Great Astronomers: Le Verrier
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.
Urbain Jean Joseph Le Verrier, 1811-1877
"The man who discovered a planet with the point of his pen."
Le Verrier's discoveries are of a very different nature than the astronomers we've read about so far. We tend to think of an astronomer as someone who looks at the stars through a telescope, but there's a lot more to it than that. No one deserves the title of astronomer more than Le Verrier, but he never made a single discovery with a telescope. In fact, his scientific achievements could have been done if he had never looked through a telescope at all.
To fully understand and appreciate the movements of celestial bodies, what's mostly needed is advanced mathematical knowledge. A mathematician needs to start with the exact position of the sun, moon, and planets at precise times, so he will either need to locate that data from another astronomer [or look at astronomy observational records] or make observations himself. Accurate observations taken carefully and recorded as purely as possible to make them free from errors ["reduced" to compensate for aberrations and other inconsistencies] are the raw data that mathematicians use for their own astronomical work. From their observed positions, he uses math to discover the true laws that regulate their movements. This takes a brilliant mathematical mind.
Le Verrier is one of the most brilliant mathematicians to work with astronomy. His mathematical genius was able to penetrate the deep, profound mysteries of nature.
Urbain Jean Joseph Le Verrier was born at Saint-Lo, in the department of Manche in France [two hours from Paris] on March 11, 1811. He was educated at the famous Ecole Polytechnique school of the higher branches of science. He earned a reputation there for his mathematical skills. When he graduated, he intended to devote his life to public service by being a civil engineer. His earliest work in that field wasn't in the field of mathematics that later made him famous. His early work involved practical chemical research in the laboratory. He became quite an expert in chemistry [under Joseph Guy-Lussac], and might have become a famous chemist if destiny hadn't led him to astronomy. He made some original discoveries as a chemist. He wrote a scientific paper on the combination of phosphorus and hydrogen, and another one on the combination of phosphorus and oxygen.
[In 1837, he married Lucile Marie Clothilde Choquet, the musical daughter of his former teacher at the Mayer Institute. They had two sons and a daughter.]
When he had been a student at Ecole Polytechnique, he had shown an ability to deal with the most subtle instruments of mathematical analysis. In 1839, at twenty-eight years of age, he made his first discovery in astronomical research. We'll look at how this happened because it began the astronomy career that became his life's work.
If there was one single planet orbiting around the sun, then the path that planet took would be a perfect, consistent ellipse that never changed position or altered its course. It would trace out its path in exactly the same manner as it followed the gravitational pull of the sun. But imagine there was a second planet also orbiting around the sun. The sun's attraction force [gravity] would also pull on this planet so that it made an elliptical orbit, but with the first planet adding its own gravitational pull, the second planet's ellipse wouldn't be completely regular or exact. Assuming the sun was enormously bigger than either of the two planets, its attraction force would be so much greater than either planet that the orbital ellipses of the two planets would be almost exact, regular ellipses, yet their orbits would be slightly influenced by the attraction force of the other planet. The general principle of nature is that every body in space attracts every other body. As long there's just a single planet, the only thing influencing that planet's orbit is the attraction force between the planet and the sun, and the planet's ellipse is undisturbed. But once a second planet is introduced, both planets not only have the influence of the sun's attraction force, but they have to deal with the attraction force of the other planet. This mutual force may be very small, but that doesn't make its effect any less real. That small force disturbs the perfect ellipse. Our solar system has several planets disturbing each other, so their orbits aren't perfect ellipses.
As well, a planet may make a general ellipse as it goes around the sun in one rotation. But as it goes around a second time, and third, and so on, the ellipse gradually varies. Its curve changes slightly, it doesn't stay on the same level plane, or it shifts its position on that plane. So if we want a full picture of the movements of the planets over great periods of time, we need to figure out how that planet's orbit is affected by any disturbances that influence it [in order to calculate changes in its ellipse over vast periods of time].
Imagine a planet is like a train engine running on a track laid out to form a huge ellipse. As the planet goes along, the shape of the track is gradually changing, although the change may be so slight that it isn't noticeable in the next revolution. The train track is also on a plane that wobbles slightly, and the whole elliptical track moves about slowly on the plane. Over short time periods, the changes and shifts as planets disturb each other's orbits are insignificant. But when we consider the effects over a long period of time -- say, thousands of years -- the accumulated displacement can have a profound change in the solar system.
[For an interesting look at the variations of the orbits: view a 3-min computer-generated model of one theory of how our solar system moves through space, and how that affects the earth's rotation around the sun on YouTube.]
