Diameter and accurate determination of the diameter of Mars presents peculiar difficulties which have not as yet been satisfactorily resolved. Ordinary telescopic measure ments of diameter give results which are too great a consequence of several disturbing factors all of which act in a positive sense. Especially to be mentioned is irradiation, a subjective disturbance of vision which causes a bright object to appear larger than it is. The effects of most of these factors can be evaluated and allowed for, but there remains the influence of the planet's atmosphere, which possesses a certain amount of visibility, especially when it contains haze, and is seen in increased thickness at the planet's edge. This latter aspect of the problem has recently excited some discussion, and will be further alluded to in the subsection relat ing to photography by light of different colors. The equatorial diameter of Mars resulting from what are regarded as the most reliable telescopic measures is 6,740 km. (4,190 miles). Photo graphs made in yellow light by R. J. Trumpler give 6,82o km. miles). Independent determinations of the solid surface by Trumpler and by W. H. Wright indicate a diameter approxi mately i8o km. (112 m.) less than this. If this difference is inter preted in terms of the atmospheric overlayer it provides a value of 90 km. (55 m.) for the atmospheric thickness, a result which may be very much in error, and is mentioned here principally for the purpose of illustrating a difficulty encountered in determining the diameter of the planet.
Measurements of the polar and equatorial diameters indicate an ellipticity of figure greater than can reasonably be accounted for by the planet's rotation, and greater also than seems com patible with the observed motion of the satellites. The usually adopted measure of ellipticity or "flattening" is the fractional excess of equatorial over polar diameter. This quantity, as derived from the measurements, is o•oio (Trumpler, Lick Obs. Bull., vol. 13, p. 31) ; and as calculated from the dynamical considerations just indicated, o.005. It appears likely at the present time that the smaller, or calculated ellipticity is correct, and that the dis crepancy in the observed value is to be ascribed to phenomena of the Martian atmosphere.
Some of the more important dynamical data not specified else where in this article are: The above are calculated for a mean Martian diameter of 6,770 km. If the diameter of the solid planet is taken to be i8o km. smaller (as explained above) the last three constants become : volume, 0.141 ; density, 4.2; gravity, 0.40.
Surface Features.—The fascination of the study of the Mar tian surface arises from a number of circumstances. Mars is the nearest and most favourably situated for observation of all the planets. Venus it is true comes closer to us, but at the time of its least distance the illuminated hemisphere is turned away, and consequently cannot be seen. Furthermore, Mars is one of the few heavenly bodies on which we see a solid surface, like that of the earth. Its solidity is shown by the substantial permanency of its configurations; the only other astronomical objects of conse quence which present to our view surfaces of a like character are the Moon, and possibly Mercury; otherwise, perhaps with such in considerable exceptions as the asteroids and the satellites of planets, our telescopes reveal only fluid masses or bodies sur rounded by clouds or impenetrable vapour. In the case of the
Moon, the only real rival of Mars in the display of surface detail, we have the advantage of greater proximity, which enables us to distinguish mountains, plains and minor features of topography, but we encounter a sameness of aspect, due in part to its apparent rotational immobility and in part to the absence of an atmosphere, which is not altogether relieved by its phase variations. And whereas we see only slightly more than half the lunar surface, Mars-although its distance is such that, even at closest approach, the ultimate nature of its markings can only be inferred-has the advantage of rotation and all parts of the planet are therefore brought successively before us; it is, besides, surrounded by an atmosphere that provides diversified phenomena of climate and weather. Mars stimulates a unique and compelling interest, as a subject both of scientific investigation and popular theorizing.
In the telescope the planet presents a ruddy or orange coloured disc relieved by darker markings of a greenish blue hue which occupy about three-eighths of the area of the planet, and are, for the most part, distributed in a belt just south of the equator. There is one considerable projection into the northern hemisphere, known as the Syrtis Major, and a few isolated dark regions occur in the northern latitudes, as shown on the chart (see Plate, fig. I I). It should perhaps be explained, in respect to the charts and other illustrative material relating to astronomical subjects, that the usual practice is followed of placing them with the south side uppermost, to correspond with the appearance in the field of an inverting telescope. The dark markings of Mars are permanent features, although some of them undergo variations in size and appearance. Extending from the dark areas into the regions of greater brightness are a number of wisp-like filaments, the so called and much discussed "canals." It seems to be generally agreed among observers who have the use of powerful telescopes that their occurrence is not limited to the bright regions (though they are, naturally, more easily seen there), but that they traverse the dark areas as well, forming a network which completely en meshes the planet. At the poles of rotation are usually seen brilliant white patches, termed polar caps. These have not the quasi-permanent character of the dark markings, for each of them waxes and wanes with the coming and going of winter in the hemisphere to which it belongs, all but disappearing in the sum mer, as shown in figs. 12 to i6 of Plate. The striking parallelism between this phenomenon and the yearly variation of the winter areas of ice and snow in our higher terrestrial latitudes suggested to early observers, that the Martian caps are polar snow-fields, and this view of their constitution appears to be the one most generally held at the present time, although an alternative explana tion in terms of frozen carbon dioxide has been suggested. An other feature presented in the telescopic view of the planet is the so-called "limb light," a narrow band of bluish green tint that outlines the bright limb or fully illuminated edge of the planet.