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 12h or 24h. But this was little better than a guess, and not nearly of the same precision as that which marked Cassini’s subsequent determination.
Mars, 1836, April 13, 9h 50m; long. 332°.
(10-inch reflector; power 252.)
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.