The more conspicuous Martian phenomena have been known since the early days of telescopic observation. In 1610 the phases were observed by Galileo. The first sketches of surface detail were made by Huygens in 1659. Huygens suggested a rotation period of 24 hours, and Cassini seven years later independently estimated 24 hours and 4o minutes, which is very close to the actual value. The latter observer also descried the polar caps. The drawings of Huygens and of Hooke (166o), though crude when judged by modern standards, have permitted the rotation period to be determined with a high degree of accuracy; the value 22.6' for the sidereal revolution cannot well be in error as much as a tenth of a second. Little of value was accomplished for a century or more but in i 783 Sir William Herschel detected sea sonal variation in the sizes of the polar caps, and determined the position of the axis of rotation. During the nineteenth especially its second half, with the increase in power and quality of telescopes, further details were revealed. In 1877 Asaph Hall dis covered the two satellites. The outstanding observer of this period was G. V. Schiaparelli, who in 1877 made the first comprehensive triangulation of the surface, and added greatly to the number of known line-like markings which he was the first to designate "canals." Schiaparelli also reported a phenomenon with respect to the reality of which his successors have not been uniformly in agreement, namely, the "gemination" or doubling of the canals at certain seasons of the Martian year. Because of the pioneer nature of his work, the excellence of his observations and the philosophi cal character of his deliberations, Schiaparelli is undoubtedly to be regarded as the pre-eminent student of the Martian surface ; in more recent years the names of E. E. Barnard, P. Lowell, E. M. Antoniadi, W. H. Pickering and many others have been associated with the telescopic study of Mars, and noteworthy photographs. have been made especially by E. C. Slipher at Lowell and by R. J Trumpler at the Lick Observatory. Finally, inquiries of two kinds, which may be regarded as departures from the traditional methods of telescopic observation, have been undertaken during the last three oppositions (1922-24-26). One of these has for its object the determination of the planet's temperature through analysis of its thermal radiation ; the other consists in the study of the planet by light of different colours. Both will be discussed.
Before undertaking a more detailed description of the planet than is provided above it will be appropriate to refer to the Mar tian atmosphere, since it is through this gaseous envelope that the planet must be viewed, and some modification in appearance might reasonably be anticipated on this account. Photographic observations indicate that the effect of atmosphere on the visibility and appearance of the surface depends on the colour of light used in observation, and that for colours to which the eye is most sensitive (yellow and spectrally contiguous colours) modification of the appearance of the surface of Mars is not, except perhaps at the polar caps, very serious ; so that it is permissible to approach the study of the planet by way of its telescopic appearance.
In fig. 6 of the Plate is a photograph taken by Barnard with the great telescope of the Yerkes Observatory. It was made by yel low light, a colour to which the eye is specially sensitive, and it illustrates, as well as a monochrome representation can, the ap pearance of the planet when seen in the telescope under good observing conditions. The south polar cap lies on the upper right edge of the disc. To the eye the cap is normally a brilliant white, while the remainder of the lighter toned area is deep yellow or orange ; the dark parts are, as has already been said, normally of a bluish green tinge. The region shown is that of the Syrtis Major, the dark triangular portion resembling the Indian peninsula, ex tending downward to the left. On the chart (see plate) the Syrtis will be found in long. 290°, lat. -1-10°. In correlating photograph and chart it is necessary to bear in mind that in the former the di rection of the axis is inclined upward to the right, as is shown by the position of the polar cap. The lower of the two dark strips, which, on the photograph, reach out to the right, is the Sabaeus Sinus, approximately in long. 345°, lat. —7°. The small excres cence, or downward projection at the extremity of the strip, is the so-called forked bay; the bifurcation is not visible in the photo graphs because of the disadvantageous position near the planet's edge, but will be readily recognized on the chart. The meridian passing through the point of bifurcation has been arbitrarily chosen as the one from which areographic longitudes are measured ("areo graphic," with respect to Mars, ="geographic" in relation to the earth; "areography" is Martian "geography").
