William Herschel Discovered Mimas With Newly Built 40-Foot Telescope

Summary: On Sept. 17, 1789, William Herschel discovered Mimas with his newly built 40-foot telescope, which thereby notched two Saturnian finds in 20 days.

German-British astronomer Sir William Herschel acknowledged that the “uncommon size” of the 40-foot telescope that he designed and built enabled him to detect Saturn’s sixth and seventh moons, both of which are much smaller than the five then-known Saturnian satellites: Royal Astronomical Society @RoyalAstroSoc, via Facebook Feb. 23, 2010

On Sept. 17, 1789, William Herschel discovered Mimas with his newly built 40-foot telescope, the optical instrument through which the German-British astronomer had discovered his first Saturnian moon only 20 days earlier.

William Herschel (Nov. 15, 1738-Aug. 25, 1822) detailed his design and construction of a reflecting telescope of “uncommon size” in a paper presented at the June 11, 1795, meeting of the Royal Society of London. The success of two years of observations through the 20-feet reflector that he had finished in 1783 revived Herschel’s interest in devising telescopes with wide apertures. As President of the Royal Society, Sir Joseph Banks (Feb. 24, 1743-June 19, 1820) secured the support of King George III (June 4, 1738-Jan. 29, 1820) for Herschel’s increased aperture project.

“In consequence of this arrangement I began to construct the 40-feet telescope, which is the subject of this paper, about the latter end of the year 1785,” Herschel wrote (page 349).

Herschel involved himself in every aspect of his new telescope’s design and construction. “In the whole of the apparatus none but common workmen were employed, for I made drawings of every part of it, by which it was easy to execute the work, as I constantly inspected and directed every person’s labour; though sometimes there were not less than 40 different workmen employed at the same time.”

The telescope’s “great mirror” was made of speculum metal, a mixture of copper and tin that polishes to high reflectivity. “While the stand of the telescope was preparing I also began the construction of the great mirror, of which I inspected the casting, grinding, and polishing,” Herschel reported.

Herschel’s 40-foot telescope was raised on the grounds of Observatory House, Herschel’s home in Slough, Berkshire, South East England. Herschel’s first view through his optical invention, which occurred on Feb. 19, 1787, was disappointing. “I do not however date the completing of the instrument till much later; for the first speculum, by a mismanagement of the person who cast it, came out thinner on the centre of the back than was intended, and on account of its weakness would not permit a good figure to be given to it” (page 350).

A second mirror cast Jan. 26, 1788, cracked during cooling. A recast on Feb. 16, “. . . with particular attention to the shape of the back, . . . proved to be of a proper degree of strength.”

Although he was able to discern Saturn on Oct. 24, 1788, Herschel remained dissatisfied with his new telescope. He persisted in finessing the great mirror over the next 10 months. Finally, on Aug. 27, 1789, “. . . it was tried upon the fixed stars, and I found it to give a pretty sharp image. Large stars were a little affected with scattered light, owing to many remaining scratches in the mirror,” Herschel noted.

The next night, Aug. 28, Herschel confirmed his suspicion of a sixth Saturnian moon, named Enceladus in 1847 by Herschel’s son, Sir John Herschel (March 7, 1792-May 11, 1871). “Having brought the telescope to the parallel of Saturn, I discovered a sixth satellite of that planet; and also saw the spots upon Saturn, better than I had ever seen them before, so that I may date the finishing of the 40-feet telescope from that time.”

Twenty days later, the keen observational astronomer claimed the seventh Saturnian moon (named Mimas by his son in 1847) as his second satellite discovery in the Saturnian system. In his account of these discoveries, read at the Royal Society’s Nov. 12, 1789, meeting, Herschel pinpointed his detection of the seventh satellite as occurring “. . . when it was at its greateſt elongation” (page 11).

Herschel considered his 40-foot telescope as critical to his closely spaced discoveries of Enceladus and Mimas. In his “Account of the Diſcovery of a Sixth and Seventh Satellite of the Planet Saturn,” Herschel stated: “It may appear remarkable, that theſe ſatellites ſhould have remained ſo long unknown to us, when, for a century and an half paſt, the planet to which they belong has been the object of almoſt every aſtronomer’s curioſity, on account of the ſingular phaenomena of its ring. But it will be ſen preſently, from the ſituation and ſize of the ſatellites, that we could hardly expect to diſcover them till a teleſcope of the dimenſions and aperture of my forty-feet reflector should be conſtructed . . .” (page 2).

The takeaways for William Herschel’s discovery of Mimas with his newly built 40-foot telescope on Sept. 17, 1789, are that the German-British astronomer’s new telescope of “uncommon size” notched detection of Enceladus and Mimas, as Saturn’s sixth and seventh known moons, within the space of only 20 days; that Herschel’s first view through his new optical instrument occurred Feb. 19, 1787; that he finessed the telescope’s great mirror for two and one-half more years; and that he dated the completion of the 40-foot telescope to his discovery of Enceladus on Aug. 28, 1789.

first speculum metal mirror, cast in 1785, for William Herschel’s 40-foot telescope; Science Museum, South Kensington, London, United Kingdom; object number 1932-567 Pt1; September 2008: Geni, CC BY SA 4.0 International, via Wikimedia Commons

Acknowledgment

My special thanks to talented artists and photographers/concerned organizations who make their fine images available on the internet.

