Appearance of the planet.—Period &c.—Phase.—Surface Configuration.—Charts and Nomenclature of Mars.—Discovery of two Satellites and of Canal-shaped markings.—Summary of Observations.—Rotation of Mars.—Further Observations required.—Changes on Mars.—The two Satellites.—Occultations of Mars.

Appearance of the Planet.—Mars is the fourth planet in the order of distance from the Sun. He revolves in an orbit outside that of the Earth, and is the smallest of the superior planets. His brilliancy is sometimes considerable when he occupies a position near to the Earth, and he emits an intense red light36, which renders his appearance all the more striking. Ordinarily his lustre does not equal that of Jupiter, though when favourably placed he becomes a worthy rival of that orb. In 1719 he shone so brightly and with such a fiery aspect as to cause a panic. The superstitious notions and belief in astrological influences prevailing at that time no doubt gave rise to the popular apprehension that the ruddy star was an omen of disaster, and thus it was regarded with feelings of terror. Fortunately the light of science has long since removed such ideas from amongst us, and celestial objects, in all their various forms, are contemplated without misgiving. They are rather welcomed as affording the means of advancing our knowledge of God’s wonderful works as displayed in the heavens.

Period &c.—Mars revolves round the Sun in 686^d 23^h 30^m 41^s, and his mean distance from that luminary is 141,500,000 miles. The orbit is one of considerable eccentricity, the distance varying between 154,700,000 and 128,360,000 miles. The apparent diameter of the planet when in conjunction with the Sun is only 4; but this may augment to 30·4 at an opposition, when the Earth and Mars occupy the least distant parts of their orbits. The real diameter of Mars is nearly 5000 miles.

Phase.—At opposition the disk of Mars is round, but when in quadrature he appears distinctly gibbous and resembles the Moon three days from full. The phase is so palpable that Galilei glimpsed it at the end of 1610. In delineations of Mars the disk is generally drawn circular, the compression being very slight and the phase too trivial to be regarded.

Surface Configuration.—This planet being singularly variable in his position relatively to the Earth, presents at times a diameter so small that the most powerful instruments are ineffective to deal with him. But at certain epochs he becomes an excellent object, with a much expanded disk, on which are displayed a number of bright and dark markings. This happens, however, with comparative rarity; for only during two months or so near every opposition, occurring at intervals of 780 days, can the planet be well seen. Generally the apparent size of Mars is very inconsiderable, and the disk not sharply defined, especially when the altitude is low. Reliable observations are seldom made at a time far removed from the date of opposition. When the planet was badly placed, in July 1882, an observer secured some observations of position,, and published them, thinking he had seen Wells’s Comet, which happened to be in the same quarter of the sky!

Mars, in nearer degree than any other member of our system, shows a configuration which may be likened to that of the Earth as regards its permanency; and in some of its outlines a general resemblance also exists, though in detail there is evidently much that is dissimilar. It is fortunate that the atmosphere of Mars is so rarefied that observers can look upon his real surface-lineaments with satisfactory perspicuity. For more than 250 years now, the telescope has been engaged in perfecting our knowledge of Martian features, and these have exhibited no mobility of form or place (apart from that due to rotation or varying inclination of the planet) so far as may be judged from a comparison of drawings. Plenty of differences exist in the latter, it is true, though similar objects are represented; but the explanation obviously lies in the inaccuracies of amateur artists, and has little if anything to do with physical changes on the planet.

When the spots were discovered in 1636 by Fontana they were, of course, very dimly glimpsed in the incompetent appliances available at that time. Huygens, in 1659, saw them better by means of his long telescopes, but still very imperfectly. Cassini, in 1666, effected a further advance in the same field, and gathered data from which he was able to announce the period of rotation. His value has proved remarkably correct, considering the means he employed to obtain it and the very short interval over which his inquiries were conducted. Huygens had previously, in 1659, witnessed the returns of a certain spot to the same approximate place on the planet, and was led to infer rotation in either 12^h or 24^h. But this was little better than a guess, and not nearly of the same precision as that which marked Cassini’s subsequent determination.

Near the poles of Mars are intensely bright patches, which have been considered to be vast areas of snow-crowned surface or fields of ice. These “polar snows” are not situated exactly at the poles, nor are they opposite to each other. Changes affect their aspect. Occasionally these or other bright markings, when on the limb, appear to protrude beyond the disk, and this curious effect of irradiation distorts the limb in a striking manner.

