Great Astronomers: Bradley
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.
James Bradley, 1693-1762
"Aberration of light" means that the light we see may not reflect the current positions of stars.
James Bradley was born in Gloucestershire to a family with ancient roots in Durham County [England] in March 1693. He went to grammar school in Northleach, and later went on to college at Oxford in March, 1711. When he was an undergraduate, he spent a lot of time with his maternal uncle, Rev. James Pound, who was well-known for his interest in science and an enthusiastic star gazer. It was most likely through spending time with his uncle that he became skilled at handling astronomical instruments, but the discoveries he made were due to his own brilliance in science.
The first evidence of Bradley's skill was seen in two observations he made in 1717 and 1718. They were published by Halley -- he recognized James Bradley's extraordinary scientific mind. Halley once said, 'In the last opposition of the sun and Mars [astronomical opposition means two celestial bodies are on opposite sides of the sky], Dr. Pound and his nephew James Bradley showed me the extreme minuteness of the sun's parallax [items in a line of sight appear to shift from different positions; see it demonstrated on YouTube], and that it was somewhere between 9 and 12 seconds.' Here's what this means: determining the sun's parallax is like determining the distance from the earth to the sun. In those days, the distance from the earth to the sun [called an astronomical unit] wasn't accurately known, but Dr. Pound and James Bradley's comments meant that, from their observations, they determined that the sun was somewhere between 94 million miles to 125 million miles away. They were very close -- the sun's distance from the earth is actually 93-94 million miles away [it varies because our orbit is elliptical]. It's stunning to think that this veteran star gazer and his brilliant nephew were able to determine the distance -- which wouldn't be accurately known for another fifty years!
Some of James Bradley's earlier observational projects were the eclipses of Jupiter's satellites. They're appealing because they're so easy to see, and Bradley enjoyed calculating the times when these eclipses would take place, and then compare his calculations with the predicted times. He was so impressive at this, and with some other projects, that he gained a reputation that got him elected as a Fellow of the Royal Society in 1718 [he was in his mid-twenties].
Until this time, Bradley had merely been an amateur astronomical enthusiast. Astronomy wasn't something one could rely on for income, so he had been considering a 'real' profession. He was thinking he would enter the Church, although he didn't take orders when he came of age. In 1719, he was offered the Vicarage of Bridstow, which is in Monmouthshire. So he became a priest and took the position in 1720. The following year, he had some additional income from the proceeds of a Welsh living. Since he was a sinecure [sinecure is a position that pays well without requiring much work], he was able to maintain both positions, and he still had plenty of time to visit his uncle in Wandsworth. His uncle was also a clergyman, and James Bradley was able to help him with his duties.
Soon James Bradley had a decision to make: to continue working for the church, or make a career in science. When Dr. John Keill died in 1721, his position as Savilian Professor of Astronomy at Oxford became vacant. The position was offered to Bradley's uncle, James Pound, but he wasn't willing to give up his clerical position. There were a couple of other candidates, but Bradley's talents were so obvious that it was offered to him. He resigned from his clerical job and took on the professorship.
James Bradley had many influential friends and could have had a very successful church career. Bishop Benjamin Hoadly had already made him his personal chaplain. But Bradley was obsessed with astronomy and willing to sacrifice his professional prospects to take on the Savilian Professorship. It wasn't that he was uninterested in spiritual matters, but he felt that his gifts would be of more use in science. So he accepted the post and read his very first lecture there in April, 1722.
In those early days of astronomy, constructing a good telescope wasn't clearly understood. To make a refracting telescope effective, it was built excessively long. James Bradley made some of his observations with a telescope that was 212 feet long! There were no tubes long enough to make such a long telescope, so the object glass [the lens closest to the star being viewed] would simply be attached to a tall pole. In spite of the inconvenience of these 'aerial telescopes' [read more about them on Wikipedia], Bradley was able to make some very precise measurements. For example, he observed the transit of Mercury across the sun in October, 1723, and the dimensions of the planet Venus. Through October and November, 1723, he made a series of observations from Wanstead of a comet that Halley had first discovered. His first contribution to the Royal Society's Philosophical Transactions [the original science journal!] was about this comet. He spent an extraordinary amount of time making calculations about this comet, which are included in his book of Calculations.
