Crossing Saturn's rings.
Over a 7 1/2-year period Saturn's rings slowly narrow from a broad ellipse to a slender thread. Then, in a matter of hours, the last hint of a streak disappears as Earth passes through the plane of the rings. These events can involve either one passage or, as in 1995-96, a sequence of three. Two of these have already occurred - on May 21st and August 10th; the last one will take place on February 11, 1996 (May issue, page 68, and August, page 72).
The August 10th event was particularly interesting to us. Because Saturn was well placed in a dark sky, it was the apparition's most favorable crossing. Also, the Sun-Saturn-Earth geometry was nearly identical to the second crossing of 1907-08, which Edward Emerson Barnard observed through the Yerkes 40-inch refractor.
Having recently completed his biography of Barnard, coauthor William Sheehan was invited by Lick Observatory director Joseph Miller and astronomical historian Donald Osterbrock to observe the August 10th event with the famous 36-inch Clark telescope, which Barnard had used in 1891 to observe another ring-plane passage.(*)
Sheehan was joined by three observers: coauthor Stephen James O'Meara, who had observed with him at Pic du Midi in 1992 (S&T: January 1993, page 20); Thomas A. Dobbins, a noted Ohio amateur and coauthor of Observing and Photographing the Solar System; and David Graham, director of the British Astronomical Association's Saturn Section. Remington Stone, a resident Lick astronomer, assisted us at the telescope.
We intended to search for faint, known, Saturnian satellites and ring anomalies with the giant glass. Another purpose was to compare what we could see to Barnard's observations of 1891 and 1907-08. Dobbins was especially interested in comparing our visual impressions with photographic and video images. Thus he came equipped with a 35-millimeter camera, a high-resolution CCD video camera with a sensitivity of less than 0.1 lux, a 9-inch monochrome monitor, and a filter wheel.
The sun-bleached domes of Lick Observatory rise prominently above the craggy oaks dotting the 4,200-foot-high summit of Mount Hamilton - an inconspicuous peak in California's sprawling Coast Range. The 36-inch Clark refractor there was the largest in the world between 1888 and 1897; it has a majesty and romance that modern instruments lack.
The airflow over the summit was often smooth and laminar during our stay. Except for our first night, we enjoyed subarcsecond seeing, which allowed the continual use of 588x and 1,176x with various filters for visual observations. We were never once tempted to stop down the lens as advocated by Percival Lowell and his disciples at the turn of the century. The great instrument was in remarkably good adjustment, and its moving mass of 13 tons could pulled along by a strong hand.
Saturn's muted belts and zones displayed attractive color with traces of subtle wavy structure. To Sheehan the Equatorial Zone appeared creamy white, while the higher latitudes seemed glazed in pale lemon-yellow light besmirched with butterscotch bands. The zone bordering the greenish skullcap of the south pole was particularly bright. The latitude of this bright southern zone corresponds to that of a white spot detected in 1994 by astronomers at Pic du Midi Observatory.
The Sun was about 1 1/2 [degrees] north of the ring plane when we began our observations on August 8th. In the initially indifferent seeing a soft-edged but prominent ring shadow with a dark core appeared to slice across the disk. Such a hazy shadow edge was noted by Percival Lowell observing with the 24-inch Clark at Flagstaff in 1907. He attributed the effect to extraplanar particles arranged in what he called "anchor-rings" or "tores."
The impression of a cloud of particles spreading out from the ring plane was certainly convincing, and had we come during a season of only average seeing we might never have realized the truth. But in the moments of exquisite seeing the effect vanished and the shadow narrowed to engraved sharpness. We concluded that the effect is caused by minor atmospheric turbulence widening the shadow and diffusing its edges. An accompanying visual effect, sharp black indentations where the ring shadow crossed Saturn's limbs, appeared similar to the famous "black drop" distortion of the disks of Mercury and Venus when they are very close to the solar limb during transits (see the sketches above).
THE RINGS: A DIARY
Our nightly observations of Saturn began around midnight. When the planet culminated, around 2:20 a.m., we focused our attention on the rings. The following notes record our impressions during the fantastic run of clear nights with the great telescope.
August 8. The rings are shadowed in darkness. Although the seeing is good during late twilight, conditions deteriorate as the evening progresses, with high winds buffeting the telescope. Still, the two of us suspect a ring segment where the western ansa of the rings should be, with Mimas hanging on it like a tiny jewel. After discussion we agree this may be a physiological illusion produced by the eye-brain combination playing a celestial game of Connect the Dots.
