The Story of Astronomy (16 page)

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.

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.

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.

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.

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 that 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.

Herschel was ably supported in his endeavors by his sister, Caroline (1750–1848), who was also a musician and a professional singer. Caroline undertook all the household duties at their Bath residence in New King Street, and she made sure that her fanatical brother was properly
fed. She also spent hours and hours helping to grind and polish the mirrors to perfection. Without the support of his sister, Herschel would never have become the most famous astronomer of his time.

When his first reflecting telescope was ready for observing, one of the first tasks tackled by William Herschel was to study the Moon. He knew the distance to the Moon, so he could estimate the height of the mountains from the shadows they cast on the surface. He made a survey and he drew maps of his findings. Then he looked deeper into space. His reflecting telescope could see far more stars than were visible with the naked eye and more than were listed in the most up-to-date star catalogs. He became interested in the different types of stars, and he studied objects such as nebulae (gas clouds) that were not stars at all. He soon realized that a large number of stars were not single stars like our Sun but were paired, or binary, stars. Others were triple stars or even clusters of several stars all rotating about each other. He went on to make a detailed study of the binary stars, and he was able to distinguish genuine binary stars from others that looked to be paired or clustered but in reality just happened to lie in the same direction and were in fact light years apart.

With his reflecting telescope William Herschel was able to see much further into the sky than others before him. His catalog of binary stars eventually grew to 700 entries, and soon the number of known double stars in the sky grew to nearly 10 percent of the number of single stars like the Sun. He discovered many new nebulae to add to the French astronomer Charles Messier's collection, and the extended catalog added up to about 250 objects. His sister Caroline was keen to do more than look after the house and polish telescope mirrors and so he supplied her with one of his telescopes. It was not long before Caroline Herschel had also discovered new objects to add to the Messier catalog, plus binary stars and several new comets.

The Herschels became well known in Bath, and sometimes received visits from local residents and others who wanted to talk about astronomy and look through the telescope. One of the visitors was none other than Nevil Maskelyne (1732–1811), the Astronomer Royal and director of the Royal Greenwich Observatory.

The Herschels made excellent progress as they continued to map the skies and build up catalogs of new objects. Both were fanatical about their work and for years they were happy to be making additions to the catalogs. Then, on March 13, 1781, William Herschel made a discovery
that was every astronomer's dream. For some time he had been watching a star that he suspected was moving against the background of the other stars. It had no tail and so it was certainly not a comet. Nor was it listed in the Messier catalog. After more observation Herschel became convinced that the object really was moving, and furthermore he was also convinced that it was moving in a nearly circular orbit. There seemed to be only one possible explanation for the motion. William Herschel had discovered a new planet!

The discovery of a new planet launched Herschel's career. The invention of the telescope had paved the way for countless additional stars to be detected by astronomers. Many new comets and even other galaxies had been discovered, but nobody had found a new planet since earliest times. It was thought that all the planets had been discovered at the dawn of astronomy, and thousands of years of gazing at the heavens since then had not changed this belief. When the position and direction of motion of Herschel's new planet had been measured it was easy for other astronomers to verify the find. As the path of the new object in the sky was plotted, there was no doubt that it was further from the Sun than Saturn. After a time it became obvious that it was obeying Kepler's and Newton's laws, and it was following an elliptical course around the Sun. Now all that remained was to name the
new planet. Some suggested that it be called planet Herschel. Others suggested the rather pompous name of Georgium Sidus to honor King George III, but after his incompetent handling of the American Revolution George was not the most popular of kings.

In the best classical style the name Uranus was eventually agreed upon. A few years later Herschel discovered two moons orbiting Uranus and they became known as Titania and Oberon, thereby seemingly disclosing his taste for the plays of William Shakespeare. William Herschel seems to have been too modest to name the moons himself and the names were actually given many years later by his son John.