Figuring out the long-term effects of planets disturbing each other's orbits requires a mind with superior mathematical gifts. It doesn't just take intellectual ability -- it takes intellectual ability combined with patience to calculate extensive grueling equations over many years of work. Le Verrier found that his peculiar gifts were perfectly suited for this kind of work. His first significant science article investigated the changes of orbits in our solar system for past ages and predicted the changes they would undergo in the future.
To illustrate this kind of research, we can take the planet we're most interested in -- Earth -- and look at the changes its orbit has undergone over time because of the gravitational disturbance of other planets. Over a hundred years, or even a thousand years, there's not enough change to be very noticeable. It takes more time than that for the change to be significant. Le Verrier gave details of the earth's journey through space at 20,000 year intervals all the way back to 100,000 years ago, and then he gave predictions about what the earth's orbit would look like at 20,000 year intervals in the future all the way to 101,800 A.D.
His skill at doing this kind of work brought him some recognition. The Paris Observatory was headed by the genius Francois Arago. Arago realized that Le Verrier was just the man who was qualified to work on a difficult astronomical problem that had been puzzling astronomers.
After Sir William Herschel discovered the planet Uranus, astronomers scrutinized its orbits with eager attention, and they were able to determine its exact position at certain times. When enough records were available, astronomers were able to confirm its orbit. But Uranus resembles a star -- that's why it was never recognized as a planet until Herschel detected its true nature. Its star-like point looked so much like a star [it has a ring, like Saturn, which probably accounts for its pointed look]. Once there was enough data, it was possible to track its exact orbit. Astronomers found that it takes eighty-four years for it to make a complete circuit around the sun. They had already inferred the shape of its orbit even before Herschel discovered that it was a planet, when they thought it was a star they were tracking. They compared this old data with their newer observations. Nobody expected both sets of data to be an exact match -- the large planets Jupiter and Saturn make their orbits in the vicinity of Uranus, and would be expected to cause some disturbance. But when they reduced the figures to compensate for Jupiter and Saturn, and all the other planets, there was still some disturbance that couldn't be accounted for. There must be something else out there causing some interference. But what?
Astronomers could only see one possible solution: there must be something else out there, probably a planet, affecting Uranus's orbit. Arago urged Le Verrier to try to figure out what it was. But locating and identifying it would have to be done differently than most astronomical research: searching the sky with a telescope wasn't going to help.
There were a few facts that could be inferred about this mysterious celestial body. It must be huge -- even larger than Earth with a much larger mass to cause the amount of disturbance that had been observed. But it must be so far away that we'd only be able to see it as a tiny object from our point of view. Uranus itself was so far away that it was almost impossible to see it with the naked eye, and this body was even farther away, with an orbit that was outside of Uranus's orbit. So it was probably not visible to our eyes. If it had been visible, it would have been recognized as a planet long ago. So it must be a planet that was so far away that it would never be seen with the unaided eye.
There's a vast physical difference between a planet and a star. A star is a fiery, burning sun that gives off light, and a planet is a dark mass that only becomes visible when sunlight falls on it. Even though a star is thousands of times larger than a planet and millions of times farther away, planets still resemble stars when viewed through a telescope. There's only one way to tell the difference by simply looking. If the planet is big enough, it will look like a disc with a definable circular outline. Stars never look like that. No matter how much they're magnified, they only show up as radiant points of light. The older and well-known planets, like Jupiter and Mars, are disc-shaped when magnified. Although they just look like points of light with the unaided eye, they have a very clear disc outline when viewed through even a simple telescope. But a planet as far away as Uranus looks like a star unless viewed with a very good telescope and a very discerning eye, like William Herschel's. That's why it was observed and recorded seventeen times and labeled as a star by experienced astronomers before Herschel recognized it as a planet. All those times before, its planetary behaviour had been overlooked, and it was taken for granted that it was a star. There was nothing significant enough about it to cause any special notice.
Since the unknown celestial object that was disturbing Uranus's orbit was so far away and would appear so tiny if it was seen at all, its actual shape might be easily missed. It was not likely to be recognized just by looking for it through a telescope even though it was very different from the stars.
There are hundreds of thousands of stars in the sky. Trying to pick out a planet from among them that looked like a star would be very complicated. There are just too many stars. If all those stars could be swept aside, it would be simple to see any planets that are bright enough to view through a telescope. But since planets look like stars among the multitude of stars, it's almost impossible. But what if there was a way to calculate the precise location where the planet should be? Then astronomers could focus their search in one spot.