We later witness a spectacular do-si-do of moons only 3 arcseconds from the west limb. Tethys is just emerging from a transit across Saturn's face as Mimas, followed by Enceladus, approaches from the opposite direction. The emergence of Tethys at first goes unnoticed in the poor seeing. For a few confusing minutes, it looks as though Mimas is wobbling back and forth next to Saturn's globe. We finally sort out the strange events: As Mimas went behind Saturn, Tethys immediately took its place. Then Tethys occulted Enceladus - just before Enceladus slipped behind Saturn's disk!
August 9. In near-perfect seeing the dark side of Saturn's rings is feebly visible to us while using averted vision and an occulting bar made from a section of a drill bit. Dobbins and Graham remain skeptical about our claim, yet we are both convinced that we see the rings lined up slightly north of the ring shadow, appearing stronger on the western side. The rings are ghostly, nebulous; we doubt whether they could be seen in smaller instruments. In the excellent seeing O'Meara records two faint, starlike beads in the ring plane between Mimas and Saturn.
August 10, about 10 hours before ring-plane crossing. Graham, with some exasperation, records in his observing book: "The vexing question of the visibility of the ring continues unabated from last night. I cannot detect any trace of it whatsoever other than the shadow of the ring on the globe which is patently obvious." Curiously, his drawing records a faint gray belt just north of the ring shadow, possibly the silhouette of the rings themselves crossing the globe.
August 11, about 11 hours after ring-plane passage [which occurred on August 10 around 2:00 p.m. PST]. Viewing the now-Sunlit face of the rings, all of us see them as a very thin, clear line the finest imaginable - extending with increasing faintness to about 2 Saturn radii. Graham writes with satisfaction, "The RING HAS RETURNED! ... This is the sight we have eagerly awaited and what a spectacle it makes." To complete the picture, a faint satellite lies off the extremity of either ansa. Surprisingly, the rings display a deep coppery tinge. Both Graham and Dobbins confirm this color, though Dobbins sees it as more tan than coppery.
Dobbins uses his 35-mm camera and occulting bar to take long-exposure photographs. They show the ring extending even beyond the outer edge of ring A (2.26 Saturn radii) to a distance of 2.9 radii on the western ansa. Apparently we captured a faint exterior ring or possibly additional unresolved satellites (see the drawing at left).
During our three hours of observations the rings continue to brighten perceptibly, until at the end they are just as prominent without the occulting bar as they had been with it. The rings are even visible in the telescope's 6-inch finderscope!
August 12-13. The rings continue to brighten and widen. They clearly extend symmetrically about 2 Saturn radii, tapered like the cross-section of a biconvex lens. The rings are continuous except for a small area close to the planet where the glare wipes them out. The line of light appears slightly north of the rings' shadow-line across the ball. On the night of the 13th the coppery tone of the previous night has vanished; the rings appear a steady, bright yellow.
August 14. A vision of beauty! As we observe, astronomers working with the nearby 120-inch Shane reflector measure subarcsecond seeing. This confirmed what we already knew: the seeing during our seven nights on Mount Hamilton had often been exceptional.
Although the shadowed face of Saturn's rings was obvious in small telescopes for some weeks before the ring-plane crossing, its visibility diminished as the time of passage neared. Just hours beforehand the rings appeared only as faint, nebulous threads of light through the great 36-inch refractor. False suggestions of the rings' presence - such as the afterimage of the dark shadow on the disk, or the eye-brain's connection of dots - were at times very convincing. Yet when the rings themselves were glimpsed they were fainter and of lower contrast than the illusory effects that mimicked them. In any case, for the two days prior to the August 10th ring-plane crossing the unilluminated face was visible to only two of us, and then just barely.
By comparison, in the week prior to the 1891 ring-plane crossing Barnard could not make out the slightest trace of the rings in either the 36-inch or 12-inch refractors at Lick. Within hours after the crossing, however, he noted that they became "easily and distinctly visible as slender threads of light."
At the reappearance of the rings in January 1908 Barnard was at Yerkes, and he attempted to observe the event in the middle of a severe Wisconsin winter. He was critically ill at the time and had to "wrap up good to go into the big dome." The night before the reappearance, he found the rings invisible in the 40-inch refractor. Hours after ring-plane crossing they had brightened up, appearing as a uniform line "of an ashy color, rather thickish and fuzzy."
Judging by Barnard's rather equivocal description of the event, we witnessed a speedier and more dramatic reappearance of the rings than that of either 1891 or 1908. This may have been due to instrumental and seeing factors. Only hours after minimal edge-on presentation the sunlit face of the rings appeared to us as an exceedingly delicate yet unmistakable thread of bright light.
It will be worthwhile to look for similar effects in your own telescopes at the February 1996 crossing. But such an opportunity as we had at Lick will not come again until 2038!
* William Sheehan, The Immortal Fire Within: The Life and Work of Edward Emerson Barnard (Cambridge University Press, 1995).