The Story of Astronomy (15 page)

Even in the ancient world, distances north and south of the equator could easily be calculated by observing the Sun. The measurement of the elevation of the midday Sun above the horizon was all that the navigator needed to find his latitude. The calculation was affected by the various seasons, but it was a relatively straightforward task to build in a correction factor to allow for the time
of the year. At night it was even easier to find latitude; it was simply a matter of measuring the height of the Pole Star above the horizon. With the invention of the astrolabe, the identification of any known star enabled the latitude to be calculated. The only time that latitude could not be found was when prolonged cloud cover meant that none of the heavenly bodies was visible.

Distances around the Mediterranean could be measured directly along the coastlines with comparative ease simply by pacing out the distance, or sometimes by triangulation. Finding the longitude was therefore not a serious problem so long as land was in sight. It was when the ships began to explore beyond the sight of land or outside the Mediterranean that finding the longitude became a requirement. The problem was not confined to sea voyages; the longitudes of distant places reached by land journeys, such as China and Japan, were sometimes estimated with errors as much as 90 degrees away from the true values. It was small wonder that when Christopher Columbus (1451–1506) sailed to the west he thought the land he discovered lay off the coast of the Indies.

Thus finding the longitude, the east–west distance from
a fixed meridian, was a far more difficult task than finding latitude, but it was still essentially a case of knowing the local time and comparing it with the time at a zero meridian. Local time could be calculated from the position of the Sun at noon, but finding the time at the zero meridian was a very different matter. The English calculated longitudes from the Greenwich meridian and the French worked from the Paris meridian. By the 17th century both countries were fully aware that there were astronomical events in the skies that could be used for determining the time at the zero meridian by an observer at another point on the Earth. The most obvious was an eclipse. Astronomers could predict the path of a total solar eclipse and the time at any point along the path. Such an eclipse was a rare event, but an eclipse of the Moon was much more common and was almost as useful. In many ways it was actually better, and it could be seen wherever the Moon was visible. The eclipse was an excellent method for determining longitude on land, but the navigator would need an eclipse every night to help with the problem of longitude at sea.

The English opted for using the position of the Moon to help navigators calculate the time at Greenwich; it was the most prominent object in the night sky and it could easily be observed from a ship at sea. As we have seen, the French developed a different method; they studied
the motion of the moons of Jupiter. They reasoned that the moons could be used as a clock in the sky, and if they could produce tables of the regular eclipses of the moons by observing them passing in or out of the shadow of the giant planet, then navigators could use the published tables to calculate the time at the Paris meridian.

The French method of using the Jovian satellites was the simpler of the two approaches. Galileo (1564–1642) was the first to observe the satellites of Jupiter using his recently invented telescope, and he was quick to realize that the satellites kept regular time as they orbited their planet. Tables were constructed to predict the times when the satellites entered the shadow of Jupiter, and Galileo went on to design a special helmet for finding the longitude—it had a telescope attached to one of the eyeholes. The method was not easy to use on land, but on the swaying deck of a ship at sea it was almost impossible to see Jupiter let alone the satellites. The observation was so difficult that even Galileo had to admit that the pounding of the observer's heart could cause the whole of the planet to jump out of the telescope's field of view.

The English fared a little better with their approach. The Moon was the most prominent object in the night sky and it moved across 12 degrees of sky in the course of a day. All that was necessary was to produce tables showing the position of the Moon at any time of day or
night. By observing the background stars nearest to the Moon its position in the sky could be calculated. The tables in the nautical almanac could then be used to calculate the time at Greenwich, and hence the longitude. In daytime, if the Sun and the Moon were both visible in the sky, then all that was required was to measure the angular distance between them.

In 1675 a decision was made by Charles II and his advisers to build an astronomical observatory in the Royal Park at Greenwich, a village about 5 miles (8 km) from the center of London. This was an obvious location for the new observatory, for it was sufficiently far from London not to be troubled by the smoke of the city, and it was right at the heart of the shipping being on the banks of the River Thames. There was a small hill in the location where in the distant past there had once stood a Norman castle, and Christopher Wren (1632–1723) thought this was the ideal site. On June 22, 1675 a royal warrant addressed to the Master General of the Ordnance outlined the plans and purpose for the new observatory:

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.

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.

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.

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.