J.E. Keeler's discovery of a gap in the outer part of the A ring

In early January 1888, James E. Keeler was one of the first astronomers to work with the very new Lick Observatory 36-in. refractor. On January 7 while observing Saturn visually on a night of very fine seeing, he discovered a narrow, dark “division” in the outer part of the A ring. Despite repeated attempts, neither Keeler nor any of the other Lick observers saw this gap again until over a year later, on March 2, 1889, another night of extremely good seeing. On that occasion not only Keeler, but also E. S. Holden, J. M. Schaeberle, and E. E. Barnard all observed “Mr. Keeler's division,” as Barnard called it. It could only be seen using very high magnification with this large telescope, at a site known to be excellent, on the nights of very best definition. This gap is not the same as the feature which J. F. Encke had earlier discovered and described as a low-contrast division nearly in the middle of the A ring, and had drawn as nearly the same width as Cassini's division. Later visual observations by B. Lyot and A. Dollfus, again on nights of fine seeing with large telescopes, showed that the Encke division is complex. To them, with the best resolution, it appeared as three wide minima of light, fuzzy, and of low contrast, with a narrow, well-marked minimum of light at its outer edge. The outer edge is just where Keeler placed his gap, although he did not see the low-contrast structure in the Encke division. The images, with much superior resolution obtained from the Pioneer and Voyager space probes, show that the Encke division is even more complex than Lyot and Dollfus realized, but confirm the narrow Keeler feature as a true gap in the outer part of the A ring.