Herschel Begins Building His Reflecting Telescope

William Herschel’s 40-foot-long reflecting telescope was used to discover two of Saturn’s moons and to observe nebula, identified as galaxies such as the Milky Way. The telescope, functional in 1789, remained the largest such instrument for more than fifty years.

Summary of Event

William Herschel learned to play a musical instrument as a child, but he also developed a keen interest in observing the sky with his father. From 1757 to 1772, while achieving success as a musician, he continued studying mathematics and made naked-eye observations of the sky. It was in 1773 that William’s interest in astronomy intensified. He read books on astronomy and purchased a quadrant, an instrument used to measure angles between the stars, as well as some lenses and mirrors. His first telescope is believed to have been a small, compact reflector of the type designed by the Scottish astronomer James Gregory, but this telescope was too small to be of any significant help. Herschel wanted a bigger, more powerful instrument, one that would gather more light and allow him to see fainter stars. However, large lenses, or mirrors, were very expensive at the time, so he had to make his own mirrors. [kw]Herschel Begins Building His Reflecting Telescope (1787)
[kw]Telescope, Herschel Begins Building His Reflecting (1787)
[kw]Reflecting Telescope, Herschel Begins Building His (1787)
[kw]Building His Reflecting Telescope, Herschel Begins (1787)
[kw]Begins Building His Reflecting Telescope, Herschel (1787)
[g]England;1787: Herschel Begins Building His Reflecting Telescope[2710]
[c]Astronomy;1787: Herschel Begins Building His Reflecting Telescope[2710]
[c]Science and technology;1787: Herschel Begins Building His Reflecting Telescope[2710]
Herschel, William
Herschel, Caroline Lucretia
George III

By 1774, Herschel had developed techniques to cast and polish mirrors superior to any that had been made previously. He constructed more than four hundred telescopes and observed the planets and their moons, the stars, and unusual objects called nebula, Nebulae which are luminous patches in the night sky. His large telescopes could resolve the individual stars in nearby nebula, but even with this telescopic power, the nebula could not be separated into individual stars and therefore looked like clouds. This led him to theorize that nebula were groups of stars, gathered together over long periods of time by the force of gravity; the Milky Way, Milky Way galaxy he added, was one of these galaxies.

Herschel’s sister, Caroline Lucretia Herschel, Herschel, Caroline Lucretia became his assistant, recording his observations and helping him grind and polish the mirrors for his new telescopes. She would become a noted astronomer as well—discovering eight comets and three nebulae—and is frequently referred to as the first woman astronomer of lasting significance. William and Caroline, still active musicians, gave their last public musical performance in 1782, after which they devoted themselves to astronomy.

Between 1786 and 1802, William Herschel published three catalogs noting the positions and characteristics of nebulae. These observations were performed mostly using his 20-foot telescope, an instrument with a focal length of 20 feet and a diameter of 18.8 inches. Herschel, however, was not satisfied with the magnifying power of this 20-foot telescope, so, in 1784, he decided to build Astronomy;reflecting telescope a much larger one, with a tube having a length of 40 feet and a mirror with a diameter of 48 inches. This project was far more expensive than he could afford. However, he was able to begin construction at his home and observatory in Slough, England, in 1785, upon receiving from King George III of England a £2,000 grant and an annual stipend of £200.

William Herschel and Caroline Lucretia Herschel, shown with William’s 40-foot reflecting telescope.

(Premier Publishing)

Constructing the 40-foot telescope was a major project, involving as many as forty workers, who removed trees, dug and prepared the ground, and laid a brick foundation. Another group prepared the tools for shaping and polishing the telescope’s mirror. During this time, William and Caroline made nighttime observations using the smaller telescopes and supervised construction of the giant telescope during the day.

After about two years of work, it seemed that Herschel’s largest telescope was complete. In early 1787, he tried to use the new telescope for the first time, but the quality of the mirror was not satisfactory. It weighed about one thousand pounds and was so thin that it distorted under its own weight, compromising the quality of the image. To take care of the problem, he ordered the casting of a new mirror disk, but this one broke while it was cooling. The third mirror disk proved successful. This mirror, 3.5 inches thick—twice as thick as the first mirror—was free of significant distortion.

