Page:Encyclopædia Britannica, Ninth Edition, v. 2.djvu/875

] able. The planet s synodical period being 779 936 days, or two years and about 49} days, it follows that the place of opposition performs a complete circuit of the ecliptic in an average period of rather more than 7 times the synodical period (for 7 x 49} days = 346 days, so that the change in the place of opposition nearly corresponds to the earth s yearly motion). The correspondence is nearer if two circuits of the ecliptic be taken; for 15 synodical revo lutions are equal to 11,699 040 days, and 32 sidereal years are equal to 11,G88 2048 days, or not quite 11 days less. But a much nearer agreement still is effected in 79 years, which differ by little more than two days from 42 sidereal revolutions of Mars. So that if at any time Mars in opposition has been very favourably placed for observa tion, the same condition of things, or very nearly so, will be brought about after 79 years. For instance, in the opposition of 1719, Mars was within 2 of perihelion, and shone so brightly as to be mis taken for a new star. He was similarly situated at the opposition of 1798, and will again be so situated when in opposition in 1877.

It is said that Fontana first noted the spots of Mars, and suspected the planet s rotation. But Fontana s credit is shaken by his alleged dis covery of seven Jovian satellites. Cassini ob served the spots at Bologna in 16GG, and having continued his observations fur a month, deter mined the planet s rotation-period as 24 h. 40 m. Observers at Rome, mistaking two opposite faces of the planet for the same face, deduced the period 13 h. Cassini in 1670 again observed the planet, obtaining the same rotation-period as before. Maraldi in 1704 noted changes in the shape of the spots; but from some which underwent little change, he deduced the rotation-period 24 h. 39 m. In 1719, when (as already mentioned) Mars was exceedingly well placed for observation, Maraldi again carefully observed the planet, deducing from the observations of 37 days a rotation-period identical with Cassini s.

Besides the dark spots, which have a somewhat greenish colour, the rest of the disk being mostly ruddy, astronomers had noticed a portion of the disk around the south pole which was brightly white. Maraldi states that this spot had been observed for GO years before his later observations were made. One part seemed fainter than the rest, and this portion was subject to considerable changes, occasionally disappearing, while the brighter part was constantly seen. Sir W. Herschel examined a similar bright region around the north pole, which had not before his time been well seen, because the northern regions of the planet are only b nved towards us when the planet i.- traversing the parts of its orbit near aphelion. Herschel s powerful telescopes, however, enabled him to examine the planet during opposi tions occurring in any part of the ecliptic. The magnifying powers he used were sometimes as high as 932, and with this the south polar spot was found to be 41&quot; in diameter. From these observations Herschel concluded that the diurnal rotation of Mars is accomplished in 24 h. 39 in. 21 s., that his equator is inclined to his orbit at an angle of 28 42, and his axis of rotation to the axis of the eclip tic at an angle of 30 18. The bright appearance so remarkable about the poles of Mars was ascribed by Herschel to the reflection of light from mountains of ice and snow accumulated in those regions.

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According to Herschel, the ratio of his equatorial and polar axes is 103 to 98. Schrdter estimates the same ratio to be that of 81 to 80, but later observers have found no perceptible compression. Observations of Mars in recent times have added largely to our knowledge of the planet. —Chart of Mars on Mercator's Projection. From drawings by Beer and Madler (1830-1857), Dawes (1856-1866), De la Rue, Secchi, and others, Proctor has constructed the above chart of the planet (fig. 40). FIG. 41. Mars, May 23, 1S73, 9 G. M. i. FIG. 4-J. Mars, May 21, 1873. 11.30 G. M. T. Fid. 43. Mars, May 19, 1873, 12.30 G. M. T. Fig. 44. Mars, May 13, 1373, 12.30G. M. T. But the latest drawings, taken during the opposition of 