William Herschel Observed Flattened Polar Regions on Uranian Disk

Summary: German-British astronomer William Herschel observed flattened polar regions on the Uranian disk during his 10-year study of the planet’s shape.

artist’s concept of Uranian ring system in polar rotation as discovered by NASA Ames C-141 Kuiper Airborne Observatory; Rick Guidice / NASA Ames Research Center; NASA ID ARC-1977-AC77-1069; date created 1977-09-12: Generally not subject to copyright in the United States, via NASA Image and Video Library

German-British astronomer William Herschel observed flattened polar regions on the Uranian disk, according to his observational journal notes on his 10-year study of the planet’s shape.

William Herschel (Nov. 15, 1738-Aug. 25, 1822) reported his findings about the shape of Uranus at the Dec. 14, 1797, meeting of The Royal Society of London (known officially as The Royal Society for Improving Natural Knowledge). His analysis of the Uranian disk also considered the possibility of a ring system as explaining observations of a not-round, or elliptical, shape.

Herschel had discovered the planet on March 13, 1781. He referred to his planetary discovery as Georgium Sidus (George’s Star) or the Georgian planet, in honor of his royal patron, George III (June 4, 1738-Jan. 29, 1820).

The first entry presented in Herschel’s 1797 report dated back to Nov. 13, 1782. The entry noted: “7-feet reflector, power 460. I perceive no flattening of the polar region” (page 67).

On April 8, 1783, however, his entry stated: “I surmise a polar flattening.”

His entry for March 4, 1787, considered: “I begin to entertain again a suspicion that the planet is not round.” This entry also suggested “a double ring” as explaining the disk’s “double, opposite points.”

Herschel’s entry for March 5, 1792, read: “I viewed the Georgian planet with a newly polished speculum, of an excellent figure. It shewed the planet very well defined, and without any suspicion of a ring. I viewed it successively with 240, 300, 480, 600, 800, 1200, and 2400; all which powers my speculum bore with great distinctness. I am pretty well convinced that the disk is flattened.”

In his report, Herschel explained that, during his March 5 viewings, “The moon was pretty near the planet” (page 69).

The last two observations that Herschel presented in his report occurred in 1794 and 1795. The entries offered clear descriptions of an elongated Uranian disk.

Herschel’s Feb. 26, 1794, entry observed: “20-feet reflector, power 480. The planet seems to be a little lengthened out, in the direction of the longer axis of the satellites’ orbits.”

His April 21, 1795, entry stated: “10-feet reflector, power 400. The telescope adjusted to a neighbouring star, so as to make it perflectly round. The disk of the planet seems to be a little elliptical. With 600, also adjusted upon the neighbouring star, the disk still seems elliptical.”

In his report, Herschel evaluated the observations that he had obtained “. . . in the course of ten years . . .” (page 70). With respect to the Georgian planet’s ring system, he determined the absence of any “. . . ring in the least resembling that, or rather those, of Saturn” (page 70).

Herschel was able to make a positive summation of his consideration of flattened polar regions on the Uranian disk. He reported:

“The flattening of the poles of the planet seems to be sufficiently ascertained by many observations. The 7-feet, the 10-feet, and the 20-feet instruments, equally confirm it; and the direction pointed out Feb. 26, 1794, seems to be conformable to the analogies that may be drawn from the situation of the equator of Saturn, and of Jupiter.

“This being admitted, we may without hesitation conclude, that the Georgian planet also has a rotation upon its axis, of a considerable degree of velocity” (pages 70-71).

A planetary disk’s flattened shape, also known as ellipticity or oblateness, is a function of the equatorial and polar radii, according to planetary physicists Jason W. Barnes and Jonathan J. Fortney’s article in the May 1, 2003, issue of The Astrophysical Journal. The planet’s axial rotation redistributes mass from the polar regions to the equator via centripetal acceleration. The attraction of more mass toward the equatorial plane ensues from Tthe altering of the planet’s gravitational field by the redistributed mass.

Uranus has an ellipticity of 0.02293 as compared with Earth’s flattening of 0.00335, according to planetary curation scientist David Richard Williams’ “Uranus Fact Sheet” on the NASA (National Aeronautics and Space Administration) GSFC (Goddard Space Flight Center) Space Science Data Coordinated Archive (NSSDCA) website. William Herschel’s ice giant planet completes a sidereal rotation in 17.24 hours, as compared with Earth’s period of 23.9345 hours.