To some extent, identifying the region of the heavens to scan for the planet was limited to a narrow area of the sky. All of the great planets confine their orbits to a specific zone in the sky. The zone is around the earth's ecliptic, the same line that the earth takes around the sun. So the search should be somewhere within that zone. It's still a huge area, with thousands of stars. There must be some way to confine the search to a smaller area.
In 1845 Le Verrier was asked to figure out where in that strip of the ecliptic zone they should focus their search for the unknown planet. The only figures he would have to work with were the discrepancies between where Uranus's undisturbed orbit should be, and the actual measurements where its position had been observed and confirmed. This was a complicated problem, and would be extremely difficult [especially since calculators wouldn't be invented for another 115 years]. But Le Verrier took on the challenge and, to the great astonishment of the world, he found the brilliant solution! We don't have space to even try to explain the mathematical investigations and equations that were necessary. We'll just give a general indication of the method he used.
Picture a planet orbiting outside of Uranus at a distance somewhat like the distances between the other planets that orbit the sun. Let's imagine that this planet has started on a regular hypothetical course that the mathematician will assign to it, and that the planet has a certain mass. As it revolves, it will disturb the orbit of Uranus, causing Uranus to make a specifically calculated detour from its regular orbit. But actual calculated predictions of this hypothetical orbit don't match Uranus's actual observed orbit. This indicates that the unknown planet's movement must be different than what it was hypothesized to be. So the mathematician starts again with a new theoretical assigned orbit. After many of these trial and error speculative orbits, Le Verrier was able to determine that, by assuming a certain size, shape, and position for the mystery planet's orbit, and by assuming a certain number for the mystery planet's mass, he could account for the actual deviations of Uranus's orbit. Gradually, Le Verrier saw that this was the only way to account for the puzzle of Uranus's orbit. A planet with the mass he had assumed, and whose orbit had the calculations he had assigned, must exist, even though no one had ever seen it. It was astonishing that a mathematician sitting at a desk could study the observed data of one planet, and use that data to not only discover the existence of another planet, but to predict its exact position, without ever using a telescope.
And that's how Le Verrier's calculations greatly narrowed down the area that was already being scanned with a telescope. Astronomers already knew that they should be searching along the ecliptic. Le Verrier calculated where on that ecliptic the planet would be found. The next episode in this search will be celebrated for as long as science endures. It was necessary for a telescope to confirm that this planet actually existed, but Le Verrier didn't have his own telescope, or even the skills of a practical astronomer. So he wrote to Johann Gottfried Galle, an astronomer in Berlin, with the coordinates, and asked him to search the vicinity with the telescope at the Berlin Observatory. He said to look for the distinguishing disc characteristic that would positively identify it as a planet among the surrounding stars.
The request reached the Berlin Observatory in September 1846. It was a clear evening, so Galle decided to start the search that very night. Recently, a diligent star-gazer named Carl Bremiker, had put together a star chart for zones around the ecliptic. The chart, called "Hora XXI Aquarius" because it mapped out stars around the constellation of Aquarius, wasn't completed yet, but the area where Le Verrier had asked Galle to search, had been added just a few years before that. So Galle had a chart of all the stars that would be visible in the area he would be looking at. That would provide an additional method of identifying the planet. Since planets move, the planet he was looking for would be in a different spot than the chart showed it. In other words, his mystery planet would be the star that had moved from its position on the star chart to Le Verrier's coordinates.
The search would compare every point on the star chart to its actual position in the sky as seen through the telescope. As Galle looked, he noticed a star-like object of the eighth magnitude [in brightness; brighter than the stars around it] that was not on the star chart. Was this the mystery planet? It seemed unlikely that such a bright star would have been missed when drawing up the star chart, especially since dimmer stars had been included. Some stars have variable brightness, so it was possible that this star had been missed because its light had fluctuated and been too dim to notice when the chart had been made. And sometimes stars are born, so it was possible that this was a new star.
There was a way to test this new object to see if was a variable star, or a new star -- or a planet. A star remains fixed in the sky, but planets move. A planet so far away would be moving so slowly from our perspective that its movement couldn't be noticed in a single night. But Dr. Galle watched that object carefully. Even during that night, he thought he could detect some slight movement. He couldn't wait to check its position again on the following nights. He was able, over the course of a few nights, to confirm that this object was, indeed, moving -- just like a planet!