Charts and Nomenclature of Mars.—It is not desirable to trace with any detail the successive labours of those who have chiefly contributed to our knowledge of areographic features. Maraldi, W. Herschel, SchrÖter, MÄdler, Schmidt, and Dawes were foremost amongst the observers of the past; while Schiaparelli and Green are the most successful observers of to-day. As telescopes improved in effectiveness the true forms and characteristics of the markings were discerned, and at the present time some thousands of delineations of this planet must be in existence. Charts of the leading and best-assured features have been formed, and the regions of light and shade (supposed to represent land and sea) have received proper names to distinguish them. Thus there is “Fontana Land,” “Maraldi Sea,” “Herschel Continent,” and others of similar import. Schiaparelli has framed a chart in which the spots are furnished with Latin names taken from classical geography. MÄdler’s plan was to designate the markings by capital letters of the alphabet, and to divide these by small letters in necessary cases. But the charts of Proctor, Green, and others, in which the names of past and present astronomers are applied, seem to find most favour, though it is admitted that this method of nomenclature is not free from objections. In some instances the names have not been wisely selected. A few years ago, when christening celestial formations was more in fashion than it is now, a man simply had to use a telescope for an evening or two on Mars or the Moon, and spice the relation of his seeings with something in the way of novelty, when his name would be pretty certainly attached to an object and hung in the heavens for all time! A writer in the ‘Astronomical Register’ for January 1879 humorously suggested that “the matter should be put into the hands of an advertizing agent” and “made the means of raising a revenue for astronomical purposes.” Some men would not object to pay handsomely for the distinction of having their names applied to the seas and continents of Mars or to the craters on the Moon. But it is all very well to disparage a system: can a better one be found? Probably not; but the lavish use of undeserving names is calculated to bring any system into contempt.

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.

Rotation of Mars.—The diurnal period of this planet is known with far greater certainty and precision than that of any other planet, the Earth excepted. It will be useful to quote the values derived since Cassini’s time:—

The last of these, by Prof. Bakhuyzen of Leyden, is probably the best. It was based on a large number of observations extending over 220 years, viz. from those of Huygens in 1659 to those of Schiaparelli in 1879.

In a terrestrial day Mars rotates through 350°·8922, according to Mr. Marth’s period. In one hour the axial motion is 14°·6, whereas on Jupiter the horary rate of rotation is 36°·7. At intervals of 40 days (during which Mars completes 39 rotations) the various features on the disk are presented at very nearly the same times as before. Mr. Marth’s ephemerides of this planet are extremely useful to those who study the markings; and these, in combination with the charts and memoirs of Schiaparelli, Green, Terby, and others, greatly facilitate and encourage the renewed study of this object.

Further Observations required.—Favourable oppositions of Mars occur every 15 years, as in 1877 and 1892. It is at such periods that this planet should be sedulously interrogated for new features, or for corroboration of those already known. Rather a high power must be employed—certainly more than 200; and if the telescope has an aperture of at least 8 inches, the observer will be sure to discern a considerable extent of detail. He should compare his views with the various charts previously alluded to, and note any inconsistencies. Fresh drawings should also be made; and if the forms are not well assured on one night, he may confirm them by coming 37 minutes later to his instrument on the following night. Or the collective issue of several nights’ work may be included in the same drawing. The bright spots on the planet should be as attentively studied as the darker regions, and given a place in every drawing; for it is probably in connection with these luminous objects that active changes may be recognized. The “canals” and their duplication form the principal markings to be looked for; though the successful elucidation of these appearances can only be expected in a case where a powerful telescope, a keen eye, and a good atmosphere operate together. Something of them may be seen under ordinary conditions, and they ought to be very generally sought for by amateurs; for it is not always that success is found where the best conditions prevail. The great telescopes at Mount Hamilton, Nice, and other observatories may be expected to command some advantages of light, power, and position; but this need not prevent competition, or induce the idea that common appliances are practically of no avail. Everyone should strive to achieve as much as is consistent with his means and opportunities; indeed there is all the more need for effort and energy in the observer when his tools are seemingly inadequate to a research, and he should endeavour to find, in his own eye and understanding, that power which shall compensate in a great measure for lack of instrumental capacity. Mr. Proctor, in his ‘Old and New Astronomy,’ has justly remarked:—“The directors of Government observatories have usually been much less successful in studying planetary details than those zealous amateurs who take delight in the study of the heavenly orbs and are ready to wait and watch for favourable opportunities.”

Changes on Mars.—Changes have been confidently reported in some of the Martian spots. Instances have been quoted in which particular markings, though very plain at certain times, have scarcely been perceptible at others. Variations in outline as well as in visibility appear to have been witnessed, and the subject is one which merits more extended notice. It has been asserted that the origin of such variations probably lies in the aerial envelope of Mars. In April and May 1888 M. Perrotin, with the great refractor at Nice, failed to re-observe the feature known as the continent “Libya” on Schiaparelli’s chart, and stated that though this formation was plainly visible in 1886, it had ceased to exist in 1888. He suggested that the obscuration was really produced by clouds or mists circulating in the atmosphere of Mars. But Prof. Holden reported, from the Lick Observatory, that the object alluded to was distinctly visible with the 36-inch refractor there at the end of July, and in the same form in which it was drawn by Prof. Schiaparelli in 1877-8. It is to be assumed, therefore, that if any change occurred it was one of transient nature.

