Telescopic Work for Starlight Evenings

by William F. Denning

Discordant Observations.—It is curious that the details of Saturn have occasioned more dissension amongst observers than those of any other planet. This may have partly arisen from the great distance of Saturn, the comparative feebleness of his light, and complexity of his structure. The planet is usually better defined than either Mars or Jupiter; but with tolerably high powers on small instruments the image is faint, and the features so diluted that the impressions received cannot always be depended upon, especially when the air is unsteady. A fluttering condition of the object is sufficient in itself to cause deception. Prof. Hall, in speaking of the work done by the 25·8-inch refractor at Washington in 1883, says:—“Saturn’s ring has been observed, but many of the strange phenomena noted by other observers have not been seen even on the best nights.” The evidence afforded by this large instrument may not always be conclusive, but in this case there can be no doubt it properly failed to show “phenomena” which had no existence.

Eccentric Position of the Rings.—The rings are slightly eccentric with regard to the ball; in other words, the ball is not situated in the centre of the rings. Differences have been observed denoting this, though the observations are not altogether satisfactory. It has been shown theoretically that the eccentricity referred to is necessary to maintain the stable equilibrium of the system; for were the rings perfectly concentric with the planet, they must coalesce with the ball. The preservation of so complicated a structure must evidently require judicious and nicely balanced conditions.

With the great 23-inch refractor at Princeton, U.S.A., the ball of Saturn was seen through the division in the ring in November 1883—an observation which had previously been made by Lassell in 1852.

Aspect of the Rings.—In different years the rings present a varying outline, owing to the fact of their inclination (28° 10´) and to changes in the relative positions of the Earth and Saturn. At intervals of about fifteen years the rings are widely open, as they were in 1855, 1869, and 1885, and will be in 1899. At similar intervals they are rendered invisible, being turned edgeways to the Earth, as in 1848, 1862, 1877, and 1891. Since 1877 the S. side of the rings has been presented to terrestrial observers; but in 1893 the N. side will come under inspection, and remain in view until 1907. The S. side of the rings is obviously more favourably visible to observers in England and other N. latitudes, because the planet is always above the equator and attains a fair altitude when it is presented. The N. side of the rings is exposed when Saturn is in S. declination, and therefore more liable to our atmospheric disturbances owing to his comparatively low altitude. The extreme narrowness of the rings is apparent at the periods when the planet crosses the node and they are situated in the plane of the line of sight. In small telescopes they become invisible, and the finest instruments only exhibit them as thread-like extensions from the equator of the planet. Sir J. Herschel says that on April 29, 1833, the disappearance of the ring was complete when observed with a reflector of 18 inches aperture and 20 feet focal length. It remained visible in 1862 as a broken line of light. At such times the satellites are seen as bright beads, threading their way along the narrow wavering line of the belts. Inequalities have been observed at such times; for the line of light into which the rings are then resolved is not uniform in breadth, but appears broken and undulatory, as though indicating a very rugged character of surface.

Sir J. Herschel estimated the thickness of the rings as 250 miles, but Bond thought it far less—about 40 miles. There are great obstacles in the way of ascertaining the exact proportions of a structure so distant and offering such an extremely slender form to our view.

Further Observations required.—The globe and rings of Saturn offer an encouraging prospect for additional discoveries. Though the more prominent details have already been descried, there remain other features, probably of more delicate outline and intermittent visibility, which will be glimpsed in future years. Small instruments will scarcely be competent to deal efficiently with this object: observers who can command at least a moderate grasp of light may, however, enter upon the work with every assurance of interesting results. In this, as in other sections of observational astronomy, the student will realize that in oft-repeated observation and comparison of records and drawings he acquires a familiarity with the appearance of the object which will enable him to discern more and more of its configuration, until ultimately he feels confident he has progressed as far as the utmost capacity of his instrument will permit. It is in the sedulous application of his powers that the observer will find the key to success. Partial devotion to a subject offers a prospect far less encouraging; for observations of a disconnected character are seldom valuable.

