William Herschel Discovered Saturnian Moon Mimas Sept. 17, 1789

Summary: German-British astronomer William Herschel discovered Saturnian moon Mimas Sept. 17, 1789, after having discovered Enceladus 20 days earlier.

German-British astronomer Sir William Herschel’s depiction of lineup of seven Saturnian moons on Oct. 18, 1789, at 9:22 p.m. (21:22.45), with his two discoveries, 6 and 7 (named Enceladus and Mimas approximately 58 years later, in 1847, by Sir William’s son, Sir John Herschel); star s = reference point; W. Herschel, Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn (1790), page 20: Public Domain, via Biodiversity Heritage Library

German-British astronomer William Herschel discovered Saturnian moon Mimas Sept. 17, 1789, less than three weeks after his discovery of sixth Saturnian moon Enceladus.

William Herschel (Nov. 15, 1738-Aug. 25, 1822) read his “Account of the Diſcovery of a Sixth and Seventh Satellite of the Planet Saturn” at the Nov. 12, 1789, meeting of the Royal Society of London for Improving Knowledge. He said of his discoveries’ primary body: “The planet Saturn is, perhaps, one of the moſt engaging objects that aſtronomy offers to our view. As ſuch it drew my attention ſo early as the year 1774; when, on the 17th of March, with a 5 1/2-feet reflector, I ſaw its ring reduced to a very minute line, as repreſented in fig. I (Tab. I.)” (page 2).

Herschel explained in his account that he had to delay confirmation of his sightings and suspicions of more than five Saturnian moons until completion of his powerful 40-foot- (12-meter-) focal length telescope.

“In the year 1788 very little could be done towards a diſcovery, as my twenty-feet ſpeculum was ſo much tarniſhed by zenith ſweeps, in which it had been more than uſually expoſed to falling dews, that I could hardly ſee the Georgian ſatellites” (page 10), Herschel explained.

He began construction of his powerful telescope “about the latter end of the year 1785,” according to his paper, “Description of a Forty-feet Reflecting Telescope” (page 349), read at the Society’s June 11, 1795, meeting. Although Herschel made his first view through the telescope on Feb. 19, 1787, he did not consider the telescope completed until two and one-half years later.

“Aug. the 28th, 1789. 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,” Herschel noted (page 350).

After his Aug. 28 discovery of Saturn’s sixth moon, Herschel continued to scrutinize Saturn: “I continued my obſervations conſtantly, whenever the weather would permit; and the great light of the forty-feet ſpeculum was now of ſo much uſe, that I alſo, on the 17th of September, detected the ſeventh ſatellite, when it was at its greateſt preceding elongation” (page 11).

Herschel compiled his observations of his two Saturnian satellite discoveries in a table of motion. From his table of the moons’ current positions, he made calculations of their previous placements. In comparing his calculated positions with previous years’ observations, Herschel “. . . ſoon found that many ſuſpicions of theſe ſatellites, in the ſhape of protuberant points on the arms, were confirmed, and ſerved to correct the tables, ſo as to render them more perfect” (page 11).

Mimas’ discoverer described it as “. . . incomparably ſmaller than the ſixth; and, even in my forty-feet reflector, appears no bigger than a very ſmall lucid point.” Yet, despite the seventh’s smallness, Herschel added: “I ſee it, however, alſo very well in the twenty-feet reflector . . .” The keenly experienced observer of distant astronomical objects then demystified the perception of previously unseen objects: “It muſt nevertheleſs be remembered, that a ſatellite once diſcovered is much eaſier to be ſeen than it was before we were acquainted with its place” (page 12).

Herschel determined that the seventh Saturnian satellite completed one sidereal revolution in 22 hours 40 minutes 46 seconds. He noted, however, that ascertaining the orbital period for the sixth Saturnian satellite was easier than projecting the seventh’s revolution. “The difficulty of having a number of obſervations is uncommonly great; for, on account of the ſmallneſs of its orbit, the ſatellite lies generally before and behind the planet and its ring, or at leaſt ſo near them that, except in very fine weather, it cannot eaſily be ſeen well enough to take its place with accuracy” (pages 12-13), Herschel stated.

The naming of William Herschel’s two Saturnian moons was accomplished by his son, English polymath Sir John Frederick William Herschel (March 7, 1792-May 11, 1871). In his survey of the Southern Hemisphere’s night skies, published in 1847, Sir John shared his personal, mythologically-inspired names for the then-known seven Saturnian moons: Iapetus, Titan, Rhea, Dione, Tethys, Enceladus and Mimas.

The takeaways for William Herschel’s discovery of Saturnian moon Mimas on Sept. 17, 1789, are that the German-British astronomer confirmed his suspicions of more than five Saturnian moons with his discoveries of Enceladus as the planet’s sixth moon on Aug. 28, 1789, and of Mimas as Saturn’s seventh moon three weeks later; that Herschel made his discoveries with his newly completed 40-foot telescope; and that his son, Sir John Herschel, is credited with naming his father’s Saturnian discoveries as Enceladus and Mimas.

engraving of 40-foot telescope with which Sir William Herschel discovered Saturnian moons Enceladus on Aug. 28, 1789, and Mimas on Sept. 17, 1789; “Fig. 1 represents a view of the telescope in a meridional situation, as it appears when seen from a convenient distance by a person placed towards the south-west of it” (page 350); W. Herschel, Description of a Forty-Feet Reflecting Telescope (1795), Figure 1, opposite page 408: Public Domain, via Biodiversity Heritage Library

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’s depiction of lineup of seven Saturnian moons on Oct. 18, 1789, at 9:22 p.m. (21:22.45), with his two discoveries, 6 and 7 (named Enceladus and Mimas approximately 58 years later, in 1847, by Sir William’s son, Sir John Herschel); star s = reference point; W. Herschel, Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn (1790), page 20: Public Domain, via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51828917

engraving of 40-foot telescope with which Sir William Herschel discovered Saturnian moons Enceladus on Aug. 28, 1789, and Mimas on Sept. 17, 1789; “Fig. 1 represents a view of the telescope in a meridional situation, as it appears when seen from a convenient distance by a person placed towards the south-west of it” (page 350); W. Herschel, Description of a Forty-Feet Reflecting Telescope (1795), Figure 1, opposite page 408: Public Domain, via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51831513

For further information:

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. “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

Herschel, William. “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

Levy, David H. Skywatching. Revised and updated. San Francisco CA: Fog City Press, 1994.

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

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