This Month in Physics History
March 13, 1781: Herschel Discovers Uranus
Frederick William Herschel
Frederick William Herschel began life modestly as one of three children in a German family of musicians, but eventually became one of the most notable observers in the history of astronomy. Best known for his discovery of the planet Uranus, it was Herschel's systematic survey of the sky that turned out to be one of his most important accomplishments, resulting in a catalogue of nebulae that increased those then known from about 100 to 2500.
Born in Hanover, Germany, in 1738, Herschel served briefly in the German military but fled to England at age 19 with the help of his father, earning a living first as a military band-master, and then as an organist and music teacher at the Octagon Chapel in Bath, England. He later began composing military music, symphonies and choral works, and performing, proving to be quite successful as a musician. In his leisure he devoted himself to the study of foreign languages, philosophy and mathematics.
However, as he matured, Herschel developed an avid interest in astronomy when he read Smith's Compleat System of Opticks and Ferguson's Astronomy. He initially rented a small reflecting telescope to observe the heavens. Since he lacked the funds to purchase a larger telescope, he decided to build his own, with the help of his brother and sister, Alexander and Caroline, who had since joined him in England. This ultimately led to the construction of his largest telescope, a 48-inch reflector. But most of his recorded observations were made with his 20-foot reflecting telescope. The large reflecting telescopes that he constructed, including one with a 40- foot focal length, far surpassed in size those of his contemporaries.
Herschel's skill at devising his own instruments helped ensure his success as an astronomer. His first major discoveries were that Mars and Jupiter exhibit axial rotation. But on March 13, 1781, while scanning the skies with a 7-inch reflecting telescope in an attempt to determine stellar parallax, Herschel observed an unusual disk- shaped object, which he initially thought was a comet. For the next few months he continued making observations and calculations, and discovered that the object's orbit was fairly circular and lay well beyond the orbit of the planet Saturn. He concluded that it was, in fact, a planet, which the astronomical community eventually named Uranus, after the mythological god of the skies. His discovery-the first new planet discovered since Antiquity- brought immediate celebrity and earned him a pension of 200 pounds a year and a knighthood from King George III, who also made him "king's astronomer." This enabled Herschel to devote himself full-time to astronomy.
Herschel's subsequent observations and discoveries were numerous. While Caroline turned her attention increasingly to comets, he observed sunspots and confirmed the gaseous nature of the sun, and later discovered two moons of Jupiter and two moons of Uranus. But his principal work centered on stars, particularly the movement of the solar system through space, and evidence that binary stars move around a common center of gravity. Herschel made one of the first attempts to measure the sun's motion through the galaxy using nearby stars, providing an important step in the gradual acceptance in the astronomical community that the Sun was not, in fact, the center of the universe. He also anticipated the work of Laplace with his development of an evolutionary theory of the universe in which, starting from a uniform "initial state," stars form and clump into nebulae. And he discovered more than 1000 binary stars.
But by far Herschel's most ambitious undertaking was an attempt to determine the structure of the Milky Way galaxy using a technique he called "star gauging": making sample counts of the stars in the field of view of his telescope. His increasingly large and powerful telescopes allowed him to resolve many of the mysterious "nebulae" into clusters of faint stars. Because his instruments lacked clock drives to keep them trained on the moving sky, his method of observation was to train his telescope on a point on the meridian and watch what crossed the field of view in a thin strip of the visible sky. He did this while standing on a ladder, calling out descriptions of whatever he saw to his sister Caroline. As the nights progressed, he would change the position of the telescope to observe another thin strip of sky, and so forth.
With Caroline's help, Herschel was ultimately able to observe all of the sky visible in Great Britain over a period of about 20 years, during which he methodically catalogued the faint patches of light now known as nebulae. After his death in 1822, Herschel's son, John, took his father's instrument to South Africa where he was able to survey the southern sky, publishing The General Catalogue of Nebulae in 1864, which was later revised and enlarged in 1888 by L.E. Dreyer as The New General Catalogue of Nebulae. Even today most non-stellar objects are still known by their General Catalogue (NCG) numbers. Herschel concluded from his star counts that the Milky Way galaxy was shaped like a disk, marked by many irregularities, and that the sun was located near its center. Later studies, of course, confirmed Herschel's deduction of shape, but found that the Sun is not near the center, and that the system is considerably larger than Herschel supposed.
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