The Story of Astronomy (18 page)

Faraday was not an astronomer, but it was through his work that James Clerk Maxwell (1831–79), the Scottish physicist, was able to discover the close relationship between magnetism and electricity. Maxwell formulated a set of equations relating the magnetic and electric fields, and toward the end of the 19th century his work became very valuable to astronomy. The equations showed that where it had been assumed that electrical and magnetic forces were different, they were in fact the same force—one could not exist without the other. To the astronomer, one important aspect of Maxwell's work was that his equations predicted the existence of other wavelengths of radiation, all traveling at the speed of light. Maxwell was working inside the electromagnetic spectrum at
wavelengths in the microwave region, yet his equations held for radiation of any wavelength. Visible light had a wavelength of between 10
^−7
and 10
^−6
meters. Radio waves, which were not discovered until after Maxwell's time, have a wavelength ranging from about 1 meter to 10
⁵
meters. X-rays were discovered soon after radio waves, and they have wavelengths in the range 10
^−9
to 10
^−2
meters. Microwaves were then discovered, and they have wavelengths between 10
^−3
and 10
^−1
meters. The electromagnetic spectrum covered an incredibly wide range of wavelengths. But all forms of radiation, regardless of their different wavelengths and corresponding frequencies, always traveled at the speed of light and obeyed the equations of James Clerk Maxwell. Until the middle of the 20th century, however, any form of radiation outside the optical spectrum meant very little to astronomers. There was also a major problem with terrestrial-based observations, and this was the Earth's atmosphere itself. Our eyes had evolved to become sensitive to light in the visible part of the spectrum precisely because these were the wavelengths that the atmosphere allowed to filter through. Radiation at other wavelengths was invisible to our eyes, but even if we had evolved organs to see the radiation it would still not be visible on the Earth's surface because the atmosphere blocked it out.

Great advances were also made in the field of chemistry during the 19th century. One of the pioneers was John Dalton (1766–1844), who was born in Cumberland and who spent much of his working life in Manchester. He revived the ancient Greek concept of atoms, but he brought the idea up to date and he developed it to become an exact science. Thus the idea of the atom was formulated in chemistry long before the physicists came to study it. By use of careful measurements Dalton was able to calculate ratios of the masses of the atoms of the elements. He studied many chemical reactions and he calculated the relative masses of all the components. He was able to create simple compounds such as carbon dioxide directly from the elements of carbon and oxygen, and he began the process whereby all chemical compounds became known by a formula giving the ratio of the elements they were made from. For example water was H
₂
O, carbon dioxide was CO
₂
and methane was CH
₄
. His book,
A New System of Chemical Philosophy
, published in 1808, was a landmark in science and well ahead of its time. The idea of elements and compounds became well established and so, too, were the atomic weights of the various elements.

Dalton identified only 20 distinct elements in his early work, but more elements were discovered as his ideas continued to develop. By the middle of the 19th century,
when the Russian scientist Dmitry Mendeleyev (1834–1907) came to study the subject, 63 elements had been identified. Mendeleyev devised a system in which he wrote the name of each element on a card and then tried to group the cards together to identify elements with similar chemical properties. He produced the earliest periodic table of the elements. In the periodic table, the known elements were put into groups that reflected their properties, similarities and differences. Sometimes Mendeleyev discovered gaps in the periodic table—this was an indication that an element was missing. In each case the missing element was eventually discovered.

It became known from experiment that every element had a set of spectral lines associated with it. These spectra were first determined in the laboratory by the simple method of burning the chemicals on a platinum wire using a Bunsen burner and focusing the light to pass through a triangular prism onto a screen. It became evident to the astronomers that very similar sets of spectral lines could also be seen in the Sun and the stars. This was proof that the stars were composed of the same elements as the ones on Earth. The most abundant element in the Sun was hydrogen, and the spectrum of hydrogen in particular proved to be of great importance.

There were other important developments in astronomy about this time, and in the 1830s and 1840s came new evidence for the scale of the universe. Two different units evolved with which to measure the distance to the stars. One was the light year: this was the distance traveled by light in a year. The other unit was the parsec: this was the term formulated by the first astronomers to measure stellar parallax. The parsec is the distance to an imaginary star from which the maximum angular separation between the Earth and the Sun would appear to be 1 second of arc. A parsec is about 3.26 light years. It is a kind of reciprocal measure, the greater the number of parsecs the smaller the angle to be measured. For example, the Earth/Sun radius subtends an angle to 1 second of arc for a star at a distance of 1 parsec, but an angle of only 1/10th of a second of arc (not 10 seconds of arc) for a star at a distance of 10 parsecs. In 1838 the German astronomer Friedrich W. Bessel (1784–1846) made a direct measurement of the distance from the Earth to the star 61-Cygni, which he calculated to be situated at 3 parsecs. This is equivalent to a distance of 11.4 light years.

Relatively few stars in the night sky are close enough to be measured by the method of parallax. Fortunately for astronomers other methods of estimating distance became available in the 20th century, but all the new
methods have to be calibrated by the stellar parallax method. The more stars that could be measured by parallax, then the more accurate the other methods became.