Image credits:

German-British astronomer Sir William Herschel acknowledged that the “uncommon size” of the 40-foot telescope that he designed and built enabled him to detect Saturn’s sixth and seventh moons, both of which are much smaller than the five then-known Saturnian satellites: Royal Astronomical Society @RoyalAstroSoc, via Facebook Feb. 23, 2010, @ https://www.facebook.com/RoyalAstroSoc/photos/a.448029939520/335261579520/

first speculum metal mirror, cast in 1785, for William Herschel’s 40-foot telescope; Science Museum, South Kensington, London, United Kingdom; object number 1932-567 Pt1; September 2008: Geni, CC BY SA 4.0 International, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:40-foot_telescope_mirror.jpg

For further information:

Dreyer, J.L.E. (John Louis Emil), comp. The Scientific Papers of Sir William Herschel Including Early Papers Hitherto Unpublished. Vol. I; Vol. II. London, England: The Royal Society and The Royal Astronomical Society, 1912.
Vol. I: Available via HathiTrust @ https://babel.hathitrust.org/cgi/pt?id=mdp.39015010954678
Vol. II: Available via HathiTrust @ https://babel.hathitrust.org/cgi/pt?id=mdp.39015010954744
Vol. I: Available via Internet Archive @ https://archive.org/details/scientificpapers032804mbp/
Vol. II: Available via Internet Archive @ https://archive.org/details/scientificpapers02hersuoft/

Herschel, William. “XXVII. Catalogue of One Thouſand new Nebulae and Cluſters of Stars. Read April 27, 1786.” Philosophical Transactions of the Royal Society of London, vol. LXXVI for the Year 1786, Part II: 457-499. London, England: Lockyer Davis and Peter Elmsly, Printers to The Royal Society, MDCCLXXXVI (1786).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/48283813

Herschel, John F.W., Sir. “Chapter VI. Observations of the Satellites of Saturn.” Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope; Being the Completion of a Telescopic Survey of the Whole Surface of the Visible Heavens: 414-430. London, England: Smith, Elder and Co., 1817.
Available via Internet Archive @ https://archive.org/details/resultsofastrono00hers/page/414

Herschel, William. “I. Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn; With Remarks on the Construction of Its Ring, Its Atmosphere, Its Rotation on an Axis, and Its Spheroidical Figure. Read November 12, 1789.” Philosophical Transactions of the Royal Society of London, vol. LXXX, for the Year 1790, Part I: 1-20. London UK: Lockyer Davis and Peter Elmsly, Printers to The Royal Society, MDCCXC (1790).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51828893

Herschel, William. “XVI. An Account of the Discovery of Two Satellites Revolving Round the Georgian Planet. Read Feb. 15, 1787.” Philosophical Transactions of the Royal Society of London, vol. LXXVII for the Year 1787, Part I: 125-129. London, England: Lockyer, Davis, and Peter Elmsly, Printers to The Royal Society, MDCCLXXXVII (1787).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51827589
Available via Internet Archive @ https://archive.org/details/philtrans05978816

Herschel, William. “XVIII. Description of a Forty-Feet Reflecting Telescope. Read June 11, 1795.” Philosophical Transactions of the Royal Society of London for the Year MDCCXCV, vol. LXXXV, Part II: 347-409. London UK: Peter Elmsly, Printer to The Royal Society, 1795.
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51831451

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Marriner, Derdriu. “Mimantean Crater Herschel Honors Mimas Discoverer William Herschel.” Earth and Space News. Wednesday, Sept. 7, 2011.
Available @ https://earth-and-space-news.blogspot.com/2011/09/mimantean-crater-herschel-honors-mimas.html

Marriner, Derdriu. “William Herschel Discovered Saturnian Moon Mimas Sept. 17, 1789.” Earth and Space News. Wednesday, Sept. 14, 2011.
Available @ https://earth-and-space-news.blogspot.com/2011/09/william-herschel-discovered-saturnian.html

Moore, Patrick, Sir. Philip’s Atlas of the Universe. Revised edition. London UK: Philip’s, 2005.

Pearson, W. (William), Rev. Plates Belonging to the Second Volume of An Introduction to Practical Astronomy. N.P., n.d.
Available @ https://digital.tcl.sc.edu/digital/collection/ariail2/id/151

Pearson, W. (William), Rev. “XV. The Herschelian Forty-Feet Telescope (Plate VIII).” An Introduction to Practical Astronomy: Containing Descriptions of the Various Instruments, That Have Been Usefully Employed in Determining the Places of the Heavenly Bodies, With an Account of the Methods of Adjusting and Using Them. Vol. II: 71-78. London, England: Printed for the Author by Messrs. Longman, Rees, Orme, Brown, and Green, 1829.
Available via HathiTrust @ https://hdl.handle.net/2027/nyp.33433090845102?urlappend=%3Bseq=101

Royal Astronomical Society @RoyalAstroSoc. “The logo of the Royal Astronomical Society. It is in the form of a roundel and features: -- The 40-foot telescope built by William Herschel (our first president) -- The year the Society was founded -- Our Latin motto, 'quicquid nitet notandum' -- which translates as 'whatever shines should be observed.'” Facebook. Feb. 23, 2010.
Available @ https://www.facebook.com/RoyalAstroSoc/photos/a.448029939520/335261579520/

The Royal Society. “William Herschel.” The Royal Society > Science in the Making.
Available @ https://makingscience.royalsociety.org/s/rs/people/fst01800987