The Story of Astronomy (17 page)

Some would say that the discovery of Uranus was the high point of Herschel's career, and in the sense that it brought him fame and fortune this is true. He was given a pension by the king and he moved to Berkshire where he was able to set up his own observatory. It was only then, when he had royal sponsorship, that he was able to make his most significant contribution to the advance of astronomy. In 1785 he and his sister Caroline moved to a house called Clay Hall in Old Windsor. Socially it was a very acceptable address, but the Herschels did not own the property and in 1786 they were obliged to move to a new residence in nearby Slough. His dwelling became known as Observatory House. He found Slough to be very
amenable, and it was there that he met his wife, a widow called Mary Pitt. On May 7, 1788 they were married at St. Laurence Church in Slough. William's sister Caroline moved into nearby lodgings and continued her work as his assistant.

During his long career it has been estimated that William Herschel made about 400 telescopes. His salary as the court astronomer was supplemented by an income from the sale of his telescopes. He became so well known as a telescope maker that his customers included the king of Spain, the prince of Canino and the Russian emperor. He made telescopes for export to Berlin and he even sold one of his instruments to China.

The largest and most famous was his great reflecting telescope with an aperture of 1.2 meters (4 ft) and a focal length of 12 meters (40 ft). The telescope was completed in February 1787 but Herschel was not satisfied with the mirror; it was too thin and it bent under its own weight of half a ton. He ordered a new disc to be cast but it cracked in the cooling process. Then he cast a third mirror 8 centimeters (3 in) thick and weighing nearly a ton. It was polished and ready for use in August 1789. It was with this telescope that Herschel discovered two moons of Saturn within the first month of observation. They
became known as Mimas and Enceladus and, as with the moons of Uranus, it was his son who chose the names long after William's death. The discovery of the moons took place just one month after news arrived in England of the storming of the Bastille in Paris and the outbreak of the French Revolution.

The great telescope needed a complex system of scaffolding and pulleys to raise it to the desired elevation. The whole structure was mounted on a large turntable that needed to be winched round to the correct azimuth to point it toward the required object in the sky. This type of mounting made it very difficult to follow a star as the Earth rotated, and consequently the great telescope was only used to follow up discoveries from smaller instruments. The smaller but more maneuverable 6-meter (20-ft) telescope was the one Herschel used for most of his observing.

Herschel wanted to know more about the distribution of the stars in the sky. To accomplish this he divided the sky into 683 regions and then set about trying to count the number of stars in each region. He produced a model showing the distribution of the stars in the Milky Way. It consisted of a great spinning disc in which he placed the Sun, our own star, near the center. His model of the
galaxy was the most advanced produced at that time but in fact it contained one of the few mistakes made by Herschel. The Sun is actually situated on one of the spiral arms of the galaxy, quite a long way from the center where Herschel put it.

Herschel was well acquainted with Newton's work on the spectrum and he was one of the first to realize that there were other colors (or wavelengths) that lay outside the visible spectrum. He passed a beam of sunlight through a glass prism and he held a thermometer just beyond the red end of the spectrum. The thermometer indicated a temperature rise. Herschel came to the conclusion that there must be an invisible form of light beyond the red wavelength. It became known as infrared radiation. Herschel was an excellent scientist but he still held some unconventional views that may have appealed to science fiction readers. He thought, for example, that every planet was inhabited by intelligent life, and he also believed that there was a temperate region beneath the surface of the Sun where there lived a race of beings with very large heads who had fully adapted to their environment.

In Herschel's time nobody had any idea of the true size of the universe. It seemed impossible that anybody would ever be able to measure a parallax for the stars. Some of the “fixed” stars were not quite in the positions given by Ptolemy and Hipparchus, however, and this gave
reliable evidence that they had moved over a time span of nearly 2000 years. The nebulous smudges of light revealed by the large telescope for the objects in the Messier catalog were thought to be clouds of luminous gas; some of the Messier objects were in fact distant galaxies much further way than the stars, but in Herschel's time no one imagined that they were any more distant than other objects in the sky. In Herschel's time the calculations of the French mathematicians Lagrange and Laplace implied that the Newtonian model of the universe reigned supreme.

In 1816 William Herschel was made a knight of the Royal Guelphic Order by the Prince Regent. In 1820 he was a founder member of the Astronomical Society of London, which became the Royal Astronomical Society in 1831. He and his wife Mary had one child, christened John, who was born at Observatory House in Slough on March 7, 1792. John Herschel had the best possible start in life for an astronomer, and he grew up to follow his father and become a leading figure in the astronomical world. While he was at Cambridge University John Herschel (1792–1871) befriended Charles Babbage (1791–1871), well known in computing circles as the designer of his difference engine, as well as the mathematician George Peacock
(1791–1858). These three started a movement to abolish what they called the “dotty notation” of the differential calculus. The notation was widely used in England but it was not as logical as the dy/dx and d
²
y/dx
²
terminology used by mathematicians on the Continent.

Between 1786 and 1802 William and Caroline Herschel compiled three catalogs giving a total of about 2500 positions of star clusters, nebulae and galaxies. Caroline's efforts as her brother's assistant were recognized by the crown, and in 1787 she was awarded a royal pension of £50 a year. She lived until 1848 and she was only a few days short of her 98th birthday when she died.

When John Herschel became involved with astronomical work one of his major tasks was the reobservation of the double stars already cataloged by his father. Using the large telescopes he was able to detect movements of some of these pairs of stars as they rotated about each other, and this was the first positive proof that Newton's law of gravitation was still valid outside the solar system. John Herschel was fortunate to find a competent collaborator called James South (1785–1867) who was wealthy enough to afford the refined instruments needed for this kind of work. The catalog compiled between 1821 and 1823 by John Herschel and James South was published by the Royal Society in 1824, and it earned them both the Gold Medal of the Royal
Astronomical Society and the Lalande Prize from the Paris Academy of Sciences.