But all of that was just a prelude to the two great discoveries that made him famous. As often happens in science, he made his first great discovery while he was researching something else. Scientists already knew that as the earth makes its orbit around the sun every year, some of the stars seem to move in relation to the change in the earth's position. The closer the star is to earth, the more it seems to shift, which seems to indicate that the star is being seen from different positions as the earth orbits. Using these shifts as the earth moves had been suggested as a way to determine the distance of the stars. But the distance to a star is so much greater than the earth's orbit that this method hadn't been very useful. In 1728, James Bradley decided to look into this. He thought that by using more powerful instruments and making more precise measurements, he might be able to distinguish displacement shifts that had been too slight to perceive before, and make the method more effective at measuring star distances. He decided to start by focusing on a single star, Beta Draconis, since it was high enough [almost directly overhead] to avoid refraction.
He used a telescope that had been set up along the chimney at the home of Samuel Molyneaux at the western end of Kew Green [near London, about 45 minutes from the Royal Observatory in Greenwich]. The eye piece was 3.5 feet above the ground floor and it had a focal length of over 24 feet. Mr. Molyneaux had set it up in 1725 to make his own observations. If there was any shifting of Beta Draconis due to the earth's orbit, the star should move less when it moved with the sun, and more when it moved in the opposite direction. The star passed the meridian [probably referring to the imaginary line from the north pole to the south pole that goes through Greenwich, right across the the courtyard of the Royal Observatory] at noon in December. Samuel Molyneaux made a note of its position on December 3. Any perceptible shift caused by parallax [parallax is the star's shift caused by the earth's movement] should have made the star seem to shift towards the north. But when James Bradley observed the star on December 17, it appeared to have shifted a little to the south! He checked and re-checked to be sure, and even scrutinized the settings of his instruments, but the star was indeed shifting south rather than north, and continued to do so into the month of March. By then it had moved twenty seconds south of where it had been on December 3. What was going on? By the middle of April, it had started to move north again. It continued moving north, and had moved 39 seconds north by September. Then it began moving south again, and by December, it was back in its original position, directly overhead.
The star's movement in the exact opposite direction of what parallax should have caused must mean that something else, something bigger, was happening to obscure the effect of parallax. But what? James Bradley was determined to find out. He decided to watch other stars to see if they exhibited the same phenomenon. He erected another telescope at Wanstead and kept an eye on several stars that passed at different distances from the zenith. In doing this, he discovered that other stars did the same thing! But why? It was a mystery that kept him puzzled until the explanation dawned on him.
One day he was out sailing and he noticed that every time the boat turned to make a different tack, the weather vane at the top of the boat's mast shifted a little as if the wind was changing direction. After this happened a few times, he commented to the sailors that it seemed like a strange coincidence for the wind to change direction every time the boat turned. The sailors said it wasn't the wind that was turning, but the weather vane moved a little with the boat's change in direction. In fact, when using the vane to determine wind direction, you had to take the boat's course in account because the vane moved in response to both the wind and the direction the boat was going in. This meant that it was possible for a person in the boat to see the vane pointing in a different direction from which the wind was blowing, depending on whether the boat was moving or stationary, and depending on which direction the boat was sailing. Bradley realized that what was happening with the boat must be somehow related to what had puzzled him about the movement of the stars.
