THE MAEKINGS AND ROTATION OF MERCURY^ 



By E. M. Antoniadi 



It is a well-known fact that the planet Mercury has been singularly 

 neglected by astronomers, so that the greatest confusion is reigning 

 as to the reality and configuration of its spots, the duration of its 

 rotation period, or even the existence of its atmosphere. This un- 

 certainty strikes its roots in the smallness of the disk of Mercury; 

 in the practical hopelessness of detecting well-defined markings on 

 his surface at the very low altitude, and consequent rippling air, of 

 twilight and dawn ; and in the difficulty of finding the planet itself 

 without an equatorial mounting, at daytime, high above the horizon, 

 and in the overpowering glare of the Sun. 



In the beginning of the nineteenth century, Schroter drew moun- 

 tains, a dark streak, and other spots on Mercury, from which Bessel 

 deduced a rotation period of 24" 0™ 53^ round an axis inclined some 

 70° to the plane of the orbit. But these objects were illusive. Yet 

 Schroter had called attention to two features which were subsequently 

 confirmed: (1) that the phase was always smaller than it ought to 

 be theoretically; and (2) that the S. cusp looked very often blunted. 

 This last phenomenon was well explained by Schiaparelli as due to 

 the presence of some dusky area near the S. pole, although he did not 

 draw any such marking on his map of the planet. 



At about the same time as Schroter, Herschel could detect no 

 spots on Mercury. Prince, in 1867, Birmingham, in 1870, called 

 attention to their observation of white areas, while Vogel seemed to 

 recognize other markings. In 1879 Flammarion saw no spots; and 

 from 1876 to 1881 Trouvelot could only catch a glimpse of a white 

 area at the N. cusp of the crescent phase. 



In 1882 De Ball drew a curved dusky shading on the morning 

 phase, which Schiaparelli later identified with one of the markings 

 seen by himself ; and, in the same year, the eminent English observer, 

 Denning, saw several spots, which I was enabled to confirm in 1927, 

 and from which he concluded that the rotation period was about 25". 



Between 1881 and 1889 Mercury was scrutinized in broad daylight 

 by Schiaparelli with his customary perseverance and with the start- 



1 Reprinted by permission, with a few additions by the writer, from The Journal of 

 the Royal Astronomical Society of Canada, vol. 27, no. 10, December 1933. 



111666—35 8 99 



