Nineteenth-Century Efforts to Catalog Stars

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Nineteenth-Century Efforts to Catalog Stars

Overview

Among the many achievements of nineteenth-century astronomy was the production of a large number of comprehensive and accurate star catalogs. Building upon the less precise observations of previous centuries, European astronomers in the nineteenth century took advantage of better telescopes and new technologies to produce their improved catalogs. Yet their projects to map, chart, and catalog stars affected more than just the science of astronomy. Their efforts would help advance navigation and exploration and promote international co-operation among astronomers of different nationalities during a period of tension between the nations of Europe.

Background

The production of star catalogs is an ancient practice. Many pre-historic societies around the world had some method of charting or recording the movements of the stars, planets, the Moon, and the Sun. These societies even learned that it was possible to navigate by using the stars. The early Greeks produced star catalogs from naked eye observations that were used for centuries in Europe, Asia, and Africa. Centuries later Nicolaus Copernicus (1473-1543) argued that Earth goes around the Sun instead of the Sun going around Earth. One implication of this new theory was that the stars must be much farther away from Earth than was previously thought. After Copernicus European astronomers began to study the stars more carefully, cataloging them with newfound interest. Even before the use of telescopes, astronomers such as Tycho Brahe (1546-1601) of Denmark and his assistant Johann Kepler (1571-1630) of Germany gathered tremendous amounts of data on star positions. Galileo Galilei (1564-1642) of Italy, perhaps the first European to turn a telescope on the stars, immediately recognized the advantage of using telescopes to explore and chart the heavens.

If the centuries following Copernicus saw an increase in the exploration of the heavens, it is interesting to note that there was a corresponding increase in the exploration of Earth. After Christopher Columbus's rediscovery of the New World in 1492, European nations began to explore and conquer the rest of the world. The ability of their ships to navigate the oceans was crucial to the task of building their overseas empires. Having accurate star catalogs was an important part of navigation for powerful countries such as Spain and England, who developed have the largest empires.

England provides us with a good example of the connection between navigation and astronomy. In the seventeenth and eighteenth centuries English astronomers John Flamsteed (1646-1719), Edmond Halley (1656-1742), and James Bradley (1692-1762) all helped to compile star catalogs that were used to aid English ships as they explored the globe. Each of these three astronomers worked as England's Astronomer Royal at some point in his career, a position that in part required the astronomer to provide the English navy with star charts for navigational purposes. Bradley in particular made some of the most accurate and extensive star observations of his day between the years 1750 and 1762. In this way the work of these English astronomers was closely tied to their country's tasks of exploration, navigation, and empire-building.

Also during the seventeenth and eighteenth centuries, newer, larger telescopes enabled astronomers such as William Herschel (1738-1822) to see farther than ever before. Some objects that previously looked like individual stars turned out to be double stars, clusters of many stars, or even nebulae (interstellar gas or dust). Both William Herschel and Charles Messier (1730-1817) produced catalogs of star clusters and nebulae, adding to the numerous star catalogs already available. Before the nineteenth century, then, there had been many star catalogs produced in Europe. Nineteenth-century astronomers would take these catalogs, build upon them, refine their measurements for accuracy, and vastly increase the number of stars and other objects cataloged.

Impact

The centuries before 1800 was a time when the cataloging of stars was a somewhat disorganized, if still productive, activity. The nineteenth century was a time of increased organization and production. With improved skills and better technologies, nineteenth century astronomers began to look at the catalogs of previous centuries and redo them with more accuracy. Two important astronomers did this: Friedrich Bessel (1784-1846) and John Herschel (1792-1871).

Friedrich Bessel started out as a merchant's clerk in Germany. But he wanted to become a sea-trader so he could have a better life, so he taught himself navigation and astronomy, the two skills needed by a sea-trader. Soon he became an expert in astronomical theory and went to work for a number of different German observatories. At one of these observatories he obtained the catalog made by James Bradley (mentioned above). Bradley's eighteenth-century observations of stars were the best available. With new instruments and mathematical techniques Bessel set about reworking Bradley's observations to make them more accurate. When he was done he had produced a catalog of more than 3,000 stars. This catalog, which he called the Fundamenta astronomiae, would be one of the most important star catalogs produced in the nineteenth century.