It had been well known for a while that light doesn't travel through space instantaneously. It takes time. Galileo guessed that the sun had probably descended to the horizon before we saw it get there, and it was obvious that physical things -- like light -- require some time to get from point A to point B. But light is so fast that there were no tools to measure its speed back then. The Danish astronomer Ole Roemer had detected irregularities in the observed times of eclipses of Jupiter's satellites. These irregularities were due to the interval of time it took for light to stretch across the space between planets. James Bradley suggested that, since light travels at a specific speed, it might be like the wind he had noticed in the boat. If the person observing it were on an object -- such as on the earth -- then the direction the light appears to be coming from might be different than where it's actually coming from as the earth moves. The earth travels slower than light -- only eighteen miles per second, while light travels around 185,000 miles per second [or 186,000 miles per second]. In other words, light is ten thousand times faster than the speed of the earth. If the wind had been blowing ten thousand times faster than the speed the boat was moving, the position of the weather vane would have changed, even if only slightly, when the boat was moving vs when it stayed still [not to mention what such a wind might do to the boat!]. So it occurred to Bradley that when a telescope was aimed so that a star was right in the middle of the view finder, that probably wasn't the star's actual position. In fact, it wasn't even where the star would actually be if the earth had been still. This concept, called the 'aberration of light,' helped to explain why stars sometimes seem to shift. [How Stuff Works says, "The traces left by raindrops on the side windows of a moving automobile provide an analogy to the aberration of light. Even if the rain is coming straight down, the traces will be at an angle."] All of the instances of stars shifting made sense if they were caused by the moving of the earth as it related to light coming from the star. But this theory did more than explain the seeming shifts of stars and how light moved. It also illustrated and confirmed Copernicus's theory that the earth revolves around the sun, and it was important in improving practical astronomy. These days, everybody knows that where a star appears to be isn't exactly where it is [by the time its light reaches our eyes, we're seeing where it was, not where it is]. But, by using Bradley's principles, it's possible to calculate where a star actually is from seeing where it appears to be. This discovery made James Bradley famous. He tested his theory in many different ways and circumstances, and was able to confirm it completely.
Edmond Halley, the Astronomer Royal, died in January, 1742, and James Bradley was suggested as his successor. He officially got the position in February, 1742. Bradley moved to the observatory in Greenwich, but he wasn't able to start work right away because the instruments were in bad condition. His first task was getting them repaired, and by July, he was able to make his first transit observations. He worked so diligently that in a single day, he took 255 transit observations all by himself! By September, 1747, he had completed the set of observations that enabled him to confirm his second great discovery -- the nutation [rocking or wobbling] of the earth's axis. The way he made this discovery shows how meticulously he conducted his observations. He noticed that after a twelve-month orbit, when a star had completed its movement caused by aberration, it never ended up back at its exact original position. At first, he thought this was due to some instrumental error, but after investigating further and studying other stars, he saw this same effect with them. There must be something else changing that isn't due to the movement of the star itself -- something related to the position from where the star is being observed [in other words, something shifting on the earth itself].
This is what was happening: the earth is not a perfectly smooth sphere; it has a slight bulge at the equator. Gravity's attraction force pulls a bit more strongly at these bulges, and that makes the earth's axis tilt so that the positions of the poles are constantly fluctuating. That means that the point in the sky to which the earth's axis is pointing is slowly and constantly changing [by a process called precession, which you can see explained on Wikipedia]. Currently, the north pole points towards the Pole Star -- but that won't always be the case. This process of precession makes a complete circle around the ecliptic pole [read more on Wikipedia] in about 25,000 years. During this time, the earth's north pole will gradually pass near one star, then near another. Over the course of 25,000 years, various stars will be in the position that the North Star appears to be in now. In about 12,000 years, for example, our north pole will be pointed towards the bright star, Vega. [View a graphic of precession at work on YouTube.] Scientists had known about this movement of the pole for a long time. But what James Bradley had discovered was that the pole wasn't making a uniform movement like they had thought -- it was following a winding, meandering course, first tilting to one side, then the other. He attributed this to fluctuations in the moon's orbit. These fluctuations undergo a continuous change over 19 year cycles. [These are called Metonic cycles.] This shows how effectively the moon acts on the bumpy surface of the earth, causing the north pole to wobble back and forth.
The discovery of this subtle effect may not be as spectacular as his discovery of the aberration of light, but astronomers consider it a testament to his astonishing diligence and skill as an observer, and this discovery had practical application in the use of astronomical instruments.
There's not much to tell about Bradley's personal life. In 1744, shortly after he became Astronomer Royal, he married Susannah Peach, from Gloucestershire. They had one child, a daughter, who married her cousin, Rev. Samuel Peach, who was the rector of Compton Beauchamp, near Berkshire.
During the last two years of his life, James Bradley was often depressed because of a dread of losing his rational mind. But what he feared never happened. He died [of 'an uncommon illness'] at the age of seventy, on July 13, 1762, and was buried at Michinghamton.
