Astronomical Education and Journalism
Column: This Rings of Confusion!
By: Jeff Medkeff
Length: 1483 Words
Release: Feb 10, 1998
Expire: Apr 30, 1998
A long procession of planets has slowly left our evening sky over the last few months, and a lonely Saturn is slowly sinking closer to the horizon at sunset. Saturn in many ways has been my favorite planet over the years, and I have followed it this apparition with interest. Although the rings are still relatively closed to us earth-based observers, I was treated late last year to an excellent look at a couple of ring divisions I had rarely seen before.
The rings themselves have a fairly well-known history and structure. The A ring, the bright ring farthest from the planet, is narrower and brighter than the B ring, which is the next one closer in. These two rings are separated by the Cassini Division, although the presence of two different rings was suspected before the discovery of the Division due to the differing brightness and color of the rings. The dusky C ring, also called the Crepe ring, is very close to Saturn's globe inside the B ring. The rest of Saturn's rings are either difficult, or impossible, to observe telescopically.
One of the divisions that I saw this apparition was a narrow dark line roughly five-sixths of the way to the outside edge of the A ring. I naturally assumed that I had observed Saturn's elusive Encke Division, or more properly Encke's Gap. Not being the sort of person to let an observation go to waste during a cloudy night, I began to research the history of ring gaps, and discovered that I might well have jumped to conclusions about which ring gap I saw! The subject is an unsettled, or at least confusing, affair, and although 'my' ring gap does indeed have a name, it is not at all the name I would have expected it to bear.
A brief look at the history of ring gap observations will reveal just how messy the whole situation is. The first ring gap, and the only one that is regularly referred to as a 'division', was discovered in 1675 by Giovanni Domenico Cassini. Cassini, who observed from the Paris Observatory, made this discovery with a 2 1/2" refractor, a fact that the small telescope user might take some encouragement from. But beyond Cassini's straightforward discovery of the Cassini Division, which separates the A and B rings, the situation becomes a little more complex.
The next observer I have found who made an observation of a gap in the rings of Saturn is Sir William Herschel. He is best known today, of course, for his ambitious sky survey projects, and for discovering the planet Uranus. But early in his career, he was an avid and regular observer of Saturn. He made only one observation of ring gaps, even though he made numerous observations of details on the planets globe. In June 1780 he recorded that a black linear marking appeared near the inner edge of the B ring - a curious fact, as most later observers noted gaps in the A ring. The instrument used was a 6" reflector, and although the instrument was a good one, he was never able to repeat the observation. Later observers, however, began to observe ring gaps in astounding profusion.
Captain Henry Kater is perhaps the first to report extensively on ring gaps other than Cassini's Division. His lengthy paper describes observations made beginning in 1825 and extending to 1830. Kater reported the A ring to be "separated by numerous dark divisions, extremely close, and one stronger than the rest, dividing the ring about equally." Many of the observations he reported were made with a 10" achromatic refractor. He specifically reported a lack of divisions in the B ring, and so did nothing to support Herschel's observation.
In 1837, Johannes Franz Encke reported an almost perfectly centered shading in the A ring. Encke was, by this time, a renowned mathematical astronomer, already having made a reputation in the calculation of cometary orbits. Though he is not remembered as being a terribly good observer, he made these observations himself, on April 25 and May 28 of that year, and took measurements of the gap's position with a micrometer. As a result of these observations, almost any gap in the A ring has since been referred to indiscriminately as Encke's Gap. It might be worth pointing out that Encke's observation is very similar to Kater's report.
The following year, Father Francesco De Vico noted several faint divisions in ring B, and one division near the outside edge of the A ring. It is difficult to say that De Vico observed the same gap as Encke did, however, as the reported position differs considerably from Encke's measurement.
William Lassell and the Rev. William R. Dawes observed another division in the outer ring on September 7, 1843. This gap was about 6/7ths of the way to the outer edge of the A ring, and observations of it were made through the 1850's. Their A ring gap, like De Vico's, was not in the position that Encke's was; on the other hand, the supposedly new gap of Lassell and Dawes was in almost the same position as the gap reported by De Vico. By the winter of 1854, however, Dawes was also observing a fine division in approximately the center of the A ring, an observation that agreed only tolerably well with Encke's observation, as it was in a slightly different position. Father Angelo Secchi, during the same apparition, observed a similar feature in yet a third, slightly different, position.
All of the observations later than these seem to be, for the most part at least, re-observations of gaps that were initially observed as described above, with one notable exception. After the discovery of the C ring, a gap was reported between it and the B ring; the existence of this feature was hotly debated for some time until it was finally conceded that the gap exists. But observations of other gaps, especially in the A ring, seemed to parallel the observations that had already been made.
Why do I say this is this confusing? Well, it has to do with how the gaps were named - particularly Encke's gap. NASA material prepared for the Cassini mission to Saturn illustrates the difficulty well. The ring gap that Lassell and Dawes observed, the one really close to the outer edge of the A ring, is the one that is called Encke's Gap, both by NASA and by most other print sources. This is curious, because Encke never even observed that gap, let alone discovered it. On the other hand, De Vico might have been the first to observe it, suggesting it should be called De Vico's gap; but if we are unprepared to credit De Vico's observation, then there are grounds for calling it the Dawes/Lassell gap, or some such. Yet, it is called Encke's gap, and should remain so called, I suppose in order to reduce confusion.
Other named gaps include the Keeler Gap, the Maxwell Gap, and the Huygens Gap. Probably none of these individuals ever saw the gap that is named for them; Huygens because he did not have a powerful enough telescope, Maxwell because his gap is not observable from Earth (besides, he was mainly a mathematician and physicist, and not so much an observational astronomer). It is a little harder to say about Keeler, the gap named for him is perhaps large enough to be observed from Earth (though NASA credits discovery of the Keeler Gap to Voyager), but I have not found a record that he observed it. On the other hand, it is clear that Keeler made several very clear observations of the Encke Gap - that is, the gap on the outside edge of the A ring, not the one in the middle. NASA goes so far as to say that Keeler made the first definitive observation of Encke's Gap.
The question of priority of discovery is complicated further, of course, by the question of whether these observers correctly interpreted their observations. An observer who thought that the gaps were dark markings on the ring "surfaces", as Herschel did for quite some time, can hardly be credited with the discovery of individual "gaps". But this matter is much harder to address, and as the landscape is complicated enough by only considering observations, I think it best not to discuss in any depth the question of interpretation. Suffice it to say that after Herschel, most astronomers considered the dark linear markings to be gaps, rather than surface features.
When it comes to the question of what I observed last year, the answer is simple. I observed the Encke Gap, the one discovered by De Vico - or maybe by Dawes and Lassell, or Keeler - but never observed by Encke. What did you see in the rings of Saturn this apparition? And - do you know its name?