There are two well-known features visible in Saturn's A-Ring, one called the Encke Minima, and the other called the Encke Division. The Encke Minima is a broad, low contrast feature that is located about halfway out in the middle of the A-Ring. The Encke Division is a narrow, high contrast feature located near the outer edge of the A-Ring, and unlike the Encke Minima is an actual division within the ring.
It has been generally accepted that Johann Franz Encke first reported seeing what is now called the Encke Minima in 1837, while James Keeler first reported seeing what is now called the Encke Division in 1888. However, there is evidence that other observers saw both the Encke Minima and Encke Division before this. This article will discuss these observations in greater detail. It will discuss also some of the important factors involved in seeing these two features, including how wide the rings were as seen from Earth, Saturn's altitude, its diameter, seeing conditions, aperture of the telescope used, and magnification.
In addition, the Encke Division is sometimes referred to as the Keeler Gap. These are actually two different features in Saturn's A-Ring, and this article will discuss the differences between these features.
Drawing by Johann Franz Encke in May of 1837 using a 9.6" refractor showing the broad, low contrast feature in the on the northern face in the middle of the A-Ring that is now called the Encke Minima. Saturn was at opposition that month, but it was relatively low in the sky in the constellation of Libra with a declination of -13° and an altitude of 27°. Saturn's equatorial diameter was 18.64", and its polar diameter was 16.63".
He observed the minima during April and May of 1837 and noted: "In the very clear night of April 25th I tried a new achromatic eyepiece made by the local skillful mechanic Duwe for the refractor on Saturn, I noticed something what I never before observed. Saturn's ring was, at first, divided in an outer and inner ring by the known separation split" (the split he was referring to is the Cassini Division). "In addition to this I saw perfectly clear the outer, less wider ring" (the less wider ring he was referring to is the A-Ring) "being separated by a stripe in two equal parts. The stripe appeared similar as the main separation is seen in less magnifying telescopes. It could still be followed a little from the outer ends towards Saturn's sphere."
He noted that his observation of the minima is very similar to an observation by Henry Kater twelve years before on December 25th 1825: "I fancied that I saw the outer ring separated by numerous dark divisions extremely close, one stronger than the rest dividing the ring about equally". Encke concluded that due to the fact that the stripe had been seen 1825 when the ring system was seen from the southern side of the ring than in 1837, the phenomena should actually be a separation. We know today that the Encke Minima is not an actual division in the ring but rather a broad, low contrast feature.
Drawing of Johann Franz Encke.
Drawing by James Keeler made in January 1888 using the Lick 36" refractor showing the high contrast feature at the edge of the A-Ring that is now called the Encke Division on the northern face of Saturn's A-Ring. Saturn was at opposition that month, and was relatively high in the sky in the constellation of Cancer with a declination of +19°, and an altitude of 69°. Saturn's equatorial diameter was 20.32", and its polar diameter was 18.13".
Photograph of James Keeler.
One of the earliest recorded observations of what is now called the Encke Minima and Encke Division was by Henry Kater. He observed Saturn often to test two reflecting telescopes that he owned. One was manufactured by Watson and had an aperture of 6-1/4" and a focal length of 40 inches, giving it a focal ratio of approximately f/6.4. Of this telescope he felt that "Unfortunately, the mirror of this telescope is too thin, and its performance, in consequence perhaps of this, is subject to much uncertainty; but under favorable circumstances of temperature, &c., the precise nature of which I have not been able to ascertain, I have never seen a telescope which gave a more perfect image." The second telescope was manufactured by Dollond, had an aperture of 6-3/4", and a focal length of 68", giving it a focal ratio of approximately f/10. He felt this telescope was a very good instrument.
Portrait of Henry Kater.
In December 1825 Kater observed Saturn and noted that the seeing conditions were very good, as there was no wind with slight fog (ground fog usually indicates a very stable atmosphere). He was using his 6-1/4" aperture reflector with his best eyepiece at relatively high magnification of approximately 280x, which was 45x per inch of aperture. He noted that there appeared to be three divisions around both ansa on the southern face of the A-Ring, with the central one being the darkest and widest, which is now known as the Encke Minima. The outer most division matches the location of the Encke Division shown in Keeler's drawing. Kater remarked "I have little doubt, from a most careful examination of some hours, that that which has been considered as the outermost ring of Saturn consists of several rings." He was unable to see this detail in the larger 6-3/4" reflector.
Saturn was at opposition earlier that month, and about this time the southern face of the rings were near their maximum presentation towards the Earth. Also, Saturn was high in the sky in the constellation of Taurus, with an altitude of almost 60°, a declination of over +21°, its equatorial diameter was 20.54", and it's polar diameter was 18.32".
Kater made a drawing of Saturn, and a requested that a friend who was observing with him to examine the ring and make a drawing of it. To his friend Ring-A appeared to be composed of a number of divisions, six in total, that seemed similar to a course line engraving. Another friend who observed Saturn that night but was not much accustomed to telescopic observations was able to see central division, but not the two minor ones. Kater felt that this was because his friend was extremely shortsighted, and that the focus had been left set for Kater's eyes. While it is true that the total number of divisions seen were different for each observer, all recorded seeing at least one division in the A-Ring. Kater was the most experienced observer of the three, and his observations match those of later observers including William Dawes, William Lassell, and Phillip Sidney Coolidge.