The takeaways for William Herschel’s observations of flattened polar regions on the Uranian disk are that the German-British astronomer presented his findings at the Dec. 14, 1797, meeting of The Royal Society of London; that he combined his examination of the shape of the Uranian disk with an evaluation of a possible Uranian ring system; and that he cited his observation on Feb. 26, 1794, as critical to his determination of the planet’s flattened polar regions.

ringless view of Uranus taken Jan. 14, 1986, by spacecraft Voyager 2’s ISS (Imaging Science Subsystem) narrow-angle camera (NAC) from an approximate distance of 7.8 million miles (12.7 million kilometers); NASA ID: PIA18182; image addition date 1986-12-18; image credit NASA / JPL (Jet Propulsion Laboratory)-Caltech: May be used for any purpose without prior permission, via NASA JPL Photojournal

Acknowledgment

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

Image credits:

artist’s concept of Uranian ring system in polar rotation as discovered by NASA Ames C-141 Kuiper Airborne Observatory; Rick Guidice / NASA Ames Research Center; NASA ID ARC-1977-AC77-1069; date created 1977-09-12: Generally not subject to copyright in the United States; may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages; general permission extends to personal Web pages, via NASA Image and Video Library @ https://images.nasa.gov/details-ARC-1977-AC77-1069

ringless view of Uranus taken Jan. 14, 1986, by spacecraft Voyager 2’s ISS (Imaging Science Subsystem) narrow-angle camera (NAC) from an approximate distance of 7.8 million miles (12.7 million kilometers); NASA ID: PIA18182; image addition date 1986-12-18; image credit NASA / JPL (Jet Propulsion Laboratory)-Caltech: May be used for any purpose without prior permission, via NASA JPL Photojournal @ https://photojournal.jpl.nasa.gov/catalog/PIA18182;
Generally not subject to copyright in the United States; may use this material for educational or informational purposes, including photo collections, textbooks, public exhibits, computer graphical simulations and Internet Web pages; general permission extends to personal Web pages, via NASA Image and Video Library @ https://images.nasa.gov/details-PIA18182

For further information:

Barnes, Jason W.; and Jonathan J. Fortney. “Measuring the Oblateness and Rotation of Transiting Extrasolar Giant Planets.” The Astrophysical Journal, vo. 588 (May 1, 2003): 545-556.
Available via IOPscience @ https://iopscience.iop.org/article/10.1086/373893/pdf

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, Mr. (William). “XXXII. Account of a Comet. Communicated by Dr. Watſon. Read April 26, 1781.” Philosophical Transactions of the Royal Society of London. Vol. LXXI. For the Year 1781. Part II: 492-501. London, England: Lockyer Davis and Peter Elmsly, Printers to The Royal Society, MDCCLXXXII (1782).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/51826184

Herschel, William. “Observations and Reports Tending to the Discovery of One or More Rings of the Georgian Planet, and the Flattening of Its Polar Regions.” Pages 67-71. “III. On the Discovery of Four Additional Satellites of the Georgium Sidus. The Retrograde Motion of its Old Satellites Announced; and the Cause of Their Disappearance at Certain Distances From the Planet Explained. Read December 14, 1797.” Philosophical Transactions of the Royal Society of London. For the Year MDCCXCVIII, [vol. LXXXVIII (88)], Part I: 47-79. London, England: Peter Elmsly, Printer to The Royal Society, MDCCXCVIII (1798).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/49109940

Herschel, William. “III. On the Discovery of Four Additional Satellites of the Georgium Sidus. The Retrograde Motion of its Old Satellites Announced; and the Cause of Their Disappearance at Certain Distances From the Planet Explained. Read December 14, 1797.” Philosophical Transactions of the Royal Society of London. For the Year MDCCXCVIII, [vol. LXXXVIII (88)]], Part I: 47-79. London, England: Peter Elmsly, Printer to The Royal Society, MDCCXCVIII (1798).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/49109920
Available via Royal Society of London Publishing @ https://royalsocietypublishing.org/doi/pdf/10.1098/rstl.1798.0005

Herschel, William. “Remarks Upon the Foregoing Observations.” Pages 69-71. “III. On the Discovery of Four Additional Satellites of the Georgium Sidus. The Retrograde Motion of its Old Satellites Announced; and the Cause of Their Disappearance at Certain Distances From the Planet Explained. Read December 14, 1797.” Philosophical Transactions of the Royal Society of London. For the Year MDCCXCVIII, [vol. LXXXVIII (88)], Part I: 47-79. London, England: Peter Elmsly, Printer to The Royal Society, MDCCXCVIII (1798).
Available via Biodiversity Heritage Library @ https://biodiversitylibrary.org/page/49109942

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

Marriner, Derdriu. “Stuart Eves Credits William Herschel With Uranian Epsilon Ring in 1789.” Earth and Space News. Wednesday, Feb. 16, 2011.
Available @

Marriner, Derdriu. “William Herschel First Glimpsed Uranian Ring System on March 4, 1787.” Earth and Space News. Wednesday, Feb. 9, 2011.
Available @ https://earth-and-space-news.blogspot.com/2011/02/william-herschel-first-glimpsed-uranian.html

Marriner, Derdriu. “William Herschel Saw Uranian Rings But Puzzling Views Created Doubt.” Earth and Space News. Wednesday, Feb. 9, 2011.
Available @ https://earth-and-space-news.blogspot.com/2011/02/william-herschel-saw-uranian-rings-but.html

Williams, David R. (Richard), Dr. “Uranus Fact Sheet.” NASA GSFC (Goddard Space Flight Center) NSSDC (NASA Space Science Data Coordinated Archive) > Solar System Exploration > Planetary Science > Uranus.
Available @ https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html