Encke's Smudge vs. Keeler's or De Vico's Division

... Akkana

Observers frequently speak of having seen Encke's division, a small division in the outer ("A") ring of Saturn. Are they really seeing Encke's division? On a few nights last fall which had fairly good seeing, I spent some time observing Saturn with a C-8. I definitely saw something about 1/3 of the way from the outside edge of the A ring, but what I saw varied from night to night.

Sometimes I saw a broad, dark division inside the outer edge, contrasting with a narrow brighter strip outside it:

Other times, I saw a dark area extending from 1/3 of the way in, all the way to the outside edge:

(Saturn sketches by me using Jeff Medkeff's Planetary Sketching Blanks.)

The feature I see, though it varies somewhat with seeing conditions, seems to correspond well to Encke's description of the phenomenon he saw. However, this is not the same thing as the fine division in the rings, visible on this satellite image (thumbnail lifted from The Nine Planets). The well-defined, narrow gap was first observed by Father Francesco De Vico, and later observed by (and credited to) James E. Keeler.

Jeff Medkeff's page on the A-ring gap seems to have disappeared, but Eric Jamison has an excellent summary of past observations of the elusive gaps.

JPL's Saturn Fact Sheet lists the Encke Division at 133.6Mm from Saturn, and the Keeler Gap at 136.5. However, their ring system diagram doesn't label the Keeler Gap, and labels the narrow gap near the outer edge of the A ring as the Encke Division, so it's not clear what they're calling the Keeler Gap.

The August 1982 issue of Sky & Telescope has an article about Keeler's Gap, with some of the drawings by Encke and Keeler. The article does not mention DeVico, but says that Lassell and Dawes may have seen the division.

Another interesting Saturnian ring is the elusive D ring: see the page at Excelsior Optics for more information.

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