There are other questions relating to the physical aspect of this planet which future observers should be able to answer. Do the markings retain their distinctness right up to the limb? Is the opaque crescent of the disk (when Mars is in quadrature) involved in any phosphorescence or glow indicating an atmosphere? Are the bright spots and luminous borders to the continents equally as stable as the dark spots, and do they maintain an equable brilliancy?

The N. hemisphere of Mars needs much further study, as it is not so familiarly known as the S. hemisphere. This is due to the circumstance that, at favourable oppositions, the region of the S. pole is suitably presented for observation. It is only when the planet is comparatively distant, and small in diameter, that his N. hemisphere comes into view.

The difference of inclination under which the features are seen at successive oppositions gives rise to many apparent changes of figure. When the S. hemisphere is exposed to the Earth numerous objects are seen which are quite invisible when the opposite hemisphere is displayed to us. These altering conditions have to be considered in their influences by every student of areography.

Satellites of Mars.—After evading the keen and searching eyes of Sir W. Herschel, and the power of his 40-foot telescope—after eluding the grasp of Lord Rosse’s 6-foot speculum, and the frequent scrutiny of Lassell with his 2-and 4-foot mirrors, the two satellites of Mars were ultimately revealed to Prof. Hall in the 25·8-inch refractor at Washington. These tiny orbs had been enabled to avoid previous discovery by their minuteness and by their close proximity to Mars. Yet as soon as they were known to exist many observers saw them, and in certain cases success was undoubtedly attained with comparatively small instruments. The late Dr. Erck picked up the outermost satellite with a 7-1/3-inch objective, and Mr. Pratt saw it with an 8-1/7-inch mirror by With. But the effect of this eye-straining may just possibly, in one or two instances, have drawn the imagination out of its normal repose. Mr. Pratt’s instrument shows stars in the group e LyrÆ which are invisible in the great Washington telescope and in the 36-inch mirror formerly used by Mr. Common; so that it may well have produced a spectral satellite of Mars. But the satellites are certainly within the occasional reach of moderate means; for they were repeatedly seen with a 9½-inch refractor at the Observatory of Princeton, U.S.A., in October and November 1879. They “were decidedly more easy to see than Mimas,” the innermost satellite of Saturn.

Phobos, the inner satellite, revolves round the planet in 7^h 39^m, in an orbit 6000 miles from the centre of Mars. At max. elongation the satellite is about 12 distant from its primary, and its opposition magnitude is 11½. Deimos, the outer satellite, revolves in 30^h 18^m, and its orbit is 15,000 miles distant from Mars. Its elongations extend to 32, and its opposition mag. is 13½. These diminutive objects are probably not more than 10 miles in diameter. They are obviously too faint for common instruments, nor are they objects on which ordinary amateurs may occupy themselves with advantage. Of course it forms a highly interesting spectacle to glimpse, just for once, it may be, the small bodies which resisted telescopic power for more than two and a half centuries; but for really useful observations, large aperture and means of accurate measurement are required.

Occultations of Mars.—The most ancient account of a planetary occultation is probably that given by Aristotle, who refers to a lunar obscuration of Mars that occurred on April 4, 357 B.C., according to the calculations of Kepler. Another occultation of Mars appears to have been recorded by the Chinese on Feb. 14, 69 B.C. Tycho Brahe observed a repetition of the event on Dec. 30, 1595. Mr. Baily describes a phenomenon of this kind which occurred on Feb. 18, 1837, when “the planet appeared of a fine yellow colour both at its ingress and egress. No projection was observed.” Mr. Snow, of Ashhurst, saw the occultation of March 12, 1854, and he states the planet “was of almost precisely the same colour as the Moon, and he could not help comparing it to a spangle on the face of the sky. Whilst it was slowly and solemnly vanishing, it gave for several seconds the notion of its being the summit of a lunar mountain, but melted gradually away.” As Mars emersed, “nothing whatever was to be seen of the two bodies, clinging together, as it were, by threads of light; nothing of the pear-shaped appearance often recorded as put on by planets under similar circumstances.” Mr. J. Tebbutt, of Windsor, N. S. W., watched an occultation of Mars in full daylight on Aug. 12, 1875, when “the rapid disappearance of the planet’s disk was an exceedingly interesting phenomenon, its extinction taking place at a considerable distance from the Moon’s illuminated disk. The line marked by the Moon’s dark limb across the disk was well defined.” At the reappearance clouds were prevalent, and “the planet was observed as a small projection on the bright limb;” but he found it difficult to fix the exact time of last contact, owing to the ill-defined character of the planet’s gibbous limb. An occultation of Mars was also seen by Prof. Grant at Glasgow on June 3, 1878.