. He realized that once the observational data had been collected, it would take the work of a mathematician to calculate the position of a binary pair and the orbit that the revolving star was taking around the other star. A good mathematician might even be able to figure out the weight of these distant stars and compare them with our own sun.
But the calculations couldn't be done without observational data. The first step was to make observations and take precise measurements of the binary stars that looked more promising for this kind of research. In 1821, John Herschel and his friend, James South, agreed to work together. James South was a medical man with an interest in science, and enough wealth to finance research. He was able to purchase the best astronomical instruments available, and he became an enthusiastic astronomer, and an energetic observer.
James South and John Herschel spent two years observing and measuring the binary stars that Sir William Herschel had first discovered. They compiled such a collection of carefully measured figures that the volume they submitted to the "Philosophical Transactions" was quite thick! Their work is still recognized as unusually accurate for its time. It furthered the progress of sidereal astronomy [astronomy that relates to stars]. The Royal Society and the Royal Astronomical Society awarded them medals for their work, and they were recognized by foreign science institutions.
But all of this was just an introduction to John Herschel's work in astronomy. His father had spent most of his life observing the 'sweeps' of the heavens. He had a twenty-foot long reflecting telescope that he would move slowly up and down through an arc of about two degrees towards and from the north pole, as the sky passed by on its daily motion while the elder Herschel made his observations. Any time a double star would pass within his field of vision, William Herschel would describe it to his sister Caroline, who acted as his assistant during these midnight observations. When there was a nebula, he would estimate its size and brightness, determine whether it had a nucleus or not, and whether there were any unusual stars with it. He would make a note of anything worth recording. Caroline, his faithful secretary, would write down the exact position of each object, using a dial placed in front of her desk that was connected to the telescope as a reference.
John Herschel took on the task of re-observing the binary stars and nebulae that his father had recorded during his nightly sessions. John was at a disadvantage without an obliging sister to assist him. He had to both sweep the skies with his eyes, and do all of the recording himself. This was a drawback and meant that he wasn't able to do as much in the same time. Not only that, but constantly adjusting his eyes from reading measurements on instruments and writing notes by the light of a lamp, to viewing distant stars in a dark sky also slowed him down. Nebulae can only be seen very faintly in the sky. It takes time for the eyes to adjust from a light environment to the dark sky through the lens of a telescope in order to see them. He must have had some method to lessen this inconvenience, but it still limited him.
In spite of that inconvenience he still did great work during his 'sweeps.' He was especially diligent about recording all the double stars he could locate. He had to decide how close two stars had to appear to each other in order to count as double stars. When he had noted all the objects in the sky that seemed promising and possibly of interest for further study, he had thousands of items. In the Royal Society's 'Philosophical Transactions,' there are six or seven memoirs describing his work in this field of astronomy.
One of the achievements he's best known for is a systematic method for determining whether two stars qualify to be called 'Double Stars.' When one star revolves around another in a gravitational orbit, it makes an ellipse. From our perspective, the ellipse will look foreshortened unless we happen to be viewing the orbit from just the right angle, and that almost never happens. So we don't see the path as a perfect ellipse, even though we know it is. John Herschel came up with a brilliant way of telling the size and position of the ellipse even though he couldn't see it properly. By studying the part of the orbit that he could see, and comparing that with measurements he had recorded, he was able to tell where and how big the actual ellipse was. In other words, he devised a way to find the actual orbits of double stars. This method is still used by astronomers who study the movement of binary stars.
[John married Margaret Stewart in 1829. They had a dozen children!]
His work, both in discovering and measuring binary stars, and in finding the orbits of the ones that revolve around other stars, brought John Herschel recognition from the Royal Society. They awarded him another gold medal in November, 1833. Prince Augustus Frederick, Duke of Sussex gave a speech in which he said,
'They say that distance of place grants the same privilege as distance of time. I would gladly give up the privilege granted to me because of Sir John Herschel's separation from his home country and his friends to express how much I admire his character in language stronger than I dare to use. If I were to compliment him with the sincerest words of my heart, it would merely sound like insincere flattery. His achievements in almost every field of human knowledge, his skill at scientific writing, his service and devotion to astronomy. his regulated behaviour that springs from high principles, and, above all, his pleasing modesty that crowns all of his other virtues -- present an accomplished scientist that can be found nowhere else. His talents deserve better words than I can come up with, no matter how much I wish I could commend him as he deserves.'
John Herschel's 'separation from his home country' refers to his time away from England. He was soon to depart on one of the greatest science expeditions in the history of astronomy.
