Telescopic Work for Starlight Evenings

by William F. Denning

Many other points in the Saturnian system require renewed attention, but some of them appear to be so doubtful as to scarcely deserve mention. Possibly the student had better commence his review of the planet without any of the bias or prejudice which former observations might occasion. But it is as well to know the true state of the case; for the judgment of a careful observer is not likely to be warped by preconception, and of course some of the doubtful observations may be amply verified at a future time. Several of these have already been briefly referred to, and a few others may here be noted. The form of the shadow thrown on the rings from the ball has been observed of a curious shape, and M. Trouvelot supposes it to be variable and occasioned by changes on the level surface of the rings. The same observer has noticed transverse notches in the edges of the inner bright ring. Evidence of variation is not entirely wanting in regard to the chief division, and observers should notice whether it appears uniformly black, as it has been suggested that a gauze ring fills the interval. Exterior to the outer ring a faint luminosity has also been suspected, as though the phenomenon of the inner ring had its counterpart here. The colour of the belts on the ball should be ascribed by careful estimates, as many such observations may give an insight into the variations occurring. Some observers have alleged that the transparent ring of Bond and Dawes is subject to very perceptible alterations. It must be remembered, however, that the visible aspect of this exceedingly delicate structure is much affected by the condition of the atmosphere, and that the inclination of the Saturnian system must obviously introduce changes. When the inclination is considerable, the globe of the planet may be discerned through this ring with greater effect than at other times, because we have to look through a thinner stratum of its material.

The observer, in seeking to elucidate some of the anomalies of former researches, will possibly himself gain a knowledge of features not hitherto recognized. Of the real existence of these he should assure himself by many critical observations before venturing to announce them.

We have hinted that further discoveries upon Saturn may be considered as practically beyond the reach of small telescopes; but the gratifying fact remains that some of the more noteworthy of the known features are visible in glasses of little pretention as regards size. With a 2-inch refractor, power about 90, not only are the rings splendidly visible, but Cassini’s division is readily glimpsed, as well as the narrow dark belt on the body of the planet. This sufficiently proves that a very small and portable instrument is capable of affording some excellent views of one of the most wonderful objects in the heavens. Grover has seen, with an aperture similar to that named, not only the belts and the shadow of the ball on the rings, but two of the satellites as well; and others may be equally successful.

Occultations of Saturn by the Moon.—Phenomena of this kind were well observed in England on May 8, 1859, April 20 and Sept. 30, 1870. Those of 1859 and Sept. 30, 1870, were observed by the Rev. S. J. Johnson, who noted that “the dull hue of the planet contrasted strikingly with the brilliant yellow of the Moon.” Dawes witnessed the occultation in 1859, and saw the opaque edge of our satellite sharply defined on the ball and rings of Saturn, without the slightest distortion of form. No dark shading was remarked by him contiguous to the Moon’s bright edge at the reappearance, such as he and others had observed on Jupiter on the occasion of his occultation, Jan. 2, 1857. Saturn was described as of a pale greenish hue, and offered a strong contrast to the brilliant yellow lustre of the Moon. On the early morning of April 20, 1870, several observers were on the qui vive for this interesting occurrence; and their experiences are reported in the ‘Monthly Notices R. A. S.’ vol. xxx. p. 175 et seq., from which the following are brief extracts:—

Mr. Ellis:—“The light of the planet, by contrast with the Moon, was very faint.” Mr. Carpenter:—“There was not the least alteration in the planet’s form.” Capt. Noble:—“Saturn appeared of a richly-greenish yellow when compared with the brilliant white light of the Moon.” Mr. G. C. Talmage:—“The difference in colour between Saturn and the Moon was most marked, the planet appearing of a yellow tint.” Mr. J. Carpenter:—“At disappearance the planet was a very dull object when in contact with the Moon; its light probably a twentieth as bright. At reappearance the planet was rather tremulous; no distortion was noticed.” On June 13, 1870, the Rev. J. Spear, of Bengal, watched the Moon pass “steadily over the planet without causing any change of form or giving any indication of the planet’s light passing through an atmospheric medium. When near the Moon’s limb Saturn assumed a sickly green hue.”

I observed the occultation of Sept. 30, 1870, at Bristol, with a 4-1/4-inch refractor; but the event offered no novel traits, the most prominent feature being the difference of brightness in the Moon and Saturn. Mr. C. L. Prince observed this event with a Tulley refractor of 6·8 inches aperture, power 250. He says there was not the slightest distortion of either body, but he noticed that “the edge of the ring lingered somewhat upon the Moon’s limb about the time of disappearance.”

Another occultation occurred soon after new Moon on April 9, 1883, and one of the observers, Mr. Loomis, described the disappearance of the rings as a spectacle of great interest, and said the impression was forcibly conveyed to his mind that the Moon was very much nearer to the eye than Saturn.

The Satellites.—The discovery of the eight moons of this planet ranged over the long period of 193 years. Five different observers share the honours between them. Our knowledge of the Saturnian satellites may almost be said to furnish us with a history of improvements in the telescope; for they were severally detected at epochs corresponding to instrumental advances. The following are the periods, distances, &c. of the satellites:—

No. and Name.	Mean Distance.		Sidereal Period. d h m	Real Diam. Miles.	Date of Discovery.	Discoverer.
	Diameters of Saturn.	Miles.
7th. Mimas	1·53	115,000	0 22 37	1000	1789, Sept. 17.	W. Herschel.
6th. Enceladus	1·97	148,000	1 8 53	...	1789, Aug. 28.	W. Herschel[46].
5th. Tethys	2·44	183,000	1 21 18	500	1684, Mar. 21.	J. D. Cassini.
4th. Dione	3·12	234,000	2 17 41	500	1684, Mar. 21.	J. D. Cassini.
3rd. Rhea	4·36	327,000	4 12 25	1200	1672, Dec. 23.	J. D. Cassini.
1st. Titan	10·12	759,000	15 22 41	3300	1655, Mar. 25.	C. Huygens.
8th. Hyperion	12·23	917,000	21 7 7	...	1848, Sept. 19.	Bond & Lassell.
2nd. Iapetus	29·61	2,221,000 79	7 53	1800	1671, Oct. 25.	J. D. Cassini.

The numbers in the first column refer to the order of discovery.