The whole scientific community commended his superb triumph. This mighty planet had been out there all along, but was only found by refined mathematical calculation. Le Verrier's name had been known by experts in the more abstract branches of astronomy, but now it became celebrated everywhere. But he had to share this fame with J. C. Adams of Cambridge. In the next chapter, we'll look at how he independently came to the same discovery through mathematical calculation at about the same time. [His calculations weren't accurate as Le Verrier's, and Le Verrier is credited with the discovery. But one of Neptune's five rings is named after Adams.]
As soon as it had been confirmed that the mystery celestial object was a planet, the great observatories added it to their working charts and recorded its position day after day. After enough observations, its exact orbit could be determined. Of course, the observations taken with the telescope were more accurate than what Le Verrier had guessed at with his mathematical equations using figures of its supposed position reflected off Uranus. Actual observation showed it in a slightly different position from Le Verrier's calculations [but only one degree off!]
What should the newly discovered planet be called? The previously known planets had been named after mythological gods, and it seemed logical to follow this convention. Since the planet was so remotely far away, "Neptune" -- the god of the remote underworld -- was suggested. And that's the name that was adopted.
Le Verrier became so famous after this discovery that when Francois Arago resigned from the Paris Observatory in 1854 [he wasn't healthy, and refused to swear allegiance to Napoleon III], the man who discovered Neptune seemed like a fitting replacement to become the director of the Paris Observatory, France's equivalent of Astronomer Royal. Admittedly, his astronomy work had been been of an abstract mathematical nature. His research had been done at a desk rather than in an observatory, and he had very little experience with astronomical equipment. But he accepted the position and determined to master the hands-on technical part of work eagerly. He tried to inspire the staff of the observatory with enthusiasm for the systematic work [mathematical reduction?] that is required to make astronomical observation of practical use. Unfortunately, Le Verrier didn't have the natural abilities [the people skills, evidently] to successfully administrate the observatory. There were disputes between him and the staff. Things became so tense that he was forced to resign. He was replaced by the mathematician Charles-Eugene Delaunay in 1870.
Now that he didn't have his observatory duties to tend to, Le Verrier went back to his beloved mathematics. He continued to work in an unofficial capacity with the observatory, researching the movements of the other planets. Three years later, Delauney died in an accidental drowning, and Le Verrier was reinstated, and held onto the position as Director of the Paris Observatory for the rest on his life [four years].
The kind of work he did after this is difficult to explain -- there are too many mathematical terms and symbols that would have to be grasped. Basically, he was mostly working with the effects that the attraction force of gravity had on the planets mutually affecting one another. His calculations were used to prepare tables and charts to predict the exact positions of planets for almanacs. It was a tedious, overwhelming task, and if he hadn't devoted himself to, it would probably never have been done at all.
Once he had solved the mysterious disturbance in the orbit of Uranus, he was curious about disturbances in the orbits of other planets. He calculated how much attraction force the planets were exerting on each other to show how that could account for their orbital irregularities. In one instance, near the beginning of his career, Le Verrier had discovered that Uranus, which was the outermost planet known at the time, was showing a mysterious influence from some unknown celestial body. Now he noticed a similar effect with Mercury, which is the innermost planet in our solar system. He couldn't account for this. What was causing it? Mercury's ellipse experienced a slow movement that made it revolve in its plane. Le Verrier didn't think this was caused by any of the known planets in our solar system. Could there be another planet between Mercury and the sun? He hoped to get someone to check his calculations with a telescope, the same way Galle had checked and found Neptune. If there was another planet, then it would have to come between the sun and earth at some point, so we should be able to see it sometimes. He was so confident that there was something there that when Edmond Modeste Lescarbault spotted a dark object in the sky in March, 1859, Le Verrier was certain it was the mystery planet. He predicted that it would be visible again in March 1877. A diligent watch was kept, but there was no planet to see. [This hypothetical planet was named Vulcan, and continued to be searched for during solar eclipses until 1908.]
Urbain Le Verrier received every honour that a scientist could receive. The last few years of his life were some of the most turbulent years in modern French history. He supported the Imperial Dynasty [the Napoleons], and the brief rule of the Paris Commune [radical socialist revolutionary government of 1871] was an anxious time for him. In fact, there was a point when there were fears for his personal safety.
In early 1877, his health, which had been gradually failing due to liver disease since 1873, took a turn for the worst. He died in September 1877 at age 66.
He was buried in a grand public funeral in the Montparnasse Cemetery. Some of his pallbearers were leading scientists from around the world, and memorials were given expressing admiration for his talents and his great service to science.
[There's another biography about Le Verrier here.]
