The rings of Saturn have fascinated astronomers ever since Galileo first directed his telescope at the giant planet in 1610. As telescopes became more powerful, details about the rings emerged. These included the discovery of moons, some of which were embedded in the rings, as well as narrow gaps that split the rings into a series of ringlets. As to whether the rings were solid or composed of a multitude of small particles, it wasn’t until 1859 when Scottish physicist James Clerk Maxwell proved mathematically that they must be small particles since a solid ring or solid ringlets would be unstable and as a result could not exist.

Observational proof of Maxwell’s theory did not come until 1895 when a young astronomer named James Edward Keeler measured the rotational velocity of Saturn's rings and found them to be consistent with small particles orbiting the planet.
Keeler’s discovery, which was made using the new field of astronomical spectroscopy, cemented his reputation as America's leading astrophysicist. But he died five years later at the age of 42, just as he was reaching the peak of his scientific career, and today his name is largely forgotten. This post traces Keeler's life from his childhood to his career-capping discovery, with the focus, of course, on his fascination with Saturn.
James Edward Keeler was the second son of machinist and watch maker William F. Keeler, who is known today for his superb letters from the USS Monitor during the Civil War [1]. James Edward was born on September 10, 1857 in La Salle, Illinois. When he was 12 years old he and his family moved to the tiny village of Mayport, Florida, where his father was the deputy collector of customs, an orange grower and a newspaper correspondent.
At a young age, Keeler showed a remarkable aptitude for science and mathematics. Beautifully detailed maps of the world made when he was 10 years old also attest to his artistic ability [2]. With no school in Mayport remotely capable of providing the kind of education he needed, he was home schooled by his parents and older brother Henry [3].
Keeler's prowess in designing and building scientific instruments and his love of science were nurtured by his father. His interest in astronomy, in particular, developed from the practical side of surveying which he also learned from him. Spending untold hours in his father's workshop in Mayport, he built a surveyor’s cross which he used to lay out lands for neighbors, an experimental windmill, and a telescope. His crowning achievement was his telescope, which he built using lenses he ordered from an optical dealer in Philadelphia.

The budding astronomer documented his observations in a small journal, whose inside front cover is adorned with the large title in block letters, “Mayport Astronomical Observatory,” below which appears “J. E. Keeler, Director!” The journal is filled with pen and ink drawings of celestial bodies, as well as detailed calculations of their positions [4].
In another journal he described his projects, in particular his telescope which he completed in late 1875: "Saturday, at work completing my telescope. The other day I tore a postage stamp into quarters & put one up on the lighthouse and saw it easily with my telescope." [5]
His excitement a few days later when he was finally able to view the heavens is palpable: “Directed my telescope to the stars and saw the rings of Saturn for the first time . . . Saw the annular nebula in Lyra, one satellite of Saturn. Divided the double star Castor with 108 . . . saw all four of the stars in the trapezium in the Great Nebula of Orion. . . .” [6]

Keeler’s unique talent was discovered by Charles Rockwell, a wealthy amateur astronomer from Tarrytown, New York and a friend of his parents from their days in La Salle, Illinois. Keeler's younger sister Lizzie, who was attending a private school in Tarrytown in the 1870s, happened to be visiting Rockwell’s private observatory and, while looking through his telescope at Saturn, remarked that she had often seen the planet in her brother’s telescope back home. Intrigued, Rockwell inquired more about her unusual brother and not long after brought him north, helping to finance his education at Johns Hopkins University in Baltimore.
Described by one of his Johns Hopkins classmates as a backwoods character with a pronounced “Cracker drawl” [7], Keeler was well liked by his fellow classmates and professors, and quickly made many close friends. He majored in physics and German, and took minors in mathematics, chemistry and astronomy, graduating four years later in 1881 with his bachelor's degree.
After graduating, Keeler worked as an assistant to Samuel P. Langley at Allegheny Observatory in Pittsburgh where he further honed his technical skills. With the exception of a year of post-graduate studies in physics in Germany (then the world center of science) from 1883-84, he worked at Allegheny Observatory until 1886 when he was offered a position as a junior astronomer at Lick Observatory in California, then under construction atop 4,200-foot Mount Hamilton.
When Keeler arrived at Mount Hamilton in April 1886 the observatory dome had not yet been completed and the telescope (lenses, tube and fittings) was still being built. When completed, the new telescope with its 36-inch lens and 56-foot focal length would be the largest in the world. Placed in charge of the observatory's spectroscopic work, Keeler was responsible for the design of the spectroscope that would be attached to the telescope.
The 36-inch refractor was finally installed in early January 1888, and the Lick astronomers waited anxiously for the skies to clear. Finally on January 8 they had their first view of the heavens, although it was so cold that the dome had frozen shut and could not be turned. Nevertheless, the dome's shutter could be opened, thus enabling them to view the stars and planets for several minutes before the Earth's rotation moved them out of the field of view. At nine o'clock the Orion nebula came into view, with a new wealth of detail, followed just after midnight by Saturn.