John Herschel was working on a revision of his father's 'sweeps,' looking for any new objects he could find in the skies that were visible. He had just about exhausted what could be seen in the northern hemisphere and completed that project. He had reviewed one zone after another from Windsor. He had added hundreds of nebulae to his list that his father had discovered. He had measured thousands of double stars. But now his great survey was done. Everything that could be seen through his twenty foot telescope had been measured and catalogued.
But John Herschel felt that this was only half of what should be done. What about the southern hemisphere? It had never been observed and catalogued. There was a part of the southern hemisphere that was visible from England, but what little had been seen only made passionate astronomers eager to see the rest of it. The glorious Great Nebula in Orion lay in the southern hemisphere, near the equator. It can be seen from both hemispheres. There are noteworthy objects that can only be seen from the southern hemisphere, and they're closer to the earth than the stars that are visible from the northern hemisphere. The closest star we can measure is Alpha Centauri, and it's visible from the southern hemisphere, as well as the bright star cluster, Omega Centauri, the Southern Cross constellation, the Magellanic Clouds, Carina Nebula, and the great rift of the Milky Way. John Herschel wanted to see these, so he decided to pack up his telescope and take a trip to the Cape of Good Hope [southern Africa]. The skies are pure there, offering the perfect views he would need on clear nights to study the delicate nebulae.
In November 1833, Sir John Herschel -- who had been knighted by King William IV. in 1831, sailed from Portsmouth [southern England] towards the Cape of Good Hope with his astronomical instruments. The voyage took two months. He landed in Table Bay [overlooking Cape Town]. After looking around at various places, he decided to set up his observatory at Feldhausen, six miles away from Cape Town, near the base of Table Mountain. There was a roomy old estate there to live in with his family [four of his children had been born by this time, and two more were born in South Africa]. A temporary building was built to contain the equatorial telescope [which rotates in the opposite direction as the earth to stay at the same latitude], but his long twenty-foot telescope had to remain outside under the open sky.
His focus continued to be measuring the relative distances of double stars and closely examining nebulae. He made many beautiful pencil drawings of nebulae in the southern skies while he was there.
During the four delightful years he worked in South Africa, he studied 1,707 nebulae and star clusters! His notes about what they looked like, their positions, measurements of double stars, and other valuable astronomical research were published in a magnificent book funded by by his travelling companion, Algernon Percy, the Duke of Northumberland ["Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope;" see a photo of a signed copy here]. This is a monumental work that will be interesting reading for anyone who has an interest in astronomy.
[Herschel didn't just study stars while he was in Africa; he also studied botany and geography on this trip, and was visited by Charles Darwin in 1836. Wikipedia notes, "Later on, Darwin would be influenced by Herschel's writings in developing his theory advanced in The Origin of Species. In the opening lines of that work, Darwin writes that his intent is 'to throw some light on the origin of species - that mystery of mysteries, as it has been called by one of our greatest philosophers,' referring to Herschel. However, Herschel ultimately rejected the theory of natural selection." Herschel also discovered and named the satellites/moons of Saturn and Uranus on this trip.]
John Herschel was lucky enough to be at the Cape of Good Hope when Halley's comet was scheduled to return in 1833. He put a lot of time into studying this comet, and the records of his observations make up one of the most interesting chapters of the book just referred to.
Sir John Herschel went home to England in 1838. He had made many friends in South Africa, and they had seen how hard he worked while he was with them. They wanted to remember his time there with some kind of marker that would bring honour to their colony. So some of the scientists in the area raised a monument -- an obelisk on the same spot where his telescope had stood in Feldhausen with an engraving of one of the nebulae he had observed and some inscriptions. [A Latin inscription stated that Herschel, the son of a famous astronomer, observed nebulae and stars on that site, and an English inscription said, "Here stood from 1834 to 1838 the reflecting telescope of Sir John Herschel, Baronet: who during a residence of four years in this colony contributed as largely by his benevolent exertions to the cause of education and humanity as by his eminent talents to the discovery of scientific truth."]
After a five year absence, his return to England caused much rejoicing among astronomy lovers. There was a banquet in his honour, and Queen Victoria made him a Baronet at her coronation [which had been in 1837, while he was still in South Africa]. Caroline, his famous aunt, was now eighty years old and still mentally alert and fully able to appreciate the honour he added to the Herschel name. Her appreciation was mixed with other feelings, too -- she loved her nephew, but she had been completely devoted to her brother, John's father, and she was jealous for William's honour and fame. She couldn't bear to hear of the praise and achievements of other astronomers, even when the other astronomer was her own nephew!