John Herschel also took the catalogs of a previous astronomer and reworked them. This astronomer was his father, William Herschel. William had cataloged thousands of double stars, star clusters, and nebulae, but only those visible in the Northern Hemisphere. John Herschel, who grew up surrounded by his father's telescopes, decided to complete his father's work by cataloging the night sky of the Southern Hemisphere. First he resurveyed the northern heavens, finding many objects that his father had overlooked. In 1833 he published a new catalog of the northern heavens that contained 2306 objects.

John then decided to go to the Southern Hemisphere to catalog the double stars, star clusters, and nebulae there. For more than four years he lived in Cape Town, an English colony at the southern tip of Africa. Although Cape Town had it's own observatory, built to help the English navigate the oceans, John had brought his own telescopes to Africa. In addition to cataloging stars and other objects in the southern sky, he spent time exploring the country, hunting for plants and animals, and watching the English and other Europeans interact with the native Africans. He explored both the heavens and the earth at Cape Town, and he considered himself to be a hunter of stars and nebulae as well as plants and animals.

But when John Herschel saw how poorly the Europeans treated the native Africans he changed this view. After he had observed how some cruel Europeans hunted the African peoples, and how the Africans in turn would attack the Europeans, he no longer wanted to think of himself as a hunter. He began to see his search for stars and nebulae to be more like that of a farmer who harvests crops rather than a hunter who kills his prey. Here, then, we have an example of a nineteenth-century astronomer whose interaction with a different culture caused him to change the way he looked at astronomy. When he was done his observations became the most important catalog ever produced of the objects in the southern heavens. Astronomers around the world would use it for years to come.

John Herschel and Friedrich Bessel are just two examples of astronomers who produced important catalogs in the nineteenth century. The second half of the nineteenth century saw the further production of still more massive star catalogs. Each of these contained thousands of entries for the positions of stars, star clusters, and nebulae. The nebulae were of special interest to astronomers, and in 1864 John Herschel published his General Catalog, which contained positions for 5,000 of these objects. In 1888 John Dreyer (1852-1926) published the New General Catalog, a larger catalog of more than 13,000 nebulae. Not everyone was concerned with nebulae, however. Two German astronomers managed to produce a gigantic catalog containing nearly 325,000 individual stars. This catalog, done by Friedrich Argelander (1799-1875) and Eduard Schönfeld (1828-1891), was called the Bonner Durchmusterung and is still used today.

We have seen that nineteenth-century catalogs included many objects besides individual stars. The observation and cataloging of double stars, star clusters, and nebulae was an important part of nineteenth-century astronomy. It is also important to remember that all of the catalogs produced represent the work of many astronomers from different centuries and different countries. Because most catalogs were built upon the work of previous astronomers, no single astronomer could ever hope to be as productive as an entire community. The efforts made to catalog stars in the nineteenth century were cooperative; they were not always the work of one or a few astronomers. There are many examples of this co-operation.

In 1868 Friedrich Argelander persuaded the German Astronomical Society to lead an international program, the goal of which was to determine star positions to a high degree of precision. This project would involve about twenty observatories and resulted in considerable international cooperation among the world-wide astronomical community. And in Paris in 1887 an international congress of astronomers agreed to begin a survey of the entire night sky using the relatively new technology of photography. Eighteen observatories around the world participated, and many stars never before seen were cataloged. Both of these international projects took place at a time when the European political situation was going from bad to worse.

Near the end of the nineteenth century the nations of Europe were arguing among themselves over the ownership of other parts of the world. But in these international astronomical projects of 1868 and 1887 we can see that an international community of European astronomers was able to work together. They united the many distant observatories, those controlled by different countries, in the common project of observing the stars and improving star catalogs. The production of star catalogs in the nineteenth century was, in the end, the result of an astronomical community that cut across time and across national borders.

STEVE RUSKIN