The telescope had “first light,” an astronomer’s term for the first attempt to observe through a new telescope, on August 28, 1789. The extraordinary power of Herschel’s telescope was immediately apparent. That first evening, Herschel quickly discovered Saturn’s Saturn (planet) sixth moon, named Enceladus. On September 17, 1789, he discovered Saturn’s seventh moon, called Mimas.

Even with all its power and reach, the 40-foot telescope was not Herschel’s favorite, for two reasons. First, it required a lot of maintenance and the mirror needed frequent polishing. Second, and even more problematic, the mounting that allowed the telescope to be aimed at different spots on the sky was difficult to handle. Herschel, therefore, continued to make most of his observations with the 20-foot telescope, using it to discover two moons of Uranus, named Titania and Oberon. The 40-foot telescope remained the world’s largest for more than fifty years, possibly because of the difficulties using it. It was not until 1845 that William Parsons, Parsons, William the third earl of Rosse, built a larger telescope, which Parsons called the Leviathan. Had Herschel’s 40-foot telescope been easier to maintain and aim, he might have discovered even more cosmic phenomena, such as the spiral nebulae, a discovery made by Parsons.

In recognition of his achievements, William Herschel was knighted in 1816. He had helped to start the Astronomical Society of London in 1820, which later became the Royal Astronomical Society. Royal Astronomical Society, London A piece of the tube of the 40-foot telescope is displayed in the garden of Greenwich Observatory in London, but the mirror has been lost.


William Herschel’s 40-foot reflecting telescope allowed him, along with his sister Caroline, to make significant astronomical discoveries. He located the planet Uranus Uranus as well as two moons of Saturn, determined the rotational period of Saturn, used the same techniques to study the rotation of other planets, observed the motion of double stars, and concluded that stars are held together by gravitation. Thus, he was able to confirm the universal nature of Newton’s laws of gravity.

The telescopes aided his nebular research as well, which suggested that there existed more than the one hundred known nebulae in the universe. His catalog of nebulae was completed by Caroline after his death and published by Caroline’s nephew (and William’s son) John Herschel. General Catalogue of Nebulae and Clusters of Stars
General Catalogue of Nebulae and Clusters of Stars (Herschel and Herschel) (1864) listed about twenty-five hundred nebulae. The nebular research also suggested that new worlds might begin from gaseous matter, which remains the accepted origin of solar systems. Herschel also concluded that the known solar system is moving through space, and he was able to determine the direction of its motion.

Further Reading

  • Armitage, Angus. William Herschel. Garden City, N.Y.: Doubleday, 1963. A 158-page account of Herschel’s life and scientific contributions, with a chapter focusing on the construction of his large telescope and the discoveries he made with it.
  • Crawford, Deborah. King’s Astronomer William Herschel. New York: Julian Messner, 2000. A 191-page biography of Herschel, discussing both his musical and scientific careers, including his development of the high-powered telescope.
  • Dreyer, John Louis Emil, ed. The Scientific Papers of Sir William Herschel. Dorset, England: Thoemmes Continuum, 2003. A massive, 1,441-page collection of Herschel’s papers, containing detailed descriptions of his astronomical observations and discoveries.
  • Hoskin, Michael A. The Herschel Partnership: As Viewed by Caroline. Cambridge, England: Science History, 2003. A biographical work that focuses on the partnership between Caroline and William.
  • _______. William Herschel and the Construction of the Heavens. New York: Norton, 1964. A nearly 200-page discussion of Herschel’s discoveries, with notes on their astrophysical significance by D. W. Dewhirst.
  • Lubbock, Constance A. The Herschel Chronicle: The Life-Story of William Herschel and His Sister, Caroline Herschel. Cambridge, England: Cambridge University Press, 1933. A biography of William and Caroline with material drawn from their own records.
  • Sidgwick, J. B. William Herschel: Explorer of the Heavens. London: Faber & Faber, 1953. An account of Herschel’s life, including his career as a musician and his later work as an astronomer.

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