Telescopic Work for Starlight Evenings

by William F. Denning

Orbits of the Satellites of Mars.

Discovery of Satellites and of Canal-shaped markings.—The interest in this planet has been accentuated in recent years by several circumstances. The discovery of two satellites in 1877 by Prof. Hall, with the 25·8-inch Washington refractor, caused the directors of large instruments to test their capacity upon these minute objects. Schiaparelli’s observations of the canal-shaped markings have afforded another attractive feature in connection with this planet. He detected a network of dark straight lines stretching generally from N. to S. across the planet; and in the winter of 1881 found these objects duplicated, i. e. the lines ran in pairs so close together that they were separated with difficulty. The study of the topography of Mars had never previously revealed structures like these; yet the Italian astronomer appears to have observed them with “comparative ease whenever the air was still.” Other observers have not wholly confirmed the appearances alluded to, but no favourable opposition has occurred since 1877, and no surprise need be felt that the delicate features visible in the pellucid sky of Italy should elude detection in less genial climes. In 1886 M. Perrotin, at Nice, using a 15-inch equatoreal, saw a number of the “canals,” and some of them were double. In 1888 the observers having charge of the 36-inch refractor at Mount Hamilton re-observed the “canals” as broad bands, but none of them appeared to be duplicated. The conditions were unfavourable, the planet being more than three months past opposition.

Prof. Schiaparelli re-observed the duple “canals” in June 1890 with a refractor by Merz of 18 inches aperture, powers 350 and 500. His observations are supported by Mr. A. S. Williams, of Brighton, who informs me that he detected forty-three of the “canals,” and seven of them were “clearly and certainly seen to be double.” Mr. Williams’s instrument is a 6½-inch reflector by Calver, and powers of 320 and 430 were successfully employed on it; magnifiers under 300 were found of little use.

Summary of Observations.—From observations at Bristol I have drawn up the following summary as to the configuration of Mars:—

1. That the “Hour-glass” or “Kaiser Sea,” and some other markings of analogous character, present very bold, dark, and clearly defined outlines, enabling them to be visible in very small telescopes. In 1873 I saw certain spots with a refractor of only 1-3/4-inch aperture. Mr. Grover, in 1867, “made a set of pencil-drawings, with a 2-inch telescope, which gave the general markings of the planet very well.” In ‘Recreative Science’ it is mentioned that on June 7, 1860, a semi-circular dark spot on the N.W. part of the disk of Mars was distinctly seen with a 1½-inch telescope, power 120.

2. There is an intricate mass of surface-markings on the planet, which, in its main features, is capable of being satisfactorily delineated, and which in its general aspect is similar to the canals depicted by Schiaparelli, though not nearly so pronounced, straight, and uniform as he has shown in his charts.

3. The detail is visible in the form of irregular streaks, condensations, and veins of shading, very faint and delicate in some parts. The veins apparently connect many of the larger spots, and here and there show condensations, which have sometimes been drawn as isolated spots. A night of good definition, however, reveals the feeble ligaments of shade connecting them.

4. That there exists on the immediate borders of many of the darker patches and veins a remarkable brightness or shimmering, which reminds one of the bright spots merging out of the dark belts on Jupiter. Just contiguous to the “Kaiser Sea,” and on its eastern limits, this brightness was so striking in March 1886 as to compare with that exhibited by the N. polar cap. In drawings by many observers these regions of special luminosity have no place, but there is little doubt they occupy a leading position in the physical configuration of Mars.

5. That there is no trace of a dense atmosphere on Mars, as some of the text-books infer. The pronounced aspect of the chief markings, their durableness and continuity of form, the ease with which they may be traced up to the limb, the absence of phenomena indicating dense cloud-bearing air-strata, and other observed facts verify the conclusion that the planet’s surface is comparatively free of vapours, and in a totally different condition to that of Jupiter and Saturn.