Never before seen with such a powerful telescope the astronomers were awestruck. In Keeler's words the telescope was "beyond the greatest telescopic spectacle ever beheld by man. The giant planet, with its wonderful rings, its belts, its satellites, shone with a splendor and distinctness of detail never before equalled." [8]
That night Keeler made the observatory's first astronomical discovery. Close to the outside edge of the outer A-ring he observed a previously unseen narrow gap which now bears his name, the "Keeler gap."
Keeler also took the opportunity to make a drawing of Saturn. He was able to sketch it from memory, which he had trained by viewing scenes for one or two minutes and then drawing them without looking back. Since astronomical photography was then in its infancy, Keeler's drawings of planets were considered outstanding representations by astronomers of the time.
Keeler devoted much of his time at Lick to stellar spectroscopy, the field of astrophysics that separates electromagnetic radiation (in Keeler's case visible light) from stars into a spectrum of wavelengths. Since matter emits or absorbs radiation at specific wavelengths, spectroscopy enables astrophysicists to determine the chemical composition of celestial objects.
He also used spectroscopy to measure the component of a star's velocity in the line of sight of the observer, which astronomers refer to as the radial velocity. This technique relied upon the Doppler principle, namely that the wavelength of a spectral line emitted by a star is shifted to shorter or longer wavelengths for a star moving toward or away from the observer. While this technique had been used before Keeler's time, his accurate radial velocity measurements were far superior to all others.
In 1891 Keeler was offered the directorship of Allegheny Observatory in Pittsburgh. In late May he made his last observation at Lick and headed east, stopping first at Oakley Plantation in West Feliciana Parish, Louisiana to marry Cora Matthews whom he got to know at Lick, and then on to Pittsburgh where the newlyweds arrived in early July.
It was in the spring of 1895 that Keeler made his outstanding discovery about Saturn. He recognized that the Doppler principle could be used to confirm Maxwell’s theory that the planet's rings were made up of a multitude of small particles orbiting the planet. If the theory was correct, which all astronomers believed it was, the rotational velocity of the rings would decrease with increasing distance from the planet due to the decrease in the gravitational force.
Keeler realized that he could test Maxwell’s theory provided his spectroscope was accurate enough. By centering the image of Saturn on the slit of the spectroscope, the radial velocities of the planet and the rings could be measured. Since Saturn rotates counter-clockwise, the spectral line he was observing would be shifted to longer wavelengths for all points on the spectrograph to the right of center, but to shorter wavelengths for all points to the left. If Maxwell’s theory was correct, the spectral shift of the light emitted from the rings (and from that their rotational velocity) would decrease in a prescribed manner as the distance from the planet increased.
On the night of April 9, 1895 Keeler aimed Allegheny’s 13-inch refractor at Saturn and made a two-hour long exposure of its spectrum. After developing the plates, the behavior he hoped to see was clearly visible in the photograph. He repeated the measurements the next night and got similar results. Delighted that he had obtained observational proof of Maxwell’s theory, he fired off the following short letter to his good friend and co-editor of the Astrophysical Journal George Ellery Hale [9]:

