William Herschel and the Discovery of the Planet Uranus

views updated

William Herschel and the Discovery of the Planet Uranus

Overview

William Herschel (1738-1822) discovered the planet Uranus in 1781. It was the first planet discovered since the beginning of recorded history. The discovery of Uranus brought Herschel much fame, which enabled him to carry out his unconventional astronomical research. His discovery of Uranus, to a small degree, even consoled England for its loss of the 13 colonies in the American Revolutionary War. Perhaps most importantly, the discovery of Uranus opened up a new phase in the discovery of the planets of our solar system.

Background

The discovery of the planets in our solar system can be said to have two distinct phases. The discovery of the planet Uranus marks the boundary between these two phases. The first phase began before recorded history. In this phase the only known planets were the five that were visible to the naked eye. Nearly every culture had knowledge of the planets we call Mercury, Venus, Mars, Jupiter, and Saturn. These five objects were first called planets by the Greeks; planet was the Greek word for wanderer. Planets were called wanderers because, unlike the stars (which do not appear to change their positions relative to one another), the planets were seen to move (or change position) relative to the fixed stars. We call this motion the orbit of planets.

Prior to the death of Nicholas Copernicus in 1543 our Earth was not considered to be one of the planets. Instead, it was thought to be the center of the universe with the Moon, Sun, planets, and stars revolving around it. No one suspected that there might be other planets besides Mercury, Venus, Mars, Jupiter, and Saturn. But there are, as we now know—Uranus, Neptune, and Pluto. And the discovery of the first planet beyond Saturn, the planet Uranus, marks the beginning of the second phase of the discovery of the planets in our solar system. Uranus was discovered in 1781 by a German astronomer living in England. His name was William Herschel.

Herschel was born in Hanover, Germany, to a family of musicians. In 1757 he moved to England. In 1766 he obtained a permanent position as an organist in the English city called Bath. But William Herschel was a man of wide interests outside of music, and astronomy was one of them. He read and mastered a number of books on astronomy. In 1767 he began to observe the night sky with small telescopes. But they were poor instruments and soon frustrated him. Unfortunately for Herschel, larger telescopes were not available. So, in 1773, he began to build his own. Working at night when his musical duties were over, he built his own telescopes and observed the heavens. Before long, with the help of his brother Alexander and sister Caroline, he was building the best telescopes in the world and seeing farther and farther into space. He was soon much more interested in astronomy than in music.

Herschel observed many different objects in the heavens. He searched for double stars, and also for the fuzzy-looking objects called nebulae (which we now know to be either star clusters, clouds of gas, or distant galaxies). He was very much like a naturalist, someone who collects and categorizes specimens. He "collected" double stars and nebulae in the same way that naturalists collect insects or plants. In this way he was rather unlike other astronomers of his day. They were interested in charting the positions of stars or calculating the orbits of planets according to the laws of gravity.

William Herschel also observed the Moon, Sun, and planets of our solar system. He believed, as did many people during his day, that the planets, Moon, and even the Sun were inhabited. It was widely believed at the time that God would have created intelligent life on other worlds. Herschel believed very strongly in the possibility of extra-terrestrial intelligent life, and he spent many nights with his telescope watching the Moon, Sun, and the planets for signs of it.

Impact

Throughout the 1770s William Herschel spent most nights observing nebulae, looking for double stars, and searching for signs of life on other worlds. Then, on the night of 13 March 1781, Herschel made an interesting discovery. While observing the area in the constellation of Gemini, he noticed a large object that he thought was a nebula or a comet. Four nights later he noticed that this object had moved, and so he concluded it was in fact a comet. He wrote of this new "comet" to astronomers in London. But the comet was difficult for those astronomers to detect because their telescopes were inferior to Herschel's. They simply couldn't see it at first. But eventually the supposed comet was observed by both English and French astronomers. They were astounded that Herschel had even seen the object, as it was so faint in their own telescopes.