Drawing by Henry Kater on December 17, 1825 showing several rings features on the southern face of Saturn's A-Ring, including the Encke Division at the outer edge of the A-Ring, the Encke Minima near the middle, and a low contrast ring feature outside of the Cassini Division.
In January 1826 Kater examined Saturn on two more occasions with the larger 6-3/4" reflector. On the first night he thought that he that the A-Ring was composed of several divisions, but they were not so distinctly seen in December with the smaller 6-1/4" reflector. On the second night the ring appeared to be made up of several rings, but he wasn't positive.
In January 1828 Kater once again examined Saturn with the 6-3/4" reflector, but did not see a trace of any divisions in the A-Ring. He therefore was no longer sure that these divisions were permanent, but due to ill health was unable to observe Saturn very often. However, he found that two other observers had noted detail in the A-Ring was that was similar to what he had seen. One was James Short, who was a well-known mirror maker. He reported observing Saturn's rings were divided into three concentric rings in the mid-1700's with one of his large (either a 12" or 18" aperture mirror with a focal length of 144" or 12 feet) reflecting telescopes. Here is a sketch of James Short observation showing the Saturn and its rings system:
Observation by James Short's showing several ring divisions in Saturn's rings. In this sketch, the ring outside of the letter A represents the Cassini Division, and he recorded two rings features outside of the Cassini Division, which appear to be the Encke Minima and Encke Division. He reported that the rings became more distinct as they approached letter B, and became points at letters C and E.
Another person was Lambert Adolphe Quetelet, who told Kater that he had seen on the southern face A-Ring divided into two concentric rings in December 1823 when observing Saturn with a 10" refractor. Saturn had been at opposition in November that year. It appears Quetelet saw the same feature in 1823 that Encke reported in 1837, and which is now called the Encke Minima.
Since other observers had seen this detail, Kater finally presented a paper on his observations to the Royal Astronomical Society on the divisions he saw in Saturn's A-Ring on May 14, 1830.
It should be noted that when he Kater was observing Saturn in January 1828 he mentioned "The quintuple belt was also distinctly seen, and the shading, or deeper yellow, of the inside edge of the inner ring". This description of a "quintuple belt" is similar to an observation that William Herschel made on November 11, 1793, which is now believed to be an early observation of the C-Ring or Crepe Ring. It is possible that Kater saw the Crepe Ring without realizing it, as Herschel had done 35 years earlier. The discovery of the C-Ring is generally credited to William Cranch Bond and George P. Bond, as well as William Dawes in 1850, but others observers including Herschel and Kater apparently saw it without understanding that they were seeing a separate ring.
In September 1843 William Lassell and William Dawes using Lassell's 9" f/12 equatorially mounted reflector reported seeing what is now called the Encke Minima near the center of the A-Ring, and another division near the outside of the ring which is now called the Encke Division on the northern face of the A-Ring. It had been a very warm day with the temperature of 76 degrees, and the seeing conditions that night appeared to have been very good, with the sky being hazy. In regards to the Encke Minima, Dawes remarked "Having obtained a fine adjustment of the focus, I presently perceived the outer ring to be divided into two. This coincided with the impression Mr. Lassell had previously received." For the Encke Division he noted "With 400 the secondary division was perceptible during occasional best views of the planet." A magnification of 400x it took to see the Encke Division that night on their 9" reflector works out to about the same magnification Kater was using per inch of aperture, around 44x per inch.
Photograph of William Dawes.
Photograph of William Lassell.
Although the northern face of the rings were relatively wide open during this time, Saturn was much lower in the sky in the constellation of Sagittarius, and its altitude was only about 14°. Saturn's declination was -22°, its equatorial diameter was 17.57", and its polar diameter was 15.68". Also, Saturn was almost two months past opposition.
Dawes mentioned that although he had heard reports of divisions in Saturn's A-Ring by Short, Quetelet, and Kater (he apparently was unaware of Encke's observation), he had been somewhat incredulous of any existing subdivisions. Now that he and Lassell had seen similar detail, he felt more inclined to report their observation. He noted also how the detail that he and Lassell saw that night was very similar to Kater's drawing from 1825, and how he wished that the planet had had an altitude of 60° as it did when Kater had observed it, rather then an altitude of 14° the night he and Lassell saw it.
It does not appear that Lassell and Dawes made a drawing of Saturn during their observation of Saturn on September 7, 1843, but this diagram made from SkyMap Pro Version 5.0 gives an idea of how wide the rings were that night.
In November and December 1850 Lassell and Dawes reported seeing the Encke Division again. At this time the southern face of the rings were not as wide open as they had been previously, but Saturn was higher in the sky in the constellation of Pisces, with an altitude of almost 41°. Saturn's declination was +3°, its equatorial diameter was 19.09", and its polar diameter was 17.04". Saturn was at opposition in October of that year.
