William Herschel Discovered First Two Uranian Moons on Jan. 11, 1787

Summary: William Herschel discovered the first two Uranian moons on Jan. 11, 1787, almost five years 10 months after discovering their primary body, Uranus.

“The Georgian Planet attended by two satellites,” William Herschel’s sketch accurately predicting placements of his two satellite discoveries on Feb. 11, 1787, at about two o’clock in the morning; S south, N north, P preceding, F following, with the Georgian Planet (Uranus) (center), first satellite (now Titania) (below) and second satellite (Oberon) (above); W. Herschel, An Account of the Discovery of Two Satellites (1787), Tab. VII, opposite page 128: Public Domain, via Biodiversity Heritage Library

German-British astronomer William Herschel discovered the first two Uranian moons on Jan. 11, 1787, five years nine months 29 days after his discovery of the satellites’ primary body, Uranus.

William Herschel (Nov. 15, 1738-Aug. 25, 1822) discovered the seventh planet from the sun on March 13, 1781. He referred to the planet as Georgium Sidus (George’s Star) or the Georgian planet, in honor of his royal patron, George III (June 4, 1738-Jan. 29, 1820).

Herschel’s observations of the Georgian system continued after his discovery. Ascertaining whether satellites circled the planet numbered among his searches. On Jan. 11, 1787, Herschel succeeded in detecting two satellites. A little over one month later, the report of Herschel’s satellite discoveries was read at The Royal Society of London’s Feb. 15, 1787. He also shared his specific observational journal entries about his satellite discoveries 28 years four months 28 days after those discoveries in a report read at the June 8, 1815, meeting of The Royal Society of London.

Herschel reported at the Feb. 15, 1787, meeting that he had “. . . ſelected a ſweep which led to the Georgian planet; and, while it paſſed the meridian, I perceived near its diſk, and within a few of its diameters, ſome very faint ſtars whoſe places I noted down with great care” (page 126).

In his report at the June 8, 1815, meeting, Herschel shared his entry describing his discoveries. “1787, January 11d 12h 13m. There is a supposed first satellite about 42 or 43 degrees south following the planet; and a second about 45 degrees north preceding” (page 304).

Herschel reported at the Feb. 15, 1787, meeting that he had wanted to have “no doubts” about his Jan. 11, 1787, discoveries. He stated: “The leaſt hazineſs, otherwiſe imperceptible, may often obſcure ſmall ſtars; and I judged, therefore, that nothing leſs than a ſeries of obſervations ought to ſatisy me, in a caſe of this importance” (page 126).

Herschel then detailed his efforts to assure the accuracy of his suspected satellite discoveries. “To this end I noticed all the ſmall ſtars that were near the planet the 14th, 17th, 18th, and 24th of January, and the 4th and 5th of February . . .”

Herschel’s strategy proved effective for one suspected satellite and convinced him to scrutinize the second suspect. “. . . and though, at the end of this time, I had no longer any doubt of the exiſtence of at leaſt one ſatellite, I thought it right to defer this communication till I could have an opportunity of ſeeing it actually in motion.”

Accordingly, Herschel tracked the strong candidate on Feb. 7 from “. . . about ſix o’clock in the evening, and kept it in view till three in the morning on Feb. the 8th . . .” He noted that “. . . during thoſe nine hours I ſaw this ſatellite faithfully attend its primary planet, and at the ſame time keep on, in its own courſe, by deſcribing a conſiderable arch of its proper orbit.”

Although he was “. . . chiefly attending to the motion of this ſatellite . . . ,” Herschel also sought to observe his other candidate. He attributed his lack of assurance concerning its motion “ . . . to my great attention to the ſormer ſatellite, or to the cloſeneſs of this latter, which was nearly hidden in the rays of the planet . . . Indeed, towards morning, when a change of place, in ſo conſiderable an interval as nine hours, would have been moſt conſpicuous, the moon interfered with the faint light of this ſatellite, ſo that I could no longer perceive it” (pages 126-127).

