Whereas, in order to the finding out of the longitude of places for perfecting navigation and astronomy, we have resolved to build a small observatory within our park at Greenwich, upon the highest ground, at or near the place where the castle stood, with lodging rooms for our astronomical observator and assistant, Our Will and Pleasure is that according to such plot and design as shall be given you by our trusty and well-beloved Sir Christopher Wren, Knight, our surveyor-general of the place and scite of the said observatory, you cause the same to be built and finished with all convenient speed, by such artificers and workmen as you shall appoint thereto, and that you give order unto our Treasurer of the Ordnance for the paying of such materials and workmen as shall be used and employed therein, out of such monies as shall come to your hands for old and decayed powder, which hath or shall be sold by our order on the 1st of January last, provided that the whole sum to be expended or paid, shall not exceed five hundred pounds; and our pleasure is, that all our officers and servants belonging to our said park be assisting to those that you shall appoint for the doing thereof, and for so doing, this shall be to you, and to all others whom it may concern, a sufficient warrant.
Thus finding a solution to the longitude problem was the main reason for the founding of the Royal Greenwich Observatory. In 1675 John Flamsteed (1646–1719) was appointed as the first Astronomer Royal. The observatory was supplied with several clocks. There were two clocks constructed with a very short swing of the pendulum, one of which was designed to show sidereal time—the rotation of the stars rather than the Sun—which was an essential requirement for such an observatory. There were seven clocks altogether in the observatory, plus a micrometer, a sextant and a mural arc. There were four telescopes with focal lengths that varied from 2.5 to 5 meters (8–16 ft). Flamsteed was given a very meager allowance for instruments, but he was prepared to put his own money into the observatory. He took a job as the incumbent of Burstow near Reigate, and this provided him with a second income.
Plotting the Motion of the Moon
Flamsteed began to work on a set of accurate observations of the Moon to determine the precise orbit. He was an excellent and meticulous observer, but he soon discovered a problem with the lunar method: it lay not in the difficulty of the observation itself but in the complex motion of the Moon. The orbit of the Moon around the Earth was analogous to the orbits of the planets around the Sun—in other words, it was an ellipse with the Earth at one focus. But the Moon was also influenced by the gravity of the Sun, and so the ellipse could only be an approximation, and its properties varied as the Moon progressed in its orbit. In the 1660s there were a number of people who claimed to have solved the problem. Flam-steed was not convinced by any of them, but he had discovered a text on the lunar motion written by the brilliant young English astronomer Jeremiah Horrocks (1618–41), better known for his observation of the transit of Venus. Flamsteed found Horrocks’ theory of the lunar motion to be the best available at the time. It was not a new theory, however, having been developed by Horrocks in the late 1630s just before his death. The great mathematician Isaac Newton (1642–1727) tried to devise a more accurate theory for the motion of the Moon, based on his inverse square law of gravitation. He failed, however, because the problem actually involved the motion of three bodies—the Sun, the Earth and the Moon—and it required a unique solution.
Flamsteed spent the rest of his life working on the problem of the lunar motion, in addition to his other astronomical duties and his job as a parish priest. Flam-steed refused to be hurried and it took him many years to publish his work. He had been measuring the position of the Moon since his investiture as Astronomer Royal in 1676, but by the end of the century he had still not published his findings. There was a controversy over the long delay. Flamsteed wanted to withhold his results until they were complete, but he was taking such a long time that other researchers were delayed and his results were urgently needed by Newton, Halley (1656–1742) and others. Isaac Newton, in his capacity as president of the Royal Society, was able to press for immediate publication. In 1704 Prince George of Denmark undertook the cost of publication. Edmond Halley edited the incomplete observations and 400 copies were printed in 1712. Flam-steed was very angry at the way in which his work had been treated. He managed to purchase 300 of the copies and to ceremoniously burn them. His own version of the star catalog, the Historiae Coelestis Britannica, was published 13 years later in 1725. It listed more than 3,000 stars and gave their positions much more accurately than in any other previous work.
A New Astronomer Royal
In 1720 Edmond Halley succeeded John Flamsteed as Astronomer Royal. Halley is best known for the comet named after him, although he made several other important contributions to astronomy.
Halley spent several years on the Atlantic island of St. Helena where he studied the skies, helping to improve the charts of the stars in the Southern Hemisphere. He took a pendulum clock with him and made two interesting discoveries. One was that his clock seemed to run more slowly at St. Helena than it did in the northern latitudes—a fact that he rightly concluded was due to a lower value of “g” (the acceleration due to gravity) near the equator than at the poles. He also took his clock to the top of a mountain and found to his satisfaction that it ran a little slower at high altitude; this effect was also caused by a small change in the value of “g.” These changes were caused by centripetal forces and the fact that the Earth’s radius is larger at the equator.
