Sir William Herschel (1738-1822)

William Herschel

No history of astronomy would be complete without an account of the contributions made by William Herschel (Friedrich Wilhelm Herschel) and his sister Caroline. William Herschel was born in 1738 in Hanover, Germany. William left Germany in 1757 and took up residence in Bath, England, where he made his living as a musician and music teacher. As a composer, William was fairly well accomplished; listed among his works are 24 symphonies, 7 violin concertos, 2 organ concertos, along with other compositions.

The Discovery of Uranus

Increasingly, William’s interests turned to astronomy. The first entry made in William’s astronomical journal was in 1766, and on March 13, 1781, William made his historic discovery of the planet Uranus. Contrary to popular belief, the discovery of Uranus was far from accidental. In fact, William had been observing Uranus as, what he believed it to be at the time, a comet for many weeks. William proposed naming the new planet “Georgium Sidus” (George’s star) after King George III, of American Revolution fame, but it was named, instead, “Uranus”. As a result of his discovery and his flattering gesture toward the king, William was knighted and made court astronomer by King George. Along with his discovery of Uranus, William is also attributed with the discovery of two of the moons of Uranus as well as two of the moons of Saturn.

Uranus

Courtesy of NASA/JPL/Caltech

Nebulae and Binary Stars

The discovery of Uranus was only one of three major contributions that William Herschel made to the study of astronomy. William had received a copy of Charles Messier’s Connoissance des temps, an almanac published in 1781, in which 103 nebulae were listed, and on October 23, 1783, he began observing these objects using the 20-foot reflector with an 18.7-inch aperture. Because William was using an instrument larger than anything that Messier had, he was able to discover new nebulae that Messier had missed, and in the course of the next several years, added 2,500 new nebulae to the list.

The German philosopher Immanuel Kant had theorized some time earlier that nebulae that could not be resolved into clusters of stars had to be “island universes” (i.e., galaxies) that were being observed at great distances, an almost prophetic insight into the nature of the universe. William Herschel, initially, accepted this idea. But, when he discovered a faint wispy object, with a star in the middle of it–which we now know to be a planetary nebula with a white dwarf at it’s center– he rejected the idea. Kant also believed that the Milky Way was itself an island universe, like the distant nebulae, and was shaped like a flat circular disk of stars. Herschel embraced this idea, which had became known as the “island universe theory” and set out to determine the shape of the Milky Way, by “gauging” the number of stars in a given region.

William and Caroline Herschel were the first astronomers in history to do a systematic survey of the night sky. In an attempt to discover the distances by means of heliocentric parallax William, using the much larger telescopes, discovered that many of the stars that he had previously observed as single stars were resolving, because of the larger instruments used, into double (or binary) stars. He also noted after having observed them over a period of time that they seemed to be moving around each other, suggesting a gravitational influence between them. This observation was very significant since it showed that Kepler’s laws of planetary motion and Newton’s law of universal gravitation applied, not only to the planets and sun of our solar system, but to distant stars as well and were truly universal. Because the stars that William was observing were so distant he was not able to determine the distances to the stars by means of parallax, so he arrived at another method for determining the shape of the Milky Way.

William had noticed in his survey of the skies that the density of stars varied depending upon what region he was looking at, at the time. Two things could account for this varied distribution of the stars: Either the stars were closer together in the regions where they appeared to be more compacted, or the stars were spaced apart more or less uniformly, and where the stars seem to be the densest is where the Milky Way extended out the furthest. It was the latter explanation that William believed to be the case and upon which he based his assessment. William observed that the greatest density of stars was around the Milky Way and that the lowest density was away from the Milky Way. From this he concluded that the shape of the Milky Way was that of a disk or grindstone.

William Herschel's Model of the Milky Way

The over all shape that William came up with for the Milky Way was wrong, but at the same time, it wasn’t that far off. He had made erroneous assumptions in determining the shape of our galaxy. For example, he incorrectly assumed that all of the dark regions that he had observed were empty spaces, when in fact, many were dark clouds of gas and dust. As a result, William concluded that there were gaps in the Milky Way where there were not, and naturally the shape of our galaxy, which he envisioned in his mind, was off as a result. But, despite the errors that William had made (he even admitted that his model of the Milky Way was probably wrong) it still must be remembered that what is important is that he helped established the idea of the Milky Way as an autonomous celestial body, and that by his systematic survey of the heavens, he extended the boundaries of the universe much further than anyone had previously thought them to be.

The realization that our solar system is merely a part of a much larger “island universe” (i.e., the Milky Way galaxy) and Kant’s idea that some of the nebulae being observed were themselves island universes were epiphanies that would later serve as stepping stones to the breakthrough made in the 20th century by Edwin Hubble, that ours was but one of many galaxies in our universe.

Caroline Lucretia Herschel (1750-1848)

Caroline Herschel

Caroline Herschel (1750-1848) joined her brother in Bath in 1772. After she arrived, Caroline served as William’s assistant in his musical career and later, in his astronomical pursuits. When William married the wealthy widow, Mary Pitt, Caroline had more time to pursue her own interests in astronomy.

Caroline, who was well respected as an astronomer in her own right, is credited with the discovery of eight new comets. Not only that, but the discovery of 14 new nebulae is attributed to her as well. She also published a catalog of 561 stars. And, like her brother, Caroline had many honors bestowed upon her, including the gold medal of the Royal Astronomical Society in 1828 and the gold medal of the Prussian Academy of Science, which she received in 1846. She was also invited to join the Academy of Science in Dublin.

The Telescopes

William Herschel, besides being a diligent observer, was also a prolific telescope maker. He built and sold a great number of 7-foot reflectors and constructed much larger telescopes as well. In 1776, William constructed a Newtonian reflector that had a 20-foot focal length and an aperture of 12 inches. Another telescope, with a 48-inch aperture and a 40-foot focal length, was completed in 1788. The construction of the 40-foot telescope was funded, in large part, by the crown.

Fourty-Foot Reflector

The 40-foot telescope (above), with a 48-inch aperture. Because of it’s tremendous size the 40-foot reflector was cumbersome and difficult to use, so as a result, William used the 20-foot reflector far more than he used this telescope. The 40-foot reflector had the distinction of being the largest telescope in the world until the 1840s, when Lord Rosse completed construction of the Leviathan of Parsonstown.

Twenty-Foot Telescope

The 20-foot reflector (above), with an 18.7-inch aperture. William began observing with this telescope on October 23, 1783. It was with this telescope that Herschel made his extensive survey of nebulae.

Seven-Foot Reflector


Copyright © Adler Planetarium and Astronomy Museum.

This telescope, which is a part of the Adler Planetarium and Astronomy Museum’s historic instruments collection, is very much like the one used by William Herschel to make his discovery of the planet Uranus.

Copyright © 2010 Eric F. Diaz

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