Dear Hale,
I made an interesting spectroscopic find last night that will make a good article for the Journal. It is a spectroscopic proof of the meteoric constitution of Saturn's rings. [Keeler's underlining]
The sketch shows the idea. The upper figure is Saturn, the observer being in the plane of the paper (the inclination of the ring simply introduces a constant factor, cos[ine] inclination. The lower figure shows the resulting line in the spectrum with the slit in the position indicated, after allowing for displacements due to rotation. The planet rotates as a whole, and the resulting [thick straight] line is inclined but straight, having an equation of the form y = ax (center of slit, or line, as origin).
If the ring rotated as a whole, the lines in its spectrum would also be straight, and inclined in the same direction. But in a fine photograph I obtained last night, the inclination is reversed. Now if the ring is made up of small particles revolving around Saturn according to Kepler's third law, the lines would be parts of a curve whose equation is of the form xy^2 = b, as shown in the figure [curved dashed lines], and this is the appearance on my photograph. The scale is too small to show the slight curvature of the lines.
I think this is the prettiest application of Doppler's principle that I have seen. . . .
Keeler’s measurement of 10.3 km/s for Saturn’s rotational speed at the equator was in close agreement with the value deduced from the period of the planet’s rotation. Likewise, his measurement of 18.0 km/s for the average rotational speed of the rings agreed well with the value computed from gravitational dynamics.
Keeler’s paper on the rings of Saturn was published in the May 1895 issue of the Astrophysical Journal. Astronomers everywhere were greatly impressed with the beauty and simplicity of his results. Three years later he was awarded the Rumford Medal of the American Academy of Arts and Sciences and the Henry Draper Medal of the National Academy of Sciences for his spectroscopic work.

Three years after making his discovery about Saturn, Keeler returned to California where he had been made director of Lick Observatory. There he refitted a 36-inch reflector telescope called the Crossley which he used for making long-exposure photographs of nebulae.
Keeler's most important work with the Crossley was on spiral nebulae, which we recognize today as spiral galaxies. His photographs showed much more detail than ever seen before. He soon discovered that many of the known nebulae were in fact spiral, and of the estimated 120,000 nebulae bright enough to be photographed by the Crossley, a very large fraction of them appeared to be spiral.
Keeler considered his program of photographing nebulae as a diversion from the spectroscopic work he was planning on doing with the Crossley. However, that work was cut short when he died of a massive stroke on August 12, 1900 at the age of 42.

At the time of his death Keeler was considered the outstanding astrophysicist of his generation. Had he not been struck down so young, he would probably have discovered the very large radial velocities of spiral nebulae and may well have reached the conclusion that they were separate and independent systems of stars.
To read more about James Edward Keeler and his family click on the links to the following two pages on my website: Lick Observatory and Allegheny Observatory Crypt.
Notes:
Much of the information used in this post is from the excellent biography of my great grandfather: James E. Keeler: Pioneer American astrophysicist and the early development of American astrophysics, Cambridge University Press, 1984 by astrophysicist and former director of Lick Observatory, Donald E. Osterbrock. This book is referred to below as Osterbrock.
[1] Ink, Dirt and Powder Smoke: The Civil War Letters of William F. Keeler, Paymaster on the USS Monitor, edited by Charles W. McLandress, Seal River Publishing, Toronto, 2023. drawings in possession of author.
[2] Henry D. Keeler to Elizabeth E. Dutton, 15 February 1873, Henry Dutton Family Papers (MS 2094), Manuscripts and Archives, Yale University Library.
[3] Mayport Astronomical Observatory Journal by James E. Keeler, 1877-85. Original in possession of author. Photocopy in James E. Keeler Papers (MS 271), Special Collections and Archives, University Library, University of California, Santa Cruz.
[4] James E. Keeler Journal, 1875-78. Original in possession of author. Photocopy in James E. Keeler Papers (MS 271), Special Collections and Archives, University Library, University of California, Santa Cruz.
[5] ibid.
[6] Osterbrock, p. 10.
[7] Osterbrock, p. 56.
[8] James E. Keeler to George E. Hale, 10 April 1895, Photographic Archive, University of Chicago Library.
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