On November 21, 1850, Lassell was observing Saturn with his 24" f/10 equatorially mounted reflector with a magnification of 219x, 567x, and 614x and noted "I several times suspected a second division of the outer ring at both ansa, but could not absolutely verify it. The appearance I saw, or suspected, was a line one-third of the breath of the outer ring from its outer edge."
Drawing of Saturn by William Lassell in December 1850 showing the Encke Division and Crepe Ring.
William Dawes was able to see the Encke Division using his 6-1/3" f/16 Merz & Mahler refractor. On November 23, 1850, he noted that with a magnification of 425x he "Sometimes suspected that the outer ring had a short and narrow line upon it near its extremity." He wondered if perhaps the division in the outer ring was becoming visible again, and made a note to ask William Lassell to look for it.
The next day he received a letter from Lassell indicating he had seen the division again. On November 25 Dawes noted that with a magnification of 282x "I was satisfied that, in the finest moments, a very narrow and short line was visible on the outer ring near its extremities; which was confirmed with power 425x." On November 29 he saw the division again, occasionally at 323x, but far more certainly with 460x. The magnification he used during these observing sessions works out to be between 45x and 73x per inch of aperture.
It was also during these observing sessions in November and December 1850 that Dawes independently discovered Saturn's C-Ring or Crepe Ring. Lassell visited Dawes in early December and verified Dawes observation. The Bond's had discovered the Crepe Ring a couple of weeks earlier, but word did not reach Dawes and Lassell until after they had seen the ring themselves. So both the Bond's and Dawes are given credit for its discovery.
Drawing of Saturn by William Dawes in December 1850 showing the Encke Division and Crepe Ring. Note that for both of these drawings the rings are much more closed up compared to Kater's observation in December 1825, or Lassell and Dawes observation in 1843.
Another observer who saw the Encke Division was Phillip Sidney Coolidge in December 1854 and January 1855 using a Merz & Mahler 15" f/16 refractor. On December 26th he was observing Saturn and noted that the seeing seemed very good as there was thick haze. He was using a magnification of 141x, 316x, and 401x, and noted that "There is certainly one division in the outer half of ring A, and I cannot be positive that that there is not a second one. If so, it is outside of the first division." On December 27th using a magnification of 401x he noted "There are two (and at times I suspect three) divisions in ring A."
Photograph of Major Phillip Sidney Coolidge.
Drawing by Phillip Sidney Coolidge on December 27, 1854 showing several rings features within Saturn's A-Ring, including the Encke Division at the outer edge of the A-Ring, the Encke Minima near the middle, and a low contrast ring feature outside of the Cassini Division.
On January 9th, 1855, he was observing Saturn again with magnifications of 360x and 401x and was able to see the these features more clearly defined:
Drawing by Phillip Sidney Coolidge in January 9, 1855 showing the Encke Division at the outer edge of the A-Ring, the Encke Minima near the middle, and a low contrast ring feature outside of the Cassini Division.
As when Kater had observed the Encke Minima and Encke Division in December 1825, Saturn was at opposition that month, was high in the sky in the constellation of Taurus, with an altitude of almost 68°. Saturn's declination was over +20°, its equatorial diameter was 20.34", and its polar diameter was 18.15". The drawings he made on December 27th and January 9th of the three ring features in the A-Ring are very similar to the drawing that Kater had made 29 years earlier.
From the currently available evidence, it appears that the broad, low contrast feature located about halfway out in the middle of the A-Ring, now called the Encke Minima, that Johann Franz Encke was given credit for first seeing in 1837, was seen previously by James Short in the mid-1700's, by Lambert Adolphe Quetelet in 1823, by and Henry Kater in 1825.
Also, the high contrast feature located out near the outer edge of the A-Ring, now called the Encke Division, that James Keeler was given credit for first seeing in 1888, was seen previously by James Short in the mid-1700's, by Henry Kater in 1825, by William Lassell and William Dawes in both 1843 and 1850, and by Phillip Sidney Coolidge in 1854 and 1855.
It should be noted that the narrow, high contrast feature located near the outer edge of the A-Ring now known officially as the Encke Division by the International Astronomical Union, was never actually observed by Johann Franz Encke. The feature he did observe, now known as the Encke Minima, does not have an official designation by the International Astronomical Union.
To confuse matters somewhat further, the International Astronomical Union named a division that is even narrower than the Encke Division, and is located at the very edge of the A-Ring, the Keeler Gap after James Keeler, even though he never saw this division because it is not visible with ground based telescopes.
The conventional wisdom seems to be that when using telescopes of less then 10" aperture, a magnification of 400x or more is needed to see the Encke Division. A better guide may be to consider the factors that worked in Kater's, Lassell's, and Dawes favor when they observed the Encke Division. These included very good to excellent seeing conditions, a magnification of ~45x per inch of aperture or higher, and the rings being relatively wide open. If the rings are not as wide open, or Saturn is lower in the sky, then it may indeed require higher magnification and/or larger aperture to see the Encke Division in addition to very good to excellent seeing conditions.
Article © 2000 - 2013, Eric Jamison, All rights reserved. May not be used without written permission of the author.