Changes are unquestionably occurring both in connection with the ball and rings of Saturn[45]. Some of the discrepancies between the observations published from time to time are only to be explained on this assumption. It should therefore be the aim of observers to obtain further evidence of such variations, and this may be best accomplished by assiduously watching the lineaments of the planet during the most favourable periods of each opposition. The collection of a number of reliable materials through a series of years would undoubtedly possess weight in removing some of the anomalies of past observation, and afford us a more thorough knowledge of the delicate markings.

The rotation-period of Saturn is probably not much different from that given by the atmospheric markings seen by Herschel and Hall. But additional determinations are very desirable for many reasons. The spots which are so plentiful on Mars and Jupiter have furnished observers with a valid and concise means of ascertaining the rate of axial motion of those planets. Saturn, however, has far more sparingly provided the data for such an investigation; for if we disregard Schröter’s uncertain figures, we have but two values for the rotation-period. These were fortunately effected by observers of exceptional ability, and the periods may be accepted without reservation; but other independent determinations are much required. By multiplying results of this nature, we have a prolific source of comparison; and comparisons, apart from being interesting, are of importance in denoting erratic results and indicating those entitled to credence. Moreover, a reliable mean value may be sometimes deduced from multiple records; hence it becomes advisable to secure as many as possible.

The planet should be frequently examined during every opposition with the highest powers that are consistent with a perfectly distinct image; and the observer should closely scan the various parts of the disk, with an endeavour to trace spots, breaks, or other irregularities in the belts. Certain inequalities of tone have been occasionally apparent in past years, and they will doubtless reappear. The recovery of these features will form a welcome addition to our knowledge, and, if adequately observed, will enable the rotation-period of the planet to be rediscussed. In an enquiry of this kind many observations are needful, and the longer the interval over which they extend the more accurate the results derived from them are likely to be. If a broken belt should appear on Saturn, the time of its passing the planet’s central meridian should be recorded, either by measurement or careful estimation, and an ephemeris computed based on a rotation-period of 10-1/4^h, which is equal to a daily rate of nearly 843°. Then it should be carefully looked for on subsequent evenings at the times given in the ephemeris, and on every occasion when re-observed its time of transit should be noted as at first. As long as the break continues visible, so long ought it to be kept in view and the times of its central passages tabulated. It would be advisable in such a case to secure cooperation from other observers, as more numerous observations would be sure to accrue, so that, on the appearance of a marking such as that alluded to, the discoverer will do well to announce it immediately to other amateurs who are engaged upon planetary work and most likely to assist him. A white or dark spot, or any condensation on the belts, would of course serve the same purpose as a broken belt. The nature of the object is not necessarily to be considered, the main requirement being that it is one of which the longitude admits of determination. Markings on the belts, if they are ever discernible, must be watched with corresponding assiduity for traces of motion; and if such motion should betray itself, the object of the observer will be to ascertain its rate.

With reference to the narrow division in the outer ring, usually termed “Encke’s division,” astronomers would regard it as a gratifying advance could the doubts overhanging this feature be removed. Is it a real division in the ring, or simply a pencil-line of shading on the flat surface? Is it constant in place and appearance, or does it frequently exhibit changes both as to intensity and position? Judging from prior experiences, this particular object would appear to be extremely fugitive, and incapable of being assigned either a definite place or aspect. Yet the more pronounced and well-attested details of Saturn show no such vagaries: Cassini’s division seems invariable. Are we therefore to surmise that the curious behaviour of Encke’s division is to be referred to errors of observation arising from the effects of unsteady air upon a very delicate object? It is for future observers to answer these questions, and this will entail no ordinary effort, for the same impediments will be encountered in the future as in the past. But fortunately our science is rapidly progressive, and there is no doubt the mystery of Encke’s division will find-its solution before long. A powerful telescope, and a keen and continuous study of the outer ring, will enable some discriminating observer to tell us the true story of its phenomena.