Once John Herschel completed his survey of the southern hemisphere, his career observing the skies ended. He no longer did any systematic research with a telescope. But that doesn't mean he stopped studying astronomy. John Herschel is probably the only astronomer who researched and advanced multiple fields of astronomy and made a name for himself in each of them. The rest of his career was spent in areas of astronomy that didn't involve looking through the lens of a telescope.
General students are most familiar with John Herschel because of his book 'Outlines of Astronomy' [online at archive.org]. In this book, he explains the difficulties of his subject with as much simplicity as is possible. The language is picturesque, and his words glorify the concepts of the universe as they unfold. Anyone who wants to learn about the mysterious fields of astronomy in which one planet's motions affect another planet should read Herschel's book. He makes this complicated subject clear without resorting to difficult mathematics. As modern astronomy has advanced, this book has been republished again and again. Although some parts of the book are outdated in certain fields, his explanations of the basic parts of astronomy are still unrivaled.
After he got back from South Africa, John advanced a project he had worked on with his father. The two of them had studied nebulae and star clusters together for many years. All of this research was scattered throughout various articles, and finding all of them was very inconvenient for anyone who wanted to use their research as a resource for their own study. John Herschel decided to compile one single catalogue of all the nebulae and star clusters they had observed. Who was more qualified for for this task than Sir John Herschel? It was quite an inventory. Each individual nebula is so vast that our entire solar system would look like a speck in comparison, and his catalogue listed thousands of them. All of the nebulae and star clusters that the two Herschels, father and son, researched in the northern hemisphere, and the ones that John Herschel researched in the southern hemisphere, are all there, arranged in systematic order. The catalogue [General Catalogue of Nebulae and Clusters of Stars, 1786] also includes the work of other astronomers. Due to space, descriptions of each object are brief. There are abbreviations to indicate whether a specific object was bright, very bright, extremely bright, faint, very faint, or extremely faint. Of course, those are relative distinctions -- even an extremely bright nebula only appears bright in comparison to other delicate nebulae, which are all rather faint, even through a telescope. In fact, most nebulae are so difficult to see that there's not much detail to describe. John Herschel's catalogue included ten times the number of nebulous objects as had been compiled before William Herschel began to observe them. And the study of nebulae still continues. Today's telescopes show twice as many as Herschel was able to see. A new edition has been published by John Louis Emil Dreyer [New General Catalogue of Nebulae and Clusters of Stars, or NGC, 1888].
One of the best examples of John Herschel's literary abilities is in the speech he gave at the Royal Astronomical Society when he presented a medal to Francis Baily to recognize his "Catalogue of 2,881 Stars." This part of the speech shows a true appreciation for the tedious work involved in Baily's task of cataloging stars:
'Why do governments maintain magnificent scientific organizations, decorate them elegantly, put them under the leadership of talented and passionate scientists? Who benefits from these establishments? Why did James Bradley work so hard? Why did Nevil Maskelyne or Giuseppi Piazzi wear themselves out researching? It wasn't just to settle some specific point about the universe. It wasn't to satisfy their intellectual vanity by asking discriminating questions about the distant mysteries of nature. It wasn't to trace our path through space or to trace our history through past and future eternities. I don't mean to criticize any of these motives, as they are all valid. Our human mind is dazzled when we contemplate such vast mysteries, and when we study them, our minds expand and grow strong enough to pursue the boldest project. Making a practical study of the stars is worthwhile simply because the heavens are so spectacular and glorious. Stars are the landmarks of the universe. Our solar system has all kinds of complicated fluctuations that aren't there simply to awe our minds, but were placed there by God to guide our thoughts towards the enduring objects in His universe. It is impossible to go overboard in our appreciation of God's ability to create changeless stars in nature. As soon as a star is recorded and has its position registered, it becomes a fixed point for the astronomer, the geographer, the navigator, the surveyor -- a marker that will never deceive him or lead him astray. It stays the same forever, never moving from its place. It stays the same for every instrument, and can be tested by any equipment that can ever be invented. That dependability can be adapted for practical use -- to synchronize the town clock, or navigate a ship to the Indies, or map the boundaries of a barony for some empire. Once a star's position has been verified and precisely recorded, it doesn't matter if the astronomer's tools rust away, or if the marble pillar he rested his telescope on topples off its base, or the astronomer himself is long forgotten -- the star's position is marked down and recorded forever and can be the sure foundation for all kinds of other work. All the labour, expense, and tedious precision it cost to identify that star's exact position will have been worth it.'
