Our Astrnomical Column

Abstract

THE ROTATION PERIOD OF MARS—The seventh volume of the Annals of the Leiden Observatory contains a very thorough and painstaking investigation by Prof. Bakhuyzen of the rotation period of the planet Mars. In previous determinations one of two courses has usually been adopted, either to compare drawings of Huygens or Hooke with the most recent observations attainable, or to discuss some modern series which seemed to promise to compensate for its restricted range by its greater accuracy. Prof. Bakhuyzen has, however, endeavoured to utilise the entite mass of observations at his disposal, so as to avoid the sources of error to which the other methods are liable, and he possesses a great advantage over earlier investigators, in having access not only to the numerous observations made in 1877 and 1879, but also to the great series of more than 200 drawings which Schroeter had prepared for his projected “Areographischen Beiträge,” and which, becoming the property of the University of Leiden in 1876, was edited and published by Prof. Bakhuyzen in 1881. Prof. Bakhuyzen, in the reduction of these drawings, has adopted provisionally Schiaparelli's position for the pole of Mars—R.A. 317° 46′.0, Dec. 53° 25′.4, mean equinox of 1833.0—and Proctor's rotation-period—24h. 37m. 22.74s.— and deduces corrections to these elements from a comparison of the results obtained by reducing the various observations at his command with them. His first step is, from a discussion of the drawings of Kaiser, Lockyer, Lord Rosse, and Dawes, made during the oppositions of 1862 and 1864, to obtain the time of transit on January 1, 1863, of his adopted prime meridian over the Martial meridian which passes through the earth's north pole, choosing as his prime meridian the one which lies 2 to the east of the centre of M¤dler's point a, corresponding almost exactly to Schiaparelli's Fastigium Aryn, or to Proctor's Dawes Forked Bay, he finds the time of transit over the meridian passing through the north pole of the earth on January i, 1863, to be 20h. 270m. ± 4.0m, Berlin M.T. The areographic longitude of the centre of the Octiliis, the conspicuous circular spot, called by Green the Terby Sea, and by Schiaparelli Lacus Solis, will be, with this prime meridian, 9087. The second section contains the determination of the areographic longitudes of ten of the most conspicuous and easily identified markings on the surface of Mars as inferred by means of the above elements from the drawings of various observers from the time of Hooke and Huygens up to 1879. For the last-named year only Schiaparelli's observations are used, but for 1877 there is an abundant supply, there being available, besides the observations of Schiaparelli, the drawings of Lohse, Green, Dreyer, and Niesten. Beer and M¤dler's drawings afford material for 1830, Herschel and Schroeter give a very full series from 1777 to 1803; and Huygens and Hooke supply a few drawings from 1659 to 1683, from which the longitude of Mddler's f, the Kaiser or Hourglass Sea, Schiaparelli's Syrtis Major, can be inferred. These longitudes are discussed in the third section, and a corrected rotation period is obtained of 24h. 37m. 2266s ± 0.0132S., a value exceedingly close to the mean of the best previous determinations, which are as follows:—