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.
Urbain Jean Joseph Le Verrier, 1811-1877
"The man who discovered a planet with the point of his pen."
Le Verrier's discoveries are of a very different nature than the astronomers we've read about so far. We tend to think of an astronomer as someone who looks at the stars through a telescope, but there's a lot more to it than that. No one deserves the title of astronomer more than Le Verrier, but he never made a single discovery with a telescope. In fact, his scientific achievements could have been done if he had never looked through a telescope at all.
To fully understand and appreciate the movements of celestial bodies, what's mostly needed is advanced mathematical knowledge. A mathematician needs to start with the exact position of the sun, moon, and planets at precise times, so he will either need to locate that data from another astronomer [or look at astronomy observational records] or make observations himself. Accurate observations taken carefully and recorded as purely as possible to make them free from errors ["reduced" to compensate for aberrations and other inconsistencies] are the raw data that mathematicians use for their own astronomical work. From their observed positions, he uses math to discover the true laws that regulate their movements. This takes a brilliant mathematical mind.
Le Verrier is one of the most brilliant mathematicians to work with astronomy. His mathematical genius was able to penetrate the deep, profound mysteries of nature.
Urbain Jean Joseph Le Verrier was born at Saint-Lo, in the department of Manche in France [two hours from Paris] on March 11, 1811. He was educated at the famous Ecole Polytechnique school of the higher branches of science. He earned a reputation there for his mathematical skills. When he graduated, he intended to devote his life to public service by being a civil engineer. His earliest work in that field wasn't in the field of mathematics that later made him famous. His early work involved practical chemical research in the laboratory. He became quite an expert in chemistry [under Joseph Guy-Lussac], and might have become a famous chemist if destiny hadn't led him to astronomy. He made some original discoveries as a chemist. He wrote a scientific paper on the combination of phosphorus and hydrogen, and another one on the combination of phosphorus and oxygen.
[In 1837, he married Lucile Marie Clothilde Choquet, the musical daughter of his former teacher at the Mayer Institute. They had two sons and a daughter.]
When he had been a student at Ecole Polytechnique, he had shown an ability to deal with the most subtle instruments of mathematical analysis. In 1839, at twenty-eight years of age, he made his first discovery in astronomical research. We'll look at how this happened because it began the astronomy career that became his life's work.
If there was one single planet orbiting around the sun, then the path that planet took would be a perfect, consistent ellipse that never changed position or altered its course. It would trace out its path in exactly the same manner as it followed the gravitational pull of the sun. But imagine there was a second planet also orbiting around the sun. The sun's attraction force [gravity] would also pull on this planet so that it made an elliptical orbit, but with the first planet adding its own gravitational pull, the second planet's ellipse wouldn't be completely regular or exact. Assuming the sun was enormously bigger than either of the two planets, its attraction force would be so much greater than either planet that the orbital ellipses of the two planets would be almost exact, regular ellipses, yet their orbits would be slightly influenced by the attraction force of the other planet. The general principle of nature is that every body in space attracts every other body. As long there's just a single planet, the only thing influencing that planet's orbit is the attraction force between the planet and the sun, and the planet's ellipse is undisturbed. But once a second planet is introduced, both planets not only have the influence of the sun's attraction force, but they have to deal with the attraction force of the other planet. This mutual force may be very small, but that doesn't make its effect any less real. That small force disturbs the perfect ellipse. Our solar system has several planets disturbing each other, so their orbits aren't perfect ellipses.
As well, a planet may make a general ellipse as it goes around the sun in one rotation. But as it goes around a second time, and third, and so on, the ellipse gradually varies. Its curve changes slightly, it doesn't stay on the same level plane, or it shifts its position on that plane. So if we want a full picture of the movements of the planets over great periods of time, we need to figure out how that planet's orbit is affected by any disturbances that influence it [in order to calculate changes in its ellipse over vast periods of time].
Imagine a planet is like a train engine running on a track laid out to form a huge ellipse. As the planet goes along, the shape of the track is gradually changing, although the change may be so slight that it isn't noticeable in the next revolution. The train track is also on a plane that wobbles slightly, and the whole elliptical track moves about slowly on the plane. Over short time periods, the changes and shifts as planets disturb each other's orbits are insignificant. But when we consider the effects over a long period of time -- say, thousands of years -- the accumulated displacement can have a profound change in the solar system.
[For an interesting look at the variations of the orbits: view a 3-min computer-generated model of one theory of how our solar system moves through space, and how that affects the earth's rotation around the sun on YouTube.]