About a month later the Astronomer Royal in London, Nevil Maskelyne (1732-1811), suggested that the object might be a planet. But it was hard to tell. In order to be sure, the object had to be observed over time to chart its orbit. And then calculations of that orbit had to be made. This took a few more months. Finally, in the summer of 1781, Anders Johan Lexell (1740-1784), a Russian astronomer who was visiting London, calculated an orbit for the comet. Lexell's calculations convinced most astronomers that Herschel's "comet" was in fact a planet—the first planet discovered since the beginning of recorded history!

Herschel's discovery of the planet made him instantly famous. He named the planet Georgium sidus, or "George's Star," for King George III of England. George III granted Herschel an annual salary, which meant that he could give up music for a living and turn to astronomy full time. Herschel moved from Bath to Windsor, near London, where the king lived. Herschel was also made a member of the Royal Society of London and given its highest award, the Copley Medal. His telescopes were suddenly in demand, and he made a small fortune selling telescopes throughout Europe. His customers included the King of Spain as well as Lucien Bonaparte, the brother of Napoleon Bonaparte.

The naming of the new planet is an interesting story. Initially, as has been mentioned, the planet was called "George's Star." In time, other names for the planet were suggested, because many astronomers outside of England thought the name "George's Star" was simply too English, an unfair extension of English terrestrial imperialism into the heavens. Some astronomers suggested calling the planet "Herschel," and others liked the name "Neptune." But in the end the name "Uranus" was chosen. This kept with the theme of naming the planets after the Roman gods: Mercury, Venus, Mars, Jupiter, and Saturn. Using the name Uranus continued the family lineage of those gods. In Roman mythology Uranus was the father of Saturn and the grandfather of Jupiter. Thus Uranus was a perfect name, because the planet Uranus was the next planet after Saturn and the second planet after Jupiter—grandfather, son, and grandson.

Even though William Herschel had become famous for discovering Uranus, he was still seen as a very different sort of astronomer from other astronomers of his day. The goal of most astronomers was to chart the positions of planets and stars in the sky using mathematics. Herschel, however, became interested in the structure of heavenly objects—how objects in the heavens came to be formed. In other words, he was interested in the evolution of stars, and not their positions. Other astronomers thought he was an oddball. Because he had discovered Uranus, however, he had money and respectability, and was able to continue with his unique program of astronomical observation. For Herschel, the discovery of Uranus was not the high point of his career. It was instead a fortunate event that brought him enough wealth to study independently

It is interesting to note how much time William Herschel spent looking for extra-terres-trial life. He was a firm believer that the Moon, Sun and planets of our solar system were inhabited. This belief was shared by many others of his day, and it became a political issue as well. In 1781, when Herschel discovered Uranus, England was at the end of the war with its colonies in North America—the war known as the American Revolution. When the war finally ended, the American colonists were victorious and England had lost a large portion of its empire. But some Englishmen thought that William Herschel had in a way opened up new territory to English control—the territory of the heavens. William Herschel's discovery of Uranus was a consolation for the loss of the American colonies. As one English physician said: "It is true that we have lost the terra firma of the Thirteen Colonies in America, but we ought to be satisfied with having gained in return by the generalship of Dr. Herschel a terra incognita of much greater extent...."

William Herschel's discovery of Uranus freed him to do astronomy his own way. And his discovery also consoled the English after their loss of the American colonies. Perhaps most important for the science of astronomy is that the discovery of Uranus began the search for other undiscovered planets in our solar system. It was the beginning of a second phase in the discovery of planets, during which Neptune and Pluto were discovered. Neptune was discovered in 1846 when astronomers were trying solve a problem with their calculations of Uranus's orbit. And Pluto, the last planet in our solar system, was discovered in 1930 with the aid of a telescope and photographic equipment. But that was not the end of the discovery of new planets. It might be said that a third phase has begun—the search recently and successfully undertaken to discover planets orbiting other stars.

STEVE RUSKIN