It has to be said that Edmond Halley and John Flam-steed were not the greatest of friends. The main reason was their differences over religion. Flamsteed was a devout churchman whereas Halley was an atheist and made no secret of his views. His atheism very nearly prevented him from advancing his career. In 1691 he failed to obtain a professorship at Oxford because of his unorthodox views. The mathematician William Whiston (1667–1752) described the event:
I will add another Thing which I also had from Dr. Bentley himself. Mr. Halley was then thought of for successor, to be in a Mathematick Professorship at Oxford; and Bishop Stillingfleet was desired to recommend him at Court; but hearing he was a sceptick, and Banterer of Religion, he scrupled to be concerned; till his Chaplain Mr. Bentley should talk with him about it; which he did. But Mr. Halley was so sincere in his infidelity, that he would not so much as pretend to believe the Christian Religion, tho’ he thereby was likely to lose a Professorship; which he did accordingly; and it was then given to Dr. Gregory: Yet was Mr. Halley afterwards chosen into the like Professorship [of Geometry] there, without any Pretence to the belief of Christianity.
When Halley returned from his voyage to the Atlantic he could curse as fluently as any seaman. This was also too much for the orthodox Flamsteed.
A Prize for Finding the Longitude
The problem of finding the longitude at sea was proving to be far more difficult than anyone had imagined, and progress was very slow. The Board of Longitude decided to offer a prize of £20,000—a fortune in the 18th century—to anyone who could build a clock to keep good time at sea or who could solve the problem of finding the longitude by any suitable method. Thus, at the same time as the astronomers at Greenwich were working on the lunar motion, one man was devoting his life to solving the longitude problem by another method. In 1693, at Foulby in Yorkshire, a child called John Harrison was born. His father was a carpenter and the family moved to live at Barrow on Humber in Lincolnshire. As a child, young John was brought up to work with wood, and he was so precise with his woodwork that before the age of 20 he had built a working clock made almost entirely from wooden pieces.
When John Harrison (1693–1776) heard about the Board of Longitude prize he devoted his whole life to winning it. He traveled to London where he managed to get an audience with George Graham (1673–1751), the premier scientific instrument maker of the times. At first Graham did not take Harrison very seriously, but he soon realized that the younger man had many ideas in his head for the new timepiece, and that he knew how
to solve the problems involved. Graham ended up being so impressed that he gave Harrison a generous loan, with instructions for Harrison to repay the money when his finances allowed.
The first problem to be solved was to find a new regulating device for a timekeeper. The pendulum was simple and accurate but it could not cope with the swaying of a ship at sea. Harrison devised a movement with oscillating brass weights and springs that kept good time on the moving deck of a ship. He knew that higher temperatures caused a clock to run more slowly because of the expansion of the metal parts. So he experimented using a balance wheel made from a bimetallic strip of brass and steel. It was designed to retain the same radius when the temperature changed.
Harrison built five timepieces in all. His first piece (H-1) was tested in May 1741 on a six-week voyage to Lisbon and back. The outward voyage was made on the Centurion, which later became the flagship for George Anson’s circumnavigation of the world. The records show that on the return journey Harrison correctly located the ship off the Lizard, whereas the official navigator, Roger Willis, believed the ship to be at Start Point. However, Willis’ reckoning was clearly wrong, since Start Point is many, many miles further east, off the Devon coast near Salcombe. However, Harrison was not offered the prize for his timepiece. This was mainly due to jealousy on the part of the Astronomer Royal, Nevil Maskelyne (1765–1811), who wanted to win the prize with his own lunar method. The Board of Longitude was sufficiently interested to provide John Harrison with subsidies, however, and he persevered with his ideas for more than 20 years. He produced a second chronometer (H-2) in 1741, but for some reason this device was never tested at sea.
It took Harrison until 1759 to produce his third chronometer, H-3. The reason why it took him so long was that he kept encountering new ideas and improvements as he was working on it. Harrison’s fourth chronometer (H-4) was completed a year later in 1760. The Board of Longitude decided to test the two chronometers together.
A Stern Test
Eventually, the chronometer H-4 was taken on board HMS Deptford and the third chronometer H-3 was taken out of the running. Thus everything depended on H-4. The Deptford set sail for Jamaica under Captain Dudley Digges with William Harrison (1728–1815), son of John Harrison, as curator of the chronometer. Out in the Atlantic Ocean a major crisis occurred when it was discovered that the ship’s supply of beer was unfit for human consumption. To the dismay of the sailors the beer had to be thrown overboard and they were reduced to drinking water. William Harrison declared that according to the chronometer they would sight the island of Madeira the following day, where they could replenish their stocks of beer. Captain Digges disagreed, however. According to his traditional method of dead reckoning Madeira was still several days sailing away. He was willing to lay a bet on his opinion. The next morning Madeira was sighted. Dudley Digges was a good loser. He was very impressed and offered to buy the next chronometer made by the Harrisons. He wrote to John Harrison with the news:
Dear Sir,
I have just time to acquaint you with the great perfection of your watch in making the island on the Meridian; According to our log we were one degree 27 minutes to the Eastward, this I made by a French map which lays down the longitude in Teneriffe, therefore I think your watch must be right.
Adieu.