At this time few astronomers thought that the Earth was as old as the universe. However, the age of the Earth was still an important factor in the science of astronomy, as was the age of the Sun. It did not need a study of the skies to work out the age of the Earth; the rocks themselves were readily on hand for our examination. The biblical date for the creation, worked out from the genealogies in the Bible, was a mere few thousand years. According to theologians, the creation was calculated to have taken place in 4004
BC
. In the 19th century this date was already under attack from archaeologists who discovered that some of the artifacts of ancient Egypt appeared to predate the creation. By the middle of the 19th century the literal interpretation of the Bible was being undermined even more, and it was seriously challenged by the theories of Charles Darwin (1809–82), formulated in the 1840s but remaining unpublished for 30 years. If Darwin's theories were true, it would have needed millions of years to allow for the evolution of species and for the development of mankind. There were
other fields where long timescales were needed to explain their occurrence. The fossil record, for example, supported Darwin's theory of evolution and it, too, required an Earth timescale of many millions of years. The geological record pointed to an Earth that was hundreds of millions of years old. There was plenty of evidence to support the idea of a very ancient Earth.

There was one factor, however, that seemed to support the biblical account and to restrict the age of the Earth to a few thousand years, and this was the age of the Sun. It was not difficult to work out the immense amount of energy produced every second by the Sun. It did not matter how efficient the Sun was at producing light. Nor did it matter what combustible materials it had in its core. There seemed no way it could continue to burn at its current rate for more than a few thousand years without exhausting all its fuel. Life on Earth was not possible without the light and heat from the Sun. Indeed, Lord Kelvin (1824–1907) had “proved” that the Sun would not be able to shine for more than a few thousand years—but of course he was basing his calculations on 19th-century science; he knew nothing of how energy is produced in the Sun by the process of nuclear fusion.

The British geologist Charles Lyell (1797–1875) was amongst the first to study the geological formations of the Earth and to try to establish dates for the geological
record. From his travels in France and Italy he was able to observe a great many varied geological features and to work out how long it took for the sedimentary layers to form. In several places he discovered fossils of sea creatures high in the mountains, and this convinced him that the Earth's crust was moving very slowly but continuously all the time. Lyell's
Principles of Geology
was published in three volumes from 1830 to 1833. His system for determining dates for the geological record was based on two propositions. One was that the processes of geological change included all the causes that have acted from the earliest time. The other axiom was that these causes have always operated at the same average levels of energy. These two propositions suggest a “steady-state” theory of the Earth. Changes in climate existed but they fluctuated around a mean, and they reflected the changes in the position of land and sea. Lyell was able to put the various rocks into a chronological order in terms of their formation, and he became convinced that the age of the Earth was measured not in terms of a few thousand years but on a timescale of hundreds of millions of years.

The technique of taking photographs of astronomical objects like the Moon, the planets, the stars and all the other bodies in space is known as astrophotography. As
photographic techniques developed, astronomers were quick to realize that the camera could be of great benefit to them in their studies of the heavens. English-born American scientist John Draper (1811–82) took the first photographs of the Moon in 1840. They showed lunar features such as the craters and “seas.” His son, Henry (1837–82), also took pictures of the Moon and was the first person to create an image of a stellar nebula when he photographed the Orion Nebula in 1880.

The first photographs taken of the stars were disappointing, however: all that could be seen of the image were pinpoints of light. But photography proved to be very useful for the measurement of stellar parallax. In this technique bright stars were photographed against their dimmer and most distant companions. Then the photograph was repeated six months later using the same telescope and camera. The Earth had moved to the opposite end of its orbit during this period, and the nearer stars had therefore appeared to move by a small amount against their background. This tiny parallax was sufficient to make an estimate of how far away they were from the Earth.

As the method developed, an even better arrangement was devised. Astronomers flashed each image alternately onto a screen. Any star that had moved during the time the two exposures were taken could be identified
immediately by its flicker. The star had a measurable parallax on the photographic plates and thus its distance could easily be calculated.

Today, astrophotography is an essential part of astronomy, and equipment such as the Hubble Space Telescope has revolutionized our understanding of space.

As photographic techniques improved astronomers soon had other instruments at their disposal to provide even more information. A good example is the spectrograph. Stars had long been categorized by color and brightness, but these were no longer the only criteria by which to measure the light from a star. The optical spectrum showed the light emitted at different frequencies, with well-defined spectral lines corresponding to hydrogen and other elements, and it enabled the stars to be classified by their different spectral types. Spectroscopy became one of the most useful tools to the astronomer.

Spectroscopy made great strides in the second half of the 19th century. In the 1880s Edward Pickering (1846–1919), working at Harvard College, established the Harvard photometry catalog. This was the first catalog to classify stars by photographic techniques. In Italy, a Jesuit priest called Angelo Secchi (1818–78) identified four distinct spectral types of star, and he became the first
to classify the stars by their spectral properties. He was expelled from Italy by the revolution in 1848 and spent a short time at Stonyhurst College in England before moving on to Georgetown University in Washington DC. The Italian government recognized his work, and they allowed him to return to Rome in 1849 where he became professor of astronomy and director of the observatory at the Roman College.

The nature of light was one of the many conundrums puzzling physicists. Did it consist of a stream of small particles, or was it a vibration in a medium that filled the whole of space? The particle theory to explain the transmission of light still prevailed, but opinion was swinging toward a theory based on wave motion. Wave motions were well understood; the sea could carry waves of widely different wavelengths and the air could carry sound waves with a wide range of frequencies. The problem for the astronomer, in attempting to understand how light traveled through space, was that waves needed a medium in which to travel. Thus, if light was a wave motion it followed that space must be filled with some kind of medium that could carry the vibrations of the wave, just as the air carried waves of sound. The medium was given a name; it was known as the ether. The ether was assumed to
exist everywhere in the universe. When a ship traveled over the sea it had a velocity with respect to the sea. When a bird flew through the air it, too, had a velocity relative to the air. When the Earth traveled through the ether then it must have a velocity with respect to the ether. Scientists wanted to measure this velocity and thereby gain knowledge of the ether.