Observations of Saturn's Ring at the time of its Disappearance in 1907, made with the 40-in. Refractor of the Yerkes Observatory. By E. E. Barnard. (Plates 9, 10.)

The position of Saturn for observation of the phenomena connected with the disappearance of the ring has been unusually favourable this year, though the south declination (5°) of the planet was somewhat unfortunate for northern observers.

At the reappearance of the ring in October 1891, Saturn rose only about two hours before the Sun, and the resulting low position and the approach of daylight made it very unfavourable for the observation of such a delicate thing as the ring when it is placed edge on towards us. At that time, with the 36-in. aud 12-in. refractors of the Lick Observatory, and under such conditions, I was unable to see the ring (M.N. for April 1892, vol. lii. p. 419).

According to Professor Hermann Struve, the following table represents the dates of the present disappearances and reappearances of the ring of Saturn.*

1907. Apr. 17. Disappearance. The Earth in the plane of the rings.

July 26. Reappearance. Oct. 4. Disappearance. 1908. Jan. 7. Reappearance.

The Sun in the plane of the rings.

The Earth in the plane of the rings. The Earth in the plane of the rings. On account of the position of Saturn in the direction of the Sun, the disappearance of the ring in April was invisible from the Earth. The reappearance in January next will be favourable for observation.

The present paper deals with the reappearance and disappearance of the ring in July and October of this year (1907). The times are central standard time, 6h slow of G.M.T.

The unusually bad spring weather prevented early observations with the 40-in. telescope. The planet was observed, however, as frequently as the weather and other circumstances would permit. It was supposed the "reappearance" of the ring, when the Sun passed through its plane on or about July 26, would be a definite phenomenon, and that the time of this reappearance could be determined with some sort of precision. But the reappearance was a remarkably gradual phenomenon, and there was no po-sible means of telling when it occurred. The ring simply very slowly and gradually got brighter, and for several days it was impossible to tell that any change had taken place; and then it became bright and almost linear. It was not, however, at this time that the greatest interest lay. It was sometime previous to the reappearance of the ring that the most important phenomena were visible.

When the planet was examined on July 2 the entire surface of the ring was easily seen, though the Sun was not then shining on its visible surface. Where it was projected on the sky, the ring * Publications of the Astronomical Society of the Pacific, No. 114, June 10, 1907.

appeared as a greyish hazy or nebulous strip, which was not well defined under the best conditions. It was about 15 times as broad as the trace of the ring on the ball. Nothing could be seen of the sunlit edge of the ring, which must have been too thin to be visible. There were two nebulous condensations of greater brightness on the ring at each side of the planet. These were quite conspicuous, but were ill defined and nebulous, and of a pale grey colour. The ring aud condensations were so pronounced that they were strongly visible at 15h 50m (36m before sunrise), when the approaching daylight was very bright. They were still visible as late as 16h 2 (24m before sunrise), and, though faint, could have been followed a little later yet. Rough measures were made of the distances of these condensations on the preceding side of the planet from the preceding limb.

The Centre of the most Distant Spot
from the Preceding Limb.

7" 7(1) [7" 6]

The Centre of the Nearer Spot
from the Preceding Limb.
2"*9(1) [2" 8]

One setting was made for the extreme diameter of the ring, and this gave 40" 9 [40"o];* while two settings gave 18" 85 for the equatorial diameter of the planet.

At 15h 4m a faint satellite (Enceladus) following was visible on the south side of the ring, close to the second condensation on that side. The satellite and condensation were of the same brightness, which will give some idea of the brightness of the ring and condensations. The trace of the ring across the ball was not black. The seeing was not good enough to tell whether part of this trace was due to the shadow of the ring, which in all probability at that time did not reach to the ball. The width of this trace was measured by setting the wires so that their outer edges were separated by the width of the trace a correction afterwards being applied for the thickness of the wires o".10. The width was o"62(1) at 14h 50m. The ring, as projected on the sky, seemed to be a little south of the trace.

=

Position of the trace of the ring on the ball at 15h 30m:

From South Limb.
8"-83(3) [863]

From North Limb.
7" 84(3) [7" 66]

This trace was without any visible irregularities.

July 5. 15h om. The condensations were again visible. The nearer condensations were estimated to be 1-3 times as bright as the distant ones. One setting gave for the width of the trace of the ring on the ball o" 70. I could see nothing of the sunlit edge of the ring.

The seeing was very poor, and the sky clouded a few minutes later and did not again clear.

The square brackets throughout this paper mean that the enclosed values have been reduced to the mean distance of Saturn from the Sun. The numbers in parentheses are the number of settings of the micrometer wires. The times are central standard times, 6h om slow of Greenwich.

July 6. 14 30m. The ring was rather faint-very much fainter, apparently, than on July 2, and perhaps somewhat

narrower.

Width of trace on ball o"62(2).

Position of the trace :

From South Limb.

8" 79(3) [853]

From North Limb.

8"*02(3) [7"*79]

There was a faint dark belt south of the trace of the ring, 66(1) from the south limb. The condensations on the ring were visible as before. Nothing could be seen of, the sunlit edge of the ring.

July 12. 14h 10m. The feebly luminous ring was visible as a thin nebulous strip on each side of the planet. Seeing very bad. There was a satellite preceding and another (Enceladus) following the ring. When best seen, the ring was about midway in brightness between these satellites. It was very thin and of a pale nebulous colour. The sunlit edge could not be seen. As late as 15h 35m (57 before sunrise) the ring was still distinctly visible. July 23. 13h 50m. Sky thick. Seeing very bad. The ring was dimly visible as a faint thin line. The sunlit edge could not be seen.