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.
James Bradley, 1693-1762
"Aberration of light" means that the light we see may not reflect the current positions of stars.
James Bradley was born in Gloucestershire to a family with ancient roots in Durham County [England] in March 1693. He went to grammar school in Northleach, and later went on to college at Oxford in March, 1711. When he was an undergraduate, he spent a lot of time with his maternal uncle, Rev. James Pound, who was well-known for his interest in science and an enthusiastic star gazer. It was most likely through spending time with his uncle that he became skilled at handling astronomical instruments, but the discoveries he made were due to his own brilliance in science.
The first evidence of Bradley's skill was seen in two observations he made in 1717 and 1718. They were published by Halley -- he recognized James Bradley's extraordinary scientific mind. Halley once said, 'In the last opposition of the sun and Mars [astronomical opposition means two celestial bodies are on opposite sides of the sky], Dr. Pound and his nephew James Bradley showed me the extreme minuteness of the sun's parallax [items in a line of sight appear to shift from different positions; see it demonstrated on YouTube], and that it was somewhere between 9 and 12 seconds.' Here's what this means: determining the sun's parallax is like determining the distance from the earth to the sun. In those days, the distance from the earth to the sun [called an astronomical unit] wasn't accurately known, but Dr. Pound and James Bradley's comments meant that, from their observations, they determined that the sun was somewhere between 94 million miles to 125 million miles away. They were very close -- the sun's distance from the earth is actually 93-94 million miles away [it varies because our orbit is elliptical]. It's stunning to think that this veteran star gazer and his brilliant nephew were able to determine the distance -- which wouldn't be accurately known for another fifty years!
Some of James Bradley's earlier observational projects were the eclipses of Jupiter's satellites. They're appealing because they're so easy to see, and Bradley enjoyed calculating the times when these eclipses would take place, and then compare his calculations with the predicted times. He was so impressive at this, and with some other projects, that he gained a reputation that got him elected as a Fellow of the Royal Society in 1718 [he was in his mid-twenties].
Until this time, Bradley had merely been an amateur astronomical enthusiast. Astronomy wasn't something one could rely on for income, so he had been considering a 'real' profession. He was thinking he would enter the Church, although he didn't take orders when he came of age. In 1719, he was offered the Vicarage of Bridstow, which is in Monmouthshire. So he became a priest and took the position in 1720. The following year, he had some additional income from the proceeds of a Welsh living. Since he was a sinecure [sinecure is a position that pays well without requiring much work], he was able to maintain both positions, and he still had plenty of time to visit his uncle in Wandsworth. His uncle was also a clergyman, and James Bradley was able to help him with his duties.
Soon James Bradley had a decision to make: to continue working for the church, or make a career in science. When Dr. John Keill died in 1721, his position as Savilian Professor of Astronomy at Oxford became vacant. The position was offered to Bradley's uncle, James Pound, but he wasn't willing to give up his clerical position. There were a couple of other candidates, but Bradley's talents were so obvious that it was offered to him. He resigned from his clerical job and took on the professorship.
James Bradley had many influential friends and could have had a very successful church career. Bishop Benjamin Hoadly had already made him his personal chaplain. But Bradley was obsessed with astronomy and willing to sacrifice his professional prospects to take on the Savilian Professorship. It wasn't that he was uninterested in spiritual matters, but he felt that his gifts would be of more use in science. So he accepted the post and read his very first lecture there in April, 1722.
In those early days of astronomy, constructing a good telescope wasn't clearly understood. To make a refracting telescope effective, it was built excessively long. James Bradley made some of his observations with a telescope that was 212 feet long! There were no tubes long enough to make such a long telescope, so the object glass [the lens closest to the star being viewed] would simply be attached to a tall pole. In spite of the inconvenience of these 'aerial telescopes' [read more about them on Wikipedia], Bradley was able to make some very precise measurements. For example, he observed the transit of Mercury across the sun in October, 1723, and the dimensions of the planet Venus. Through October and November, 1723, he made a series of observations from Wanstead of a comet that Halley had first discovered. His first contribution to the Royal Society's Philosophical Transactions [the original science journal!] was about this comet. He spent an extraordinary amount of time making calculations about this comet, which are included in his book of Calculations.