Sir John Herschel wrote other books. His "Treatise on Meteorology" is considered a fundamental resource for tracking the weather. He also wrote articles on various subjects [including photographic processes with cyanotype, platinum salts, and fixatives; he coined the terms emulsion, positive, and negative] as a way to relax from his more rigid science research. Like other mathematicians, he also wrote poetry and published a translation of The Iliad [in 1862; page images online at Hathi Trust]. He was the first to use the Julian calendar as a dating system for astronomy.
In his later years, Sir John Herschel lived a retired life. He was offered the position of Master of the Mint [making coins, as Isaac Newton had done] in 1850 and tried that for a few years, but the tedious routine and stress of the job didn't suit him, especially as his health was failing [he had a nervous breakdown!], so he resigned and went back to his research in his beautiful home at Collingwood, in Kent.
Towards the end, John Herschel suffered from gout and bronchitis and died in May, 1871 at the age of 79. He was buried in Westminster Abbey near Isaac Newton and Charles Lyell. A decade later, Charles Darwin was buried next to him. [His wife, Margaret, died thirteen years later, in 1884.]
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.
Sir John Frederick William Herschel, 1792-1871
Studied binary stars, nebulae, and explored the skies in the southern hemisphere.
This famous son of a famous father was born at Slough, a large town in Berkshire [England] near Windsor on March 7, 1792. Sir William Herschel, who we read about in an earlier chapter, had married late in life [at age fifty], and John was his only child.
The environment young John was raised in was excellent training for his future career as an astronomer, which brought him almost as much fame as his father's. He had one great advantage that his father did not have: even from a very young age, he was allowed to pursue his own intellectual interests as much as he wanted. William Herschel had had to snatch what time he could during his busy life as a musician. But John enjoyed learning, and he had the time and the financial means to study. His early years have been described by Professor Charles Pritchard in his 1872 "Report of the Council of the Royal Astronomical Society:"
'A few characteristics of John Herschel's childhood that he himself mentioned later in his life were remembered fondly by those who were dear to him. His home was unique, and a perfect environment to nurture the gifts of a talented child like John Herschel. The head of the household was his old, observant, quiet father. His aunt Caroline, whose work and fame are still recognized as a brilliant counterpart to her famous brother, was always nearby. Young John passed his boyhood in the company of these two remarkable persons and under the shadow of his father's wonderful telescope. His father and aunt were always quietly at work, busy at research which didn't seem to be important outside the walls of the house, although, years later, William was able to eloquently teach his countrymen to appreciate the wonders of the natural world. We can catch a glimpse of the kinds of conversations the father and son had from a couple of incidents that were impressed on the boy's mind. He once asked his father what was the oldest thing in the world. The father, in the Socratic method of answering with a question, asked, 'What do you think is the oldest thing?' John didn't give a satisfactory answer, so William picked up a stone from the garden path. 'This stone, son, is the oldest thing I know of.' Another time William asked his son, 'What sort of things, do you think, are most alike?' The slight, blue-eyed boy thought for a moment, and then said, 'Leaves from the same tree are the most like each other.' The father replied, 'Then go and gather up a handful of leaves from that tree and pick the two that are most alike.' Young John wasn't able to find two leaves alike! He hid that lesson in his heart, and it influenced his later research in astronomy. These are trifling incidents, but John, who was naturally reserved, said later that these made a strong impression on his mind. We can trace his grasp and understanding of a few concepts in the incident with the stone, and the fine, subtle discrimination of an individual thing even when surrounded with similar things. These traits characterized John's philosophy.'
John Herschel went to St. John's College, Cambridge, when he was seventeen. He studied science, much to his father's satisfaction. He graduated as a Senior Wrangler [best grades in his mathematics class] in 1813. George Peacock, who later became the Dean of Ely Cathedral and remained life-long close friends with John Herschel, was in second place.
Soon after he got his degree, John Herschel showed evidence of a strong aptitude for scientific investigation. He sent an article about mathematics to the Philosophical Transactions of the Royal Society. His famous name might have helped him to get it published and brought him some attention so that his talents were recognized early in his career. He was made a Fellow [member] of the Royal Society at the unusually young age of twenty-one. But John Herschel still had not decided to devote his life to science. He continued to prepare himself for a career in law. He entered The Honourable Society of the Middle Temple [a barrister/lawyer's association] as a student and began reading with a practicing barrister [as part of his training, probably like an internship].