Figuring out the long-term effects of planets disturbing each other's orbits requires a mind with superior mathematical gifts. It doesn't just take intellectual ability -- it takes intellectual ability combined with patience to calculate extensive grueling equations over many years of work. Le Verrier found that his peculiar gifts were perfectly suited for this kind of work. His first significant science article investigated the changes of orbits in our solar system for past ages and predicted the changes they would undergo in the future.
To illustrate this kind of research, we can take the planet we're most interested in -- Earth -- and look at the changes its orbit has undergone over time because of the gravitational disturbance of other planets. Over a hundred years, or even a thousand years, there's not enough change to be very noticeable. It takes more time than that for the change to be significant. Le Verrier gave details of the earth's journey through space at 20,000 year intervals all the way back to 100,000 years ago, and then he gave predictions about what the earth's orbit would look like at 20,000 year intervals in the future all the way to 101,800 A.D.
His skill at doing this kind of work brought him some recognition. The Paris Observatory was headed by the genius Francois Arago. Arago realized that Le Verrier was just the man who was qualified to work on a difficult astronomical problem that had been puzzling astronomers.
After Sir William Herschel discovered the planet Uranus, astronomers scrutinized its orbits with eager attention, and they were able to determine its exact position at certain times. When enough records were available, astronomers were able to confirm its orbit. But Uranus resembles a star -- that's why it was never recognized as a planet until Herschel detected its true nature. Its star-like point looked so much like a star [it has a ring, like Saturn, which probably accounts for its pointed look]. Once there was enough data, it was possible to track its exact orbit. Astronomers found that it takes eighty-four years for it to make a complete circuit around the sun. They had already inferred the shape of its orbit even before Herschel discovered that it was a planet, when they thought it was a star they were tracking. They compared this old data with their newer observations. Nobody expected both sets of data to be an exact match -- the large planets Jupiter and Saturn make their orbits in the vicinity of Uranus, and would be expected to cause some disturbance. But when they reduced the figures to compensate for Jupiter and Saturn, and all the other planets, there was still some disturbance that couldn't be accounted for. There must be something else out there causing some interference. But what?
Astronomers could only see one possible solution: there must be something else out there, probably a planet, affecting Uranus's orbit. Arago urged Le Verrier to try to figure out what it was. But locating and identifying it would have to be done differently than most astronomical research: searching the sky with a telescope wasn't going to help.
There were a few facts that could be inferred about this mysterious celestial body. It must be huge -- even larger than Earth with a much larger mass to cause the amount of disturbance that had been observed. But it must be so far away that we'd only be able to see it as a tiny object from our point of view. Uranus itself was so far away that it was almost impossible to see it with the naked eye, and this body was even farther away, with an orbit that was outside of Uranus's orbit. So it was probably not visible to our eyes. If it had been visible, it would have been recognized as a planet long ago. So it must be a planet that was so far away that it would never be seen with the unaided eye.
There's a vast physical difference between a planet and a star. A star is a fiery, burning sun that gives off light, and a planet is a dark mass that only becomes visible when sunlight falls on it. Even though a star is thousands of times larger than a planet and millions of times farther away, planets still resemble stars when viewed through a telescope. There's only one way to tell the difference by simply looking. If the planet is big enough, it will look like a disc with a definable circular outline. Stars never look like that. No matter how much they're magnified, they only show up as radiant points of light. The older and well-known planets, like Jupiter and Mars, are disc-shaped when magnified. Although they just look like points of light with the unaided eye, they have a very clear disc outline when viewed through even a simple telescope. But a planet as far away as Uranus looks like a star unless viewed with a very good telescope and a very discerning eye, like William Herschel's. That's why it was observed and recorded seventeen times and labeled as a star by experienced astronomers before Herschel recognized it as a planet. All those times before, its planetary behaviour had been overlooked, and it was taken for granted that it was a star. There was nothing significant enough about it to cause any special notice.
Since the unknown celestial object that was disturbing Uranus's orbit was so far away and would appear so tiny if it was seen at all, its actual shape might be easily missed. It was not likely to be recognized just by looking for it through a telescope even though it was very different from the stars.
There are hundreds of thousands of stars in the sky. Trying to pick out a planet from among them that looked like a star would be very complicated. There are just too many stars. If all those stars could be swept aside, it would be simple to see any planets that are bright enough to view through a telescope. But since planets look like stars among the multitude of stars, it's almost impossible. But what if there was a way to calculate the precise location where the planet should be? Then astronomers could focus their search in one spot.