But all of that was just a prelude to the two great discoveries that made him famous. As often happens in science, he made his first great discovery while he was researching something else. Scientists already knew that as the earth makes its orbit around the sun every year, some of the stars seem to move in relation to the change in the earth's position. The closer the star is to earth, the more it seems to shift, which seems to indicate that the star is being seen from different positions as the earth orbits. Using these shifts as the earth moves had been suggested as a way to determine the distance of the stars. But the distance to a star is so much greater than the earth's orbit that this method hadn't been very useful. In 1728, James Bradley decided to look into this. He thought that by using more powerful instruments and making more precise measurements, he might be able to distinguish displacement shifts that had been too slight to perceive before, and make the method more effective at measuring star distances. He decided to start by focusing on a single star, Beta Draconis, since it was high enough [almost directly overhead] to avoid refraction.
He used a telescope that had been set up along the chimney at the home of Samuel Molyneaux at the western end of Kew Green [near London, about 45 minutes from the Royal Observatory in Greenwich]. The eye piece was 3.5 feet above the ground floor and it had a focal length of over 24 feet. Mr. Molyneaux had set it up in 1725 to make his own observations. If there was any shifting of Beta Draconis due to the earth's orbit, the star should move less when it moved with the sun, and more when it moved in the opposite direction. The star passed the meridian [probably referring to the imaginary line from the north pole to the south pole that goes through Greenwich, right across the the courtyard of the Royal Observatory] at noon in December. Samuel Molyneaux made a note of its position on December 3. Any perceptible shift caused by parallax [parallax is the star's shift caused by the earth's movement] should have made the star seem to shift towards the north. But when James Bradley observed the star on December 17, it appeared to have shifted a little to the south! He checked and re-checked to be sure, and even scrutinized the settings of his instruments, but the star was indeed shifting south rather than north, and continued to do so into the month of March. By then it had moved twenty seconds south of where it had been on December 3. What was going on? By the middle of April, it had started to move north again. It continued moving north, and had moved 39 seconds north by September. Then it began moving south again, and by December, it was back in its original position, directly overhead.
The star's movement in the exact opposite direction of what parallax should have caused must mean that something else, something bigger, was happening to obscure the effect of parallax. But what? James Bradley was determined to find out. He decided to watch other stars to see if they exhibited the same phenomenon. He erected another telescope at Wanstead and kept an eye on several stars that passed at different distances from the zenith. In doing this, he discovered that other stars did the same thing! But why? It was a mystery that kept him puzzled until the explanation dawned on him.
One day he was out sailing and he noticed that every time the boat turned to make a different tack, the weather vane at the top of the boat's mast shifted a little as if the wind was changing direction. After this happened a few times, he commented to the sailors that it seemed like a strange coincidence for the wind to change direction every time the boat turned. The sailors said it wasn't the wind that was turning, but the weather vane moved a little with the boat's change in direction. In fact, when using the vane to determine wind direction, you had to take the boat's course in account because the vane moved in response to both the wind and the direction the boat was going in. This meant that it was possible for a person in the boat to see the vane pointing in a different direction from which the wind was blowing, depending on whether the boat was moving or stationary, and depending on which direction the boat was sailing. Bradley realized that what was happening with the boat must be somehow related to what had puzzled him about the movement of the stars.