But it was not John Herschel's destiny to become a lawyer. He was associated with some of the leading scientific men of his era, and soon realized that he was more interested in science than law. He ended up abandoning his original plans and pursued science research exclusively. This is not so surprising, considering how skilled he was at mathematics. By the time he was twenty-nine, he had published so many articles about mathematics, and his research was so valued, that the Royal Society awarded him the Copley Medal, the highest honor they had.
His father, William Herschel, died in 1822, and John inherited his estate, including his father's telescopes and astronomical instruments. Between having these tools at his disposal and his honour for his father, he decided to become an astronomer and complete the great survey of the heavens that his father had contributed so much of his life to.
The first systematic task he worked on was measuring some "Double Stars." Double stars [also called binary stars] appear as one bright star to the unaided eye, but they are actually two stars that appear so close together that the eye sees them as a single point of light. Sometimes they can be seen as two separate stars with a telescope, but some are so close together that it takes an extremely powerful instrument to distinguish them.
Even though two stars are actually separated by vast distances, if they're in our same line of vision, they can seem like one really bright star. This is probably the case for most double stars. In the early days of astronomy when very few double stars were known and telescopes weren't strong enough to show both stars clearly, it was generally assumed that double stars were simply due to perspective with two stars being in the same line of vision. It was never even suspected that there might be a physical connection between the two stars. It was just a coincidental placement that made them look to us as if they were in the same line of vision.
Sir William Herschel [John's father] had accepted this explanation. Since they were in the same line of vision, he thought they might be useful as reference points for helping to determine how far away various stars are from the earth. He realized that as the earth made its annual orbit around the sun, the earth's displacement would make the closer of the two stars seem to shift a little closer to the farther star. If that shift could be measured, then the distance of the closer star could be measured somewhat precisely.
But the effect he was looking for wasn't the effect that actually happened. If the earth's movement made the closer star seem to move, then that shift should have been a yearly event, and then the stars should both appear back in their original positions. William Herschel waited for this to happen so he could measure the shift. And he did observe that type of shift in many of the double stars. But in some pairs, it looked like both the distance and the relative positions of the two stars were shifting. Even more surprising was that this wasn't happening on a yearly basis with the earth's orbit around the sun. In some cases, it looked like one star was revolving around the other one in an orbit that took more than a year! This was a remarkable discovery. This couldn't be caused by the earth's displacement as it orbited the sun. Something else was going on. But what? William Herschel had discovered that some of the double stars, or binary stars, had a relationship where one orbited around the other one in a similar way as the earth orbiting around the sun. So these stars didn't just fool the eye by looking like they were the same distance away from us -- they actually were relatively close to each other. The distance between them was vastly less than their distance from the earth. The heavens contained pairs of twin suns doing a celestial dance!
[YouTube has a video that zooms in on the R Aquarii binary star system with a super strong telescope. For further study, the YouTube channel Astronimate has "Five Minute Solar System" videos, including one on Supernovas that explains some different types of stars and the physics of how stars burn.]
After this discovery, astronomers were eager to focus their research on binary stars. Naturally, young John wanted to focus on binary stars, too, out of admiration for his father's discovery. His mathematical abilities made him an ideal candidate to pursue this kind of research. He realized that once the observational data had been collected, it would take the work of a mathematician to calculate the position of a binary pair and the orbit that the revolving star was taking around the other star. A good mathematician might even be able to figure out the weight of these distant stars and compare them with our own sun.
But the calculations couldn't be done without observational data. The first step was to make observations and take precise measurements of the binary stars that looked more promising for this kind of research. In 1821, John Herschel and his friend, James South, agreed to work together. James South was a medical man with an interest in science, and enough wealth to finance research. He was able to purchase the best astronomical instruments available, and he became an enthusiastic astronomer, and an energetic observer.
James South and John Herschel spent two years observing and measuring the binary stars that Sir William Herschel had first discovered. They compiled such a collection of carefully measured figures that the volume they submitted to the "Philosophical Transactions" was quite thick! Their work is still recognized as unusually accurate for its time. It furthered the progress of sidereal astronomy [astronomy that relates to stars]. The Royal Society and the Royal Astronomical Society awarded them medals for their work, and they were recognized by foreign science institutions.