To some extent, identifying the region of the heavens to scan for the planet was limited to a narrow area of the sky. All of the great planets confine their orbits to a specific zone in the sky. The zone is around the earth's ecliptic, the same line that the earth takes around the sun. So the search should be somewhere within that zone. It's still a huge area, with thousands of stars. There must be some way to confine the search to a smaller area.
In 1845 Le Verrier was asked to figure out where in that strip of the ecliptic zone they should focus their search for the unknown planet. The only figures he would have to work with were the discrepancies between where Uranus's undisturbed orbit should be, and the actual measurements where its position had been observed and confirmed. This was a complicated problem, and would be extremely difficult [especially since calculators wouldn't be invented for another 115 years]. But Le Verrier took on the challenge and, to the great astonishment of the world, he found the brilliant solution! We don't have space to even try to explain the mathematical investigations and equations that were necessary. We'll just give a general indication of the method he used.
Picture a planet orbiting outside of Uranus at a distance somewhat like the distances between the other planets that orbit the sun. Let's imagine that this planet has started on a regular hypothetical course that the mathematician will assign to it, and that the planet has a certain mass. As it revolves, it will disturb the orbit of Uranus, causing Uranus to make a specifically calculated detour from its regular orbit. But actual calculated predictions of this hypothetical orbit don't match Uranus's actual observed orbit. This indicates that the unknown planet's movement must be different than what it was hypothesized to be. So the mathematician starts again with a new theoretical assigned orbit. After many of these trial and error speculative orbits, Le Verrier was able to determine that, by assuming a certain size, shape, and position for the mystery planet's orbit, and by assuming a certain number for the mystery planet's mass, he could account for the actual deviations of Uranus's orbit. Gradually, Le Verrier saw that this was the only way to account for the puzzle of Uranus's orbit. A planet with the mass he had assumed, and whose orbit had the calculations he had assigned, must exist, even though no one had ever seen it. It was astonishing that a mathematician sitting at a desk could study the observed data of one planet, and use that data to not only discover the existence of another planet, but to predict its exact position, without ever using a telescope.
And that's how Le Verrier's calculations greatly narrowed down the area that was already being scanned with a telescope. Astronomers already knew that they should be searching along the ecliptic. Le Verrier calculated where on that ecliptic the planet would be found. The next episode in this search will be celebrated for as long as science endures. It was necessary for a telescope to confirm that this planet actually existed, but Le Verrier didn't have his own telescope, or even the skills of a practical astronomer. So he wrote to Johann Gottfried Galle, an astronomer in Berlin, with the coordinates, and asked him to search the vicinity with the telescope at the Berlin Observatory. He said to look for the distinguishing disc characteristic that would positively identify it as a planet among the surrounding stars.
The request reached the Berlin Observatory in September 1846. It was a clear evening, so Galle decided to start the search that very night. Recently, a diligent star-gazer named Carl Bremiker, had put together a star chart for zones around the ecliptic. The chart, called "Hora XXI Aquarius" because it mapped out stars around the constellation of Aquarius, wasn't completed yet, but the area where Le Verrier had asked Galle to search, had been added just a few years before that. So Galle had a chart of all the stars that would be visible in the area he would be looking at. That would provide an additional method of identifying the planet. Since planets move, the planet he was looking for would be in a different spot than the chart showed it. In other words, his mystery planet would be the star that had moved from its position on the star chart to Le Verrier's coordinates.
The search would compare every point on the star chart to its actual position in the sky as seen through the telescope. As Galle looked, he noticed a star-like object of the eighth magnitude [in brightness; brighter than the stars around it] that was not on the star chart. Was this the mystery planet? It seemed unlikely that such a bright star would have been missed when drawing up the star chart, especially since dimmer stars had been included. Some stars have variable brightness, so it was possible that this star had been missed because its light had fluctuated and been too dim to notice when the chart had been made. And sometimes stars are born, so it was possible that this was a new star.
There was a way to test this new object to see if was a variable star, or a new star -- or a planet. A star remains fixed in the sky, but planets move. A planet so far away would be moving so slowly from our perspective that its movement couldn't be noticed in a single night. But Dr. Galle watched that object carefully. Even during that night, he thought he could detect some slight movement. He couldn't wait to check its position again on the following nights. He was able, over the course of a few nights, to confirm that this object was, indeed, moving -- just like a planet!