It had been well known for a while that light doesn't travel through space instantaneously. It takes time. Galileo guessed that the sun had probably descended to the horizon before we saw it get there, and it was obvious that physical things -- like light -- require some time to get from point A to point B. But light is so fast that there were no tools to measure its speed back then. The Danish astronomer Ole Roemer had detected irregularities in the observed times of eclipses of Jupiter's satellites. These irregularities were due to the interval of time it took for light to stretch across the space between planets. James Bradley suggested that, since light travels at a specific speed, it might be like the wind he had noticed in the boat. If the person observing it were on an object -- such as on the earth -- then the direction the light appears to be coming from might be different than where it's actually coming from as the earth moves. The earth travels slower than light -- only eighteen miles per second, while light travels around 185,000 miles per second [or 186,000 miles per second]. In other words, light is ten thousand times faster than the speed of the earth. If the wind had been blowing ten thousand times faster than the speed the boat was moving, the position of the weather vane would have changed, even if only slightly, when the boat was moving vs when it stayed still [not to mention what such a wind might do to the boat!]. So it occurred to Bradley that when a telescope was aimed so that a star was right in the middle of the view finder, that probably wasn't the star's actual position. In fact, it wasn't even where the star would actually be if the earth had been still. This concept, called the 'aberration of light,' helped to explain why stars sometimes seem to shift. [How Stuff Works says, "The traces left by raindrops on the side windows of a moving automobile provide an analogy to the aberration of light. Even if the rain is coming straight down, the traces will be at an angle."] All of the instances of stars shifting made sense if they were caused by the moving of the earth as it related to light coming from the star. But this theory did more than explain the seeming shifts of stars and how light moved. It also illustrated and confirmed Copernicus's theory that the earth revolves around the sun, and it was important in improving practical astronomy. These days, everybody knows that where a star appears to be isn't exactly where it is [by the time its light reaches our eyes, we're seeing where it was, not where it is]. But, by using Bradley's principles, it's possible to calculate where a star actually is from seeing where it appears to be. This discovery made James Bradley famous. He tested his theory in many different ways and circumstances, and was able to confirm it completely.
Edmond Halley, the Astronomer Royal, died in January, 1742, and James Bradley was suggested as his successor. He officially got the position in February, 1742. Bradley moved to the observatory in Greenwich, but he wasn't able to start work right away because the instruments were in bad condition. His first task was getting them repaired, and by July, he was able to make his first transit observations. He worked so diligently that in a single day, he took 255 transit observations all by himself! By September, 1747, he had completed the set of observations that enabled him to confirm his second great discovery -- the nutation [rocking or wobbling] of the earth's axis. The way he made this discovery shows how meticulously he conducted his observations. He noticed that after a twelve-month orbit, when a star had completed its movement caused by aberration, it never ended up back at its exact original position. At first, he thought this was due to some instrumental error, but after investigating further and studying other stars, he saw this same effect with them. There must be something else changing that isn't due to the movement of the star itself -- something related to the position from where the star is being observed [in other words, something shifting on the earth itself].
This is what was happening: the earth is not a perfectly smooth sphere; it has a slight bulge at the equator. Gravity's attraction force pulls a bit more strongly at these bulges, and that makes the earth's axis tilt so that the positions of the poles are constantly fluctuating. That means that the point in the sky to which the earth's axis is pointing is slowly and constantly changing [by a process called precession, which you can see explained on Wikipedia]. Currently, the north pole points towards the Pole Star -- but that won't always be the case. This process of precession makes a complete circle around the ecliptic pole [read more on Wikipedia] in about 25,000 years. During this time, the earth's north pole will gradually pass near one star, then near another. Over the course of 25,000 years, various stars will be in the position that the North Star appears to be in now. In about 12,000 years, for example, our north pole will be pointed towards the bright star, Vega. [View a graphic of precession at work on YouTube.] Scientists had known about this movement of the pole for a long time. But what James Bradley had discovered was that the pole wasn't making a uniform movement like they had thought -- it was following a winding, meandering course, first tilting to one side, then the other. He attributed this to fluctuations in the moon's orbit. These fluctuations undergo a continuous change over 19 year cycles. [These are called Metonic cycles.] This shows how effectively the moon acts on the bumpy surface of the earth, causing the north pole to wobble back and forth.
The discovery of this subtle effect may not be as spectacular as his discovery of the aberration of light, but astronomers consider it a testament to his astonishing diligence and skill as an observer, and this discovery had practical application in the use of astronomical instruments.
There's not much to tell about Bradley's personal life. In 1744, shortly after he became Astronomer Royal, he married Susannah Peach, from Gloucestershire. They had one child, a daughter, who married her cousin, Rev. Samuel Peach, who was the rector of Compton Beauchamp, near Berkshire.
During the last two years of his life, James Bradley was often depressed because of a dread of losing his rational mind. But what he feared never happened. He died [of 'an uncommon illness'] at the age of seventy, on July 13, 1762, and was buried at Michinghamton.
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