But all of this was just an introduction to John Herschel's work in astronomy. His father had spent most of his life observing the 'sweeps' of the heavens. He had a twenty-foot long reflecting telescope that he would move slowly up and down through an arc of about two degrees towards and from the north pole, as the sky passed by on its daily motion while the elder Herschel made his observations. Any time a double star would pass within his field of vision, William Herschel would describe it to his sister Caroline, who acted as his assistant during these midnight observations. When there was a nebula, he would estimate its size and brightness, determine whether it had a nucleus or not, and whether there were any unusual stars with it. He would make a note of anything worth recording. Caroline, his faithful secretary, would write down the exact position of each object, using a dial placed in front of her desk that was connected to the telescope as a reference.
John Herschel took on the task of re-observing the binary stars and nebulae that his father had recorded during his nightly sessions. John was at a disadvantage without an obliging sister to assist him. He had to both sweep the skies with his eyes, and do all of the recording himself. This was a drawback and meant that he wasn't able to do as much in the same time. Not only that, but constantly adjusting his eyes from reading measurements on instruments and writing notes by the light of a lamp, to viewing distant stars in a dark sky also slowed him down. Nebulae can only be seen very faintly in the sky. It takes time for the eyes to adjust from a light environment to the dark sky through the lens of a telescope in order to see them. He must have had some method to lessen this inconvenience, but it still limited him.
In spite of that inconvenience he still did great work during his 'sweeps.' He was especially diligent about recording all the double stars he could locate. He had to decide how close two stars had to appear to each other in order to count as double stars. When he had noted all the objects in the sky that seemed promising and possibly of interest for further study, he had thousands of items. In the Royal Society's 'Philosophical Transactions,' there are six or seven memoirs describing his work in this field of astronomy.
One of the achievements he's best known for is a systematic method for determining whether two stars qualify to be called 'Double Stars.' When one star revolves around another in a gravitational orbit, it makes an ellipse. From our perspective, the ellipse will look foreshortened unless we happen to be viewing the orbit from just the right angle, and that almost never happens. So we don't see the path as a perfect ellipse, even though we know it is. John Herschel came up with a brilliant way of telling the size and position of the ellipse even though he couldn't see it properly. By studying the part of the orbit that he could see, and comparing that with measurements he had recorded, he was able to tell where and how big the actual ellipse was. In other words, he devised a way to find the actual orbits of double stars. This method is still used by astronomers who study the movement of binary stars.
[John married Margaret Stewart in 1829. They had a dozen children!]
His work, both in discovering and measuring binary stars, and in finding the orbits of the ones that revolve around other stars, brought John Herschel recognition from the Royal Society. They awarded him another gold medal in November, 1833. Prince Augustus Frederick, Duke of Sussex gave a speech in which he said,
'They say that distance of place grants the same privilege as distance of time. I would gladly give up the privilege granted to me because of Sir John Herschel's separation from his home country and his friends to express how much I admire his character in language stronger than I dare to use. If I were to compliment him with the sincerest words of my heart, it would merely sound like insincere flattery. His achievements in almost every field of human knowledge, his skill at scientific writing, his service and devotion to astronomy. his regulated behaviour that springs from high principles, and, above all, his pleasing modesty that crowns all of his other virtues -- present an accomplished scientist that can be found nowhere else. His talents deserve better words than I can come up with, no matter how much I wish I could commend him as he deserves.'
John Herschel's 'separation from his home country' refers to his time away from England. He was soon to depart on one of the greatest science expeditions in the history of astronomy.
John Herschel was working on a revision of his father's 'sweeps,' looking for any new objects he could find in the skies that were visible. He had just about exhausted what could be seen in the northern hemisphere and completed that project. He had reviewed one zone after another from Windsor. He had added hundreds of nebulae to his list that his father had discovered. He had measured thousands of double stars. But now his great survey was done. Everything that could be seen through his twenty foot telescope had been measured and catalogued.
But John Herschel felt that this was only half of what should be done. What about the southern hemisphere? It had never been observed and catalogued. There was a part of the southern hemisphere that was visible from England, but what little had been seen only made passionate astronomers eager to see the rest of it. The glorious Great Nebula in Orion lay in the southern hemisphere, near the equator. It can be seen from both hemispheres. There are noteworthy objects that can only be seen from the southern hemisphere, and they're closer to the earth than the stars that are visible from the northern hemisphere. The closest star we can measure is Alpha Centauri, and it's visible from the southern hemisphere, as well as the bright star cluster, Omega Centauri, the Southern Cross constellation, the Magellanic Clouds, Carina Nebula, and the great rift of the Milky Way. John Herschel wanted to see these, so he decided to pack up his telescope and take a trip to the Cape of Good Hope [southern Africa]. The skies are pure there, offering the perfect views he would need on clear nights to study the delicate nebulae.