The whole scientific community commended his superb triumph. This mighty planet had been out there all along, but was only found by refined mathematical calculation. Le Verrier's name had been known by experts in the more abstract branches of astronomy, but now it became celebrated everywhere. But he had to share this fame with J. C. Adams of Cambridge. In the next chapter, we'll look at how he independently came to the same discovery through mathematical calculation at about the same time. [His calculations weren't accurate as Le Verrier's, and Le Verrier is credited with the discovery. But one of Neptune's five rings is named after Adams.]
As soon as it had been confirmed that the mystery celestial object was a planet, the great observatories added it to their working charts and recorded its position day after day. After enough observations, its exact orbit could be determined. Of course, the observations taken with the telescope were more accurate than what Le Verrier had guessed at with his mathematical equations using figures of its supposed position reflected off Uranus. Actual observation showed it in a slightly different position from Le Verrier's calculations [but only one degree off!]
What should the newly discovered planet be called? The previously known planets had been named after mythological gods, and it seemed logical to follow this convention. Since the planet was so remotely far away, "Neptune" -- the god of the remote underworld -- was suggested. And that's the name that was adopted.
Le Verrier became so famous after this discovery that when Francois Arago resigned from the Paris Observatory in 1854 [he wasn't healthy, and refused to swear allegiance to Napoleon III], the man who discovered Neptune seemed like a fitting replacement to become the director of the Paris Observatory, France's equivalent of Astronomer Royal. Admittedly, his astronomy work had been been of an abstract mathematical nature. His research had been done at a desk rather than in an observatory, and he had very little experience with astronomical equipment. But he accepted the position and determined to master the hands-on technical part of work eagerly. He tried to inspire the staff of the observatory with enthusiasm for the systematic work [mathematical reduction?] that is required to make astronomical observation of practical use. Unfortunately, Le Verrier didn't have the natural abilities [the people skills, evidently] to successfully administrate the observatory. There were disputes between him and the staff. Things became so tense that he was forced to resign. He was replaced by the mathematician Charles-Eugene Delaunay in 1870.
Now that he didn't have his observatory duties to tend to, Le Verrier went back to his beloved mathematics. He continued to work in an unofficial capacity with the observatory, researching the movements of the other planets. Three years later, Delauney died in an accidental drowning, and Le Verrier was reinstated, and held onto the position as Director of the Paris Observatory for the rest on his life [four years].
The kind of work he did after this is difficult to explain -- there are too many mathematical terms and symbols that would have to be grasped. Basically, he was mostly working with the effects that the attraction force of gravity had on the planets mutually affecting one another. His calculations were used to prepare tables and charts to predict the exact positions of planets for almanacs. It was a tedious, overwhelming task, and if he hadn't devoted himself to, it would probably never have been done at all.
Once he had solved the mysterious disturbance in the orbit of Uranus, he was curious about disturbances in the orbits of other planets. He calculated how much attraction force the planets were exerting on each other to show how that could account for their orbital irregularities. In one instance, near the beginning of his career, Le Verrier had discovered that Uranus, which was the outermost planet known at the time, was showing a mysterious influence from some unknown celestial body. Now he noticed a similar effect with Mercury, which is the innermost planet in our solar system. He couldn't account for this. What was causing it? Mercury's ellipse experienced a slow movement that made it revolve in its plane. Le Verrier didn't think this was caused by any of the known planets in our solar system. Could there be another planet between Mercury and the sun? He hoped to get someone to check his calculations with a telescope, the same way Galle had checked and found Neptune. If there was another planet, then it would have to come between the sun and earth at some point, so we should be able to see it sometimes. He was so confident that there was something there that when Edmond Modeste Lescarbault spotted a dark object in the sky in March, 1859, Le Verrier was certain it was the mystery planet. He predicted that it would be visible again in March 1877. A diligent watch was kept, but there was no planet to see. [This hypothetical planet was named Vulcan, and continued to be searched for during solar eclipses until 1908.]
Urbain Le Verrier received every honour that a scientist could receive. The last few years of his life were some of the most turbulent years in modern French history. He supported the Imperial Dynasty [the Napoleons], and the brief rule of the Paris Commune [radical socialist revolutionary government of 1871] was an anxious time for him. In fact, there was a point when there were fears for his personal safety.
In early 1877, his health, which had been gradually failing due to liver disease since 1873, took a turn for the worst. He died in September 1877 at age 66.
He was buried in a grand public funeral in the Montparnasse Cemetery. Some of his pallbearers were leading scientists from around the world, and memorials were given expressing admiration for his talents and his great service to science.
[There's another biography about Le Verrier here.]
Comments
Post a Comment