Harrison’s fourth chronometer was accurate to within an error of only 1 mile (1.6 km) in the test voyage to Jamaica—easily close enough to win the prize offered by the Board of Longitude. On a second voyage to Barbados three years later the watch did not perform quite so well but the error was still less than 10 miles (16 km). In 1765 Harrison was paid only half the £20,000 reward although he had clearly met all the requirements. He only received the other half after a protracted legal wrangle that was settled by a private Act in 1773. Even then it needed the intervention of the king before Harrison was paid. The reason for this was that the Board of Longitude was comprised mainly of jealous astronomers and mathematicians who thought that their own lunar method was the solution to the problem of finding the longitude. And indeed, the astronomical method of finding longitude was at last coming to fruition. By a strange coincidence, the two methods became available at almost the same time, although evidence shows that John Harrison was the winner by a narrow margin. While we must acknowledge the time-consuming effort put in over many years by those seeking to provide an accurate astronomical method for finding latitude, we must also applaud the perseverance and skill of John Harrison.
In the 1750s the major advance in finding the longitude by the lunar method was made when the German mathematician Tobias Mayer (1723–62), guided by the Swiss mathematician Leonhard Euler (1707–83), produced a more accurate theory for the lunar motion. Eventually, in the 1760s, the first nautical almanac was published with instructions for finding longitude from the position of the Moon in the sky—nearly a century after the foundation of the Royal Greenwich Observatory.
An exact duplicate of Harrison’s fourth chronometer was made by Larcum Kendall for the Admiralty at a cost of £450. The cost of manufacturing the chronometers reduced quickly and few years later Thomas Earnshaw (1749–1829), the inventor of a modified design, was producing chronometers for less than one-tenth of Kendall’s price.
Using the Nautical Almanac
In 1768 when Captain James Cook (1728–79) embarked on his first voyage of discovery to the Southern Hemisphere, he carried with him a professional astronomer called Charles Green (1735–71). He also carried with him a copy of the nautical almanac published by the Royal Greenwich Observatory for the determination of longitude at sea. Green was one of the few men who could find the longitude from the position of the Moon by using the tables in the almanac. Thus Cook’s Endeavour became the first ship to enter and cross the Pacific Ocean and to know her longitude on the surface of the Earth. The voyage was also intended to make another important contribution to astronomical knowledge. This was the determination of the Earth–Sun distance—the astronomical unit—from the observation of the planet Venus on the face of the Sun. The transit of Venus was successfully observed from the island of Tahiti, but the observers could not agree about the precise timing of the entry of Venus onto the face of the Sun. The problem was a phenomenon known as the “tear drop” effect, an optical illusion that showed the shadow of Venus still joined by a “thread” to the rim of the Sun when in fact it was already into the transit.
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WILLIAM HERSCHEL
Gazing Deeper into Space
There is a well-known correlation between music and mathematics, amply demonstrated in the account of the life of William Herschel (1738–1822). The son of a German musician, William Herschel was himself a professional organist before becoming fascinated by the stars—a fascination that led him to become one of the greatest of all astronomers and a pioneer in the improvement of telescopes used for watching the night sky.
Isaak Herschel was a musician in a German military regiment. In 1755, when the regiment was transferred to England for several months, Isaak took his family with him. His young son, Wilhelm, also played in the regimental orchestra, but he had not signed up as a soldier and when his father’s regiment was transferred he decided to stay in England in the hope of making a living from his musical talents. Wilhelm was successful in this endeavor, and he was employed to play in the Duke of Richmond’s private orchestra. He moved to Leeds where he remained for four years, before moving to Halifax for a short time. In December 1766 Herschel was appointed as the organist at the Octagon Chapel in the fashionable city of Bath. Wilhelm Herschel was then 28 years of age, and about this time he changed his Christian name from Wilhelm to the anglicized version of William.
Improving the Telescope
However, it was not as a musician that William Herschel became famous. But it was through the patterns in his music that Herschel became interested in mathematics, and this in turn led him on to an interest in astronomy. Soon he became fascinated by what he could see in the night sky, and he found t
hat he was spending all his spare time studying the stars. Simple telescopes were easy to come by, but after a short time they were not good enough for Herschel’s observations. No matter how far into the sky he could see he always wanted something better so he could see further. The local opticians and spectacle makers supplied him with better lenses and eyepieces, but even they were found to be of limited use.
He knew that reflecting telescopes could be purchased from London instrument makers, but they were very expensive and Herschel could not afford one. He also knew that in the previous century Isaac Newton (1642–1727) had made his own reflecting telescope. Herschel knew that the reflecting telescope had many advantages over the refracting telescopes of the time. The mirrors were free from the colored fringes that appeared in the refracting telescope, a defect known as chromatic aberration. The mirrors were easier to manufacture than lenses, and because they could be supported from the back they could also be made much larger. Herschel’s only solution was to set about casting and grinding his own mirrors. It was painstaking work. He cast larger and larger mirrors for his telescopes, but he also met with many failures. The larger the mirror the more likely it was to crack during the cooling process. Even when Herschel had cast a perfect mirror his problems were not over. He had to spend hours and hours polishing the mirrors to achieve the perfect parabolic surface. He mastered this and other techniques, and he also made his own eyepieces. He persevered with larger and larger telescopes year after year, and by the time he had mastered all the optical techniques he was building the best telescopes in the world.
The Story of Astronomy Page 12