In November 1833, Sir John Herschel -- who had been knighted by King William IV. in 1831, sailed from Portsmouth [southern England] towards the Cape of Good Hope with his astronomical instruments. The voyage took two months. He landed in Table Bay [overlooking Cape Town]. After looking around at various places, he decided to set up his observatory at Feldhausen, six miles away from Cape Town, near the base of Table Mountain. There was a roomy old estate there to live in with his family [four of his children had been born by this time, and two more were born in South Africa]. A temporary building was built to contain the equatorial telescope [which rotates in the opposite direction as the earth to stay at the same latitude], but his long twenty-foot telescope had to remain outside under the open sky.
His focus continued to be measuring the relative distances of double stars and closely examining nebulae. He made many beautiful pencil drawings of nebulae in the southern skies while he was there.
During the four delightful years he worked in South Africa, he studied 1,707 nebulae and star clusters! His notes about what they looked like, their positions, measurements of double stars, and other valuable astronomical research were published in a magnificent book funded by by his travelling companion, Algernon Percy, the Duke of Northumberland ["Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope;" see a photo of a signed copy here]. This is a monumental work that will be interesting reading for anyone who has an interest in astronomy.
[Herschel didn't just study stars while he was in Africa; he also studied botany and geography on this trip, and was visited by Charles Darwin in 1836. Wikipedia notes, "Later on, Darwin would be influenced by Herschel's writings in developing his theory advanced in The Origin of Species. In the opening lines of that work, Darwin writes that his intent is 'to throw some light on the origin of species - that mystery of mysteries, as it has been called by one of our greatest philosophers,' referring to Herschel. However, Herschel ultimately rejected the theory of natural selection." Herschel also discovered and named the satellites/moons of Saturn and Uranus on this trip.]
John Herschel was lucky enough to be at the Cape of Good Hope when Halley's comet was scheduled to return in 1833. He put a lot of time into studying this comet, and the records of his observations make up one of the most interesting chapters of the book just referred to.
Sir John Herschel went home to England in 1838. He had made many friends in South Africa, and they had seen how hard he worked while he was with them. They wanted to remember his time there with some kind of marker that would bring honour to their colony. So some of the scientists in the area raised a monument -- an obelisk on the same spot where his telescope had stood in Feldhausen with an engraving of one of the nebulae he had observed and some inscriptions. [A Latin inscription stated that Herschel, the son of a famous astronomer, observed nebulae and stars on that site, and an English inscription said, "Here stood from 1834 to 1838 the reflecting telescope of Sir John Herschel, Baronet: who during a residence of four years in this colony contributed as largely by his benevolent exertions to the cause of education and humanity as by his eminent talents to the discovery of scientific truth."]
After a five year absence, his return to England caused much rejoicing among astronomy lovers. There was a banquet in his honour, and Queen Victoria made him a Baronet at her coronation [which had been in 1837, while he was still in South Africa]. Caroline, his famous aunt, was now eighty years old and still mentally alert and fully able to appreciate the honour he added to the Herschel name. Her appreciation was mixed with other feelings, too -- she loved her nephew, but she had been completely devoted to her brother, John's father, and she was jealous for William's honour and fame. She couldn't bear to hear of the praise and achievements of other astronomers, even when the other astronomer was her own nephew!
Once John Herschel completed his survey of the southern hemisphere, his career observing the skies ended. He no longer did any systematic research with a telescope. But that doesn't mean he stopped studying astronomy. John Herschel is probably the only astronomer who researched and advanced multiple fields of astronomy and made a name for himself in each of them. The rest of his career was spent in areas of astronomy that didn't involve looking through the lens of a telescope.
General students are most familiar with John Herschel because of his book 'Outlines of Astronomy' [online at archive.org]. In this book, he explains the difficulties of his subject with as much simplicity as is possible. The language is picturesque, and his words glorify the concepts of the universe as they unfold. Anyone who wants to learn about the mysterious fields of astronomy in which one planet's motions affect another planet should read Herschel's book. He makes this complicated subject clear without resorting to difficult mathematics. As modern astronomy has advanced, this book has been republished again and again. Although some parts of the book are outdated in certain fields, his explanations of the basic parts of astronomy are still unrivaled.