July 24. Full moon. Seeing poor. 14h 20m. The full extent

It

of the ring was visible as a narrow, almost thread-like strip. was faint and nebulous, without any irregularities. A faint satellite (Enceladus) was close following the following edge of the ring; when best seen, the ring was perhaps one magnitude less bright than this satellite. There was no appearance of direct sunlight falling on the ring or its edge. The following ansa seemed to be the most distinct-perhaps a little brighter.

July 25. Full moon. 15h 10m. The ring fairly well seen as a faint nebulous strip. 15h 15m. There was a small satellite (Enceladus) preceding the preceding end of the ring; it was 1magnitude brighter than the ring, which was of a faint grey colour. Seeing fair. With an occulter in the eyepiece the ring was quite easily visible. There was no appearance of direct sunlight, however, but the same pale illumination seen previously. There did not seem to be any irregularities on the ring, which was very narrow, and quite distinct when best seen. The trace of the ring on the ball was not black. At 15h 45m (1h om before sunrise) the ring could still be fairly well seen.

July 26. 15h om. Seeing fair. Slight haze. Bright moonlight. The ring was a little brighter, but whether due to less moonlight or not, it was not possible to tell. A small satellite (Enceladus) near and preceding the preceding end of the ring was but little brighter than the ring, perhaps as much as half a magnitude.

15h 15. The ring was decidedly brighter than on the 25th. This was perhaps due to the direct sunlight shining on it. It was very thin. A faint satellite (Tethys) followed the ball, and

was a little south of the ring. It was but a little brighter than the ring near it.

15h 30m. The small satellite (Tethys) following was much brighter than the ring near it-perhaps one magnitude brighter. The ring on the sky was broader than the trace.

A power of 700 was then applied. With this the ring was pale and nebulous. The illumination did not then look like direct

sunlight.

Position of trace of ring on ball at 15h 45m—

From South Limb.

8" 77(2) [8" 22]

The thickness of the trace was o′′·52(4).

From North Limb.

8" 23(2) [772]

16h om (45m before sunrise). The ring was still easily seen. It was but little less bright than the satellite (Enceladus) preceding. The ring was still faintly visible. The sky was white

16h 2m. from daylight. July 28. 1լի 40. The ring was bright, but much less bright than the ball of the planet. The illumination looked like true sunlight. The projection of the ring on the sky was south of the projection on the ball.

14h 30m. Thickness of trace on ball o"44(4). The illumination of the ring did not look like direct sunlight. It was a pale grey nebulous light.

15h 35m. The ring could not be traced up to the ball, but was discontinuous for some few seconds.

15h 45m. The following ansa seemed to be a little thicker than the preceding. A very faint belt on the ball was seen north. The ring on the sky was about twice as thick as its trace on the ball. August 6. 13h 45m. Trace of ring on ball:

15h om. The ring was much less bright than the ball. It looked like a bar of light on each side of the planet. Seeing very poor.

September 1. 11h 5m. A faint belt was visible on each side of the trace of the ring.

September 3.

13h om. The trace on ball seemed to be thinner than usual, and did not seem very distinct. When best seen the ring was clean cut, like a bar sharply pointed at the ends. The thickness of the ring on the sky was roughly o"67(1).

September 8.

Ioh 45m. The trace of the ring on the ball was

occasionally seen. It was very narrow. The ring was bright and

narrow.

September 10. 12h 5m. The trace of the ring was very faint and narrow. The planet was occasionally well seen, but the trace on the ball could scarcely be made out.

October 1. 11h 6m. Seeing bad.

ashy colour and comparatively faint. looked like a nebulous strip.

The ring was of a pale It was not very thin, and

October 4. 8h 30m. The ring was entirely invisible, with magnifying powers of 460 and 700.

8h 40m. Feeble traces of the ring following the planet were suspected, but not certain. The presence of Titan near interfered. A satellite preceding the planet prevented any chance of seeing the ring on that side. There was a faint satellite (Mimas) following, which seemed to be on the ring. Seeing poor.

9h 10m. With an occulter in the field, in the moments of best seeing, feeble traces of the ring were visible following.

9h 25m. With occulter, the following part of the ring was feebly glimpsed, but it was very difficult.

At 9h 45m a hexagonal diaphragm was put on over the O G. This collected the stray light into six rays, leaving clearer sky between. An occulter was used in the eyepiece. With this, very feeble traces of the ring could be seen following, but not near the planet. It could also be seen feebly once in a while preceding. In moments of steadiness, the trace of the ring or shadow of the ring on the globe could be seen. But this would have been easy if the seeing had been good.

Ioh om. The seeing had got too bad to do anything with the planet.

October 5.

11h 40m. The diaphragm removed, occulter on. The seeing very bad; could see nothing of the ring in the steadiest moments. Ioh om. A few minutes' opportunity was offered to observe Saturn with the 40-in. The seeing was fair, and I think better than on the 4th. The ring was faintly visible with the occulter. It seemed to be decidedly brighter than on the 4th. It was very slender and faint. A satellite was close preceding the planet. The shadow of the ring on the ball was sharply marked, but the seeing was not good enough to show it black. The ring was much fainter than a faint satellite (Mimas) near its following end. October 6. 9h 25m. The seeing was extremely bad, with a very high wind.

10h 30m. Seeing very bad, but by glimpses with the occulter the ring could be seen occasionally very feebly. Even with good seeing the ring would have been faint.

10h 50m. In moments of steadiness, fairly good glimpses of the ring were had. It was certainly brighter than on the 4th, and perhaps somewhat brighter than on the 5th. The shadow on the ball could also be seen.

The planet was examined later, but the seeing was too bad for any observations of value.