In 1829 John Herschel married Margaret Stewart, who was to bear him a large family of three sons and nine daughters. He was very keen to complete his father's work in astronomy and he knew that his father's researches were confined to the Northern Hemisphere. He therefore decided to undertake a journey south of the equator to survey the skies not visible in England. He began planning his expedition in 1832 while he was still living at Observatory House in Slough. He took his wife and children with him, and in November of that year John and his family set sail for South Africa, taking with them a large reflecting telescope for observing faint nebulae. They also carried an extensive selection of astronomical instruments including a refracting telescope for observing double stars. The Herschel family set up home and established themselves in a farmhouse a short distance to the south of Cape Town.

For four years John Herschel studied the clear skies of the Southern Hemisphere and made excellent progress during that time. By the time he and his family embarked for home in March of 1838, John Herschel had recorded the locations of thousands of stars and in addition he had
amassed long catalogs of nebulae and double stars. He added 1700 new entries to his father's catalog, making a total number of 68,948 stars in the Herschel catalogs. After his return to England John Herschel was given the post of Master of the Mint—after Isaac Newton had held this post it was not uncommon for it to be offered to a scientist. John Herschel did not enjoy the pressures of the job at the Royal Mint, however, and he suffered a nervous breakdown in 1854.

The 19th century was a time of yet more crucial advances in the science of astronomy, marked by important events such as the discovery of another planet in the solar system. However, it was also a period of intense exploration, discovery and even controversy in the fields of physics, chemistry and geology—advances in all of which were critical for the progress of astronomy and cosmology in the decades to come.

In 1845 John Couch Adams (1819–92), a British mathematician and astronomer, had been carefully studying the irregularities in the motion of the planet Uranus. He correctly deduced that the deviations from the elliptical orbit were caused by another planet beyond Uranus, and he was able to calculate in which part of the sky the new planet could be found. He gave his data to James Challis (1803–82),
director of the Cambridge Observatory, but although the data was accurate the Cambridge astronomers were unable to find the planet. The following year the French astronomer Urbain-Jean-Joseph Le Verrier (1811–77) made a very similar calculation and he, too, predicted that there was another planet beyond the orbit of Uranus. In September 1846 Le Verrier passed his information to the Berlin astronomer Johann Gottfried Galle (1812–1910). Galle and his assistant Heinrich Louis d'Arrest (1822–75) who painstakingly constructed a star map of the part of the sky suggested by Le Verrier, and in so doing they identified what they thought to be an uncharted star. The next night the “star” had moved relative to the background stars. It was not a star at all, but the planet we now know as Neptune. Thus the British were narrowly beaten in their search to find and identify the next new planet, but on this occasion the French, who had so often lost narrowly to the British, certainly deserved their success.

At this time there was much discussion about the nature of gravity. It seemed obvious that the force of gravity was one of the most important factors in astronomy and it was this force, acting over great distances, which determined the motions of the heavens. It was obvious that light crossed the distances between the stars, but gravity
also managed to cross the distances between the planets. Moreover, according to Newton, every particle of matter in the universe attracted every other particle of matter, however small the attraction and however far apart they were from each other. Gravity was not the only force in the universe, however. In the 19th century scientists knew of two other forces, both of which had been known in one form or another for centuries. One of these was magnetism—the same force that enables a magnet to attract metal objects. The Earth itself has a magnetic field. It was easily shown that the whole Earth behaved like a giant magnet, and the north-seeking pole of the compass needle had been used for centuries to find direction at sea. It was of great value to navigation. The other force was discovered by the developing science of electrostatics. When particular substances like glass and amber were rubbed with certain kinds of cloth they acquired an electric charge. Rods charged in this way were able to attract small particles that clung to them.

Gravity seemed to be a very weak force compared with magnetism. It was possible to measure the gravitational effect of a mountain, but generally speaking it needed an object the size of the Earth to create a sizeable gravitational force. Nevertheless, gravity kept the planets in their orbits, and it seemed obvious that it was of more importance than the other forces when it came
to the science of astronomy. Magnetism, on the other hand, did not need anything as large as a planet to produce a significant effect; it could be produced in quite small objects. It also differed from gravity in one other very important aspect. A north magnetic pole could not exist without a south magnetic pole. Opposite poles attracted each other but similar poles repelled. While it was easy to demonstrate the existence of these opposing magnetic forces, no similar effect had been detected with gravity.

The electric force differed from both the gravitational and the magnetic forces. It appeared as both positive and negative charges and, as with magnetism, opposite charges were attracted to each other but like charges were repelled. Where the magnetic poles appeared in pairs of north and south, the electrostatic charges could be created in isolation without their counterpart. By the early part of the 19th century, scientists thought that electricity and magnetism must be related to each other, although the exact nature of the relationship was not known.

Two British scientists are given much of the credit for advancing our knowledge of electricity and magnetism. One was Michael Faraday (1791–1867), who was born at Newington in Kent, the son of a local blacksmith. Faraday was a self-educated man who in 1812 became
the laboratory assistant to Sir Humphrey Davy (1778–1829) at the Royal Institution in London. Faraday was the embodiment of the practical genius; his father taught him a lot about working metals, and he was an excellent laboratory assistant and a very skilled craftsman. Moreover, he was much more than a technician. He formulated theories of his own and he contrived experiments to prove his theories. He knew that magnetism could be used to create electricity, and he also knew that electricity could produce magnetism. He put his knowledge to practical use when he built a machine to generate electricity from rotary motion. It was the first electrical generator.