After he got back from South Africa, John advanced a project he had worked on with his father. The two of them had studied nebulae and star clusters together for many years. All of this research was scattered throughout various articles, and finding all of them was very inconvenient for anyone who wanted to use their research as a resource for their own study. John Herschel decided to compile one single catalogue of all the nebulae and star clusters they had observed. Who was more qualified for for this task than Sir John Herschel? It was quite an inventory. Each individual nebula is so vast that our entire solar system would look like a speck in comparison, and his catalogue listed thousands of them. All of the nebulae and star clusters that the two Herschels, father and son, researched in the northern hemisphere, and the ones that John Herschel researched in the southern hemisphere, are all there, arranged in systematic order. The catalogue [General Catalogue of Nebulae and Clusters of Stars, 1786] also includes the work of other astronomers. Due to space, descriptions of each object are brief. There are abbreviations to indicate whether a specific object was bright, very bright, extremely bright, faint, very faint, or extremely faint. Of course, those are relative distinctions -- even an extremely bright nebula only appears bright in comparison to other delicate nebulae, which are all rather faint, even through a telescope. In fact, most nebulae are so difficult to see that there's not much detail to describe. John Herschel's catalogue included ten times the number of nebulous objects as had been compiled before William Herschel began to observe them. And the study of nebulae still continues. Today's telescopes show twice as many as Herschel was able to see. A new edition has been published by John Louis Emil Dreyer [New General Catalogue of Nebulae and Clusters of Stars, or NGC, 1888].
One of the best examples of John Herschel's literary abilities is in the speech he gave at the Royal Astronomical Society when he presented a medal to Francis Baily to recognize his "Catalogue of 2,881 Stars." This part of the speech shows a true appreciation for the tedious work involved in Baily's task of cataloging stars:
'Why do governments maintain magnificent scientific organizations, decorate them elegantly, put them under the leadership of talented and passionate scientists? Who benefits from these establishments? Why did James Bradley work so hard? Why did Nevil Maskelyne or Giuseppi Piazzi wear themselves out researching? It wasn't just to settle some specific point about the universe. It wasn't to satisfy their intellectual vanity by asking discriminating questions about the distant mysteries of nature. It wasn't to trace our path through space or to trace our history through past and future eternities. I don't mean to criticize any of these motives, as they are all valid. Our human mind is dazzled when we contemplate such vast mysteries, and when we study them, our minds expand and grow strong enough to pursue the boldest project. Making a practical study of the stars is worthwhile simply because the heavens are so spectacular and glorious. Stars are the landmarks of the universe. Our solar system has all kinds of complicated fluctuations that aren't there simply to awe our minds, but were placed there by God to guide our thoughts towards the enduring objects in His universe. It is impossible to go overboard in our appreciation of God's ability to create changeless stars in nature. As soon as a star is recorded and has its position registered, it becomes a fixed point for the astronomer, the geographer, the navigator, the surveyor -- a marker that will never deceive him or lead him astray. It stays the same forever, never moving from its place. It stays the same for every instrument, and can be tested by any equipment that can ever be invented. That dependability can be adapted for practical use -- to synchronize the town clock, or navigate a ship to the Indies, or map the boundaries of a barony for some empire. Once a star's position has been verified and precisely recorded, it doesn't matter if the astronomer's tools rust away, or if the marble pillar he rested his telescope on topples off its base, or the astronomer himself is long forgotten -- the star's position is marked down and recorded forever and can be the sure foundation for all kinds of other work. All the labour, expense, and tedious precision it cost to identify that star's exact position will have been worth it.'
Sir John Herschel wrote other books. His "Treatise on Meteorology" is considered a fundamental resource for tracking the weather. He also wrote articles on various subjects [including photographic processes with cyanotype, platinum salts, and fixatives; he coined the terms emulsion, positive, and negative] as a way to relax from his more rigid science research. Like other mathematicians, he also wrote poetry and published a translation of The Iliad [in 1862; page images online at Hathi Trust]. He was the first to use the Julian calendar as a dating system for astronomy.
In his later years, Sir John Herschel lived a retired life. He was offered the position of Master of the Mint [making coins, as Isaac Newton had done] in 1850 and tried that for a few years, but the tedious routine and stress of the job didn't suit him, especially as his health was failing [he had a nervous breakdown!], so he resigned and went back to his research in his beautiful home at Collingwood, in Kent.
Towards the end, John Herschel suffered from gout and bronchitis and died in May, 1871 at the age of 79. He was buried in Westminster Abbey near Isaac Newton and Charles Lyell. A decade later, Charles Darwin was buried next to him. [His wife, Margaret, died thirteen years later, in 1884.]
Comments
Post a Comment