Feb. 9 brought closure on his two candidates. “. . . I ſaw my firſt diſcovered ſatellite nearly in the place where I expected to find it. I perceived alſo, that the next ſuppoſed ſatellite was not in the ſituation where I had left it on the 7th, and could now diſtinguiſh very plainly that it had advanced in its orbit, ſince that day, in the ſame direction with the other ſatellite, but at a quicker rate.”

Herschel decided that, because this ſatellite’s “more contracted orbit” placed it closer to the Georgian planet, he would “. . . call it in future the firſt ſatellite, though laſt diſcovered, or rather laſt aſcertained . . .” (page 127). Based upon his observations between Jan. 11 and Feb. 11, he provisionally placed the first satellite’s orbital period at “. . . about eight days and three-quarters, and the ſecond in nearly thirteen days and an half” (page 128).

Herschel’s approximations of the orbital periods of his two Uranian satellite discoveries were close to accurate. The first satellite revolves around Uranus in 8.705867 days, according to the Uranian Satellite Fact Sheet on the NASA (National Aeronautics and Space Administration) Space Science Data Coordinated Archive (NSSDCA) website. The second satellite completes its orbit of its primary body in 13.463234 days.

The two Uranian satellites with which William Herschel is credited received names in 1852 from their discoverer’s son, English polymath Sir John Herschel (March 7, 1792-May 11, 1871). Titania, the first satellite, and Oberson, the second satellite, were named after the queen and king of the fairies, respectively, in A Midsummer Night’s Dream by Elizabeth playwright William Shakespeare (bapt. April 26, 1564-April 23, 1616).

The takeaways for William Herschel’s discovery of the first two Uranian moons on Jan. 11, 1787, are that the German-British astronomer made his satellite discoveries approximately five years 10 months after his March 13, 1781, discovery of their primary body, the planet Uranus; that his son, English polymath Sir John Herschel, named them Titania and Oberson, after the queen and king of the fairies in Elizabethan playwright William Shakespeare’s A Midsummer Night’s Dream; and that William Herschel discovered Oberon first but designated Titania as the “firſt ſatellite” because of its closeness to “the Georgian planet.”

The Uranian system includes five major moons, two of which (Titania and Oberon) are credited, for discovery, to Uranus discoverer William Herschel; Atlas Image of Uranus obtained June 7, 1998, by 2MASS Southern Facility; Atlas Image courtesy of 2MASS (Two Micron All Sky Survey), a joint project of the University of Massachusetts (UMass) and the Infrared Processing and Analysis Center / California Institute of Technology (IPAC-Caltech), funded by the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF): 2MASS Atlas Image Gallery, Public Domain, via Caltech IPAC / 2MASS (2 Micron All Sky Survey)

Acknowledgment

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

Image credits:

“The Georgian Planet attended by two satellites,” William Herschel’s sketch accurately predicting placements of his two satellite discoveries on Feb. 11, 1787, at about two o’clock in the morning; S south, N north, P preceding, F following, with the Georgian Planet (Uranus) (center), first satellite (now Titania) (below) and second satellite (Oberon) (above); W. Herschel, An Account of the Discovery of Two Satellites (1787), Tab. VII, opposite page 128: Public Domain, via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51827593

The Uranian system includes five major moons, two of which (Titania and Oberon) are credited, for discovery, to Uranus discoverer William Herschel; Atlas Image of Uranus obtained June 7, 1998, by 2MASS Southern Facility; Atlas Image courtesy of 2MASS (Two Micron All Sky Survey), a joint project of the University of Massachusetts (UMass) and the Infrared Processing and Analysis Center / California Institute of Technology (IPAC-Caltech), funded by the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF): 2MASS Atlas Image Gallery, Public Domain, via Wikimedia Commons @ https://commons.wikimedia.org/wiki/File:Uranus,_Neptune,_and_their_moons_(2MASS).jpg; via Caltech IPAC / 2MASS (2 Micron All Sky Survey) @ https://old.ipac.caltech.edu/2mass/gallery/images_ss.html

For further information:

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Available via Oxford Academic Journals @ https://academic.oup.com/mnras/article/3/3/19/1018015

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