The dark areas of the planet were regarded by the early ob servers as expanses of water, and were called "maria" or "seas."
Even so recent an observer as Schiaparelli held this view, but it has now been generally abandoned, at least with respect to most of the dark markings, though a number of the smaller and more variable are by some observers still regarded as lakes. Despite this change in interpretation the names mare and "sea" have con tinued in general use as designations of the dark areas. Conclu sive proof that these areas are not bodies of water is found in the fact that we do not see the reflection of the sun in them, as we certainly should, at times of proper presentation, if they ex posed free surfaces of water; other indications are that the dark markings are far from uniform in strength, that detail is visible in them, and that some are subject to marked alterations, as may be seen by intercomparing the illustrations of groups 6 to 8 and 12 to 16 of the accompanying plate. The several photo graphs of each group were taken at approximately equivalent presentation, and either group provides ample proof of alteration in surface markings. For the purpose of discussing some of these alterations without dependence on the rather cumbersome classical names by which they are ordinarily designated, advantage will be taken of the suggestion which fig. 7 affords of the outline of a stag's head and antlers. The resemblance will be more apparent if the plate is turned to the left through a right angle. It will now be seen that the antlers in figs. 6 and 7 are conformable but that in fig. 8 the object which serves as the antler on the reader's left is not the comparatively erect one that functions in the first two cases. A closer examination of fig. 8 will reveal a vestige of the old antler (Pandorae Fretum), while the new one can be discerned in reduced strength in fig. 7, and as a trace in fig. 6. Other dif ferences are found at the back of the stag's neck, and in the appendage to the chin (Thoth). In fact the disparities are so great that fig. 6 bears little resemblance to a head of any kind. It seems doubtful whether the earth would exhibit, to a planetary observer, alterations of surface features comparable in magnitude with those shown in these photographs. In drawing conclusions from such phenomena as these it is necessary to assure oneself that they are not the consequences of passing clouds, or of atmos pheric obscurations of one kind or another, and fig. 7 shows, in fact, in the light spot at the stag's throat, what is in all likelihood a cloud, while temporary obscurations of one part or another of the surface have occasionally been observed that are most readily accounted for as effects of mists or other atmospheric abnormali ties. There seems to be little ground, however, for doubting that much of the variation seen on Mars is real, and it becomes a matter of importance to inquire whether it synchronizes with the Martian seasons. Schiaparelli seems to have been the first to sus pect seasonal variations, and, while evidence on this point can be supplied only by observations extending over many years, experi enced observers consider that some of the changes are seasonal and others not, though the evidence is not altogether consistent (Publ. Astron. Soc. Pacific, vol. 39, I I, 108-109). Many observers of the planet, especially Lowell and members of the group of astronomers who were at one time or another associated with him at the observatory which he established, have reported sea sonal alterations of a specific character. They find the dark areas to be stronger in the spring and summer of the hemisphere in which they are situated, the intensification starting in high lati tudes, close to the shrinking cap, and working toward the equator, sometimes crossing it. They also report changes in shade or colour accompanying these alterations in intensity, the colour usually passing from bluish green to yellow, as the darkening is reduced, though a change from blue-green to chocolate-brown is stated to be characteristic of a certain region of the planet lying to the south of the Syrtis Major. The photographs in figs. 12 to 16 taken by Mr. E. C. Slipher at the Lowell Observatory have kindly been provided by him to illustrate seasonal alterations in the strength of certain of the markings.' A seasonal effect, the sig nificance of which has been emphasized in interpretations of the phenomena of the caps, is a dark margin said to surround the shrinking cap. This has been explained as flooding consequent upon the melting of the cap. The relationship of areographic mutation to the yearly cycle has been the subject of painstaking inquiry on the part of nearly every observer of the planet ; the literature relating to it is, as a consequence, very voluminous, and from its nature, difficult to abridge or summarize; for fuller infor mation the reader is referred to W. H. Pickering, Popular Astron. vol. 35, p. 195 (1927) and Antoniadi, Bull. Soc. Astron. France, vol. 38, p. 199; vol. 4o, 278.