SCENES ON THE PLANET MERCURY. 65 



an uncertain and flaring aspect which appears to the naked eye as a 

 strong scintillation. For this reason the ancients called it SrtX/W, 

 or the scintillating star. No other resource is left than to essay 

 observations in broad daylight, in the presence of the sun always 

 near, and in an always illuminated atmosphere. 



Some efforts I made in 1881 persuaded me that it was possible 

 both to see the spots of Mercury and to get sufficiently connected 

 and continuous observations of them in broad daylight, and I de- 

 cided in the beginning of 1882 to make a regular study of this 

 planet. During the eight years since then, I have had Mercury in 

 the field of my telescope several hundred times ; often, it is true, 

 with little profit and at the expense of great loss of time, either be- 

 cause of the agitation of the atmosphere, which is often strong 

 during the day — especially in the summer months — or on account 

 of the insufficient transparency of the air. But by patience I have 

 succeeded in seeing the spots on the planet one hundred and fifty 

 times with more or less precision, and in making also fairly satis- 

 factory drawings of them, employing at first, for the purpose, our 

 eight-inch Merz equatorial, but afterward our great eighteen-inch 

 Munich instrument. 



I found the rotation of the planet quite different from what it 

 has hitherto been supposed to be, on the basis of insufficient obser- 

 vations made with imperfect telescopes a hundred years ago. I 

 may describe it in a few words by saying that Mercury revolves 

 around the sun in the same manner as the moon revolves around 

 the earth. As the moon's journey around the earth is performed 

 in such a way that it always shows nearly the same face and the 

 same spots, so Mercury, in traversing its orbit around the sun, 

 constantly presents nearly the same hemisphere to that source 

 of light. I say nearly — not exactly — the same hemisphere. For 

 Mercury is subject, like the moon, to the phenomenon of libration. 

 In observing the full moon, even with a small telescope, we re- 

 mark that the same spots generally occupy the central regions of 

 its disk ; but, if we study them and their distances from the east- 

 ern and western borders more minutely, we shall soon perceive, 

 as Galileo first did about two hundred years ago, that they oscil- 

 late to a considerable degree, now toward the right and now 

 toward the left — exemplifying the phenomenon called libraiion in 

 longitude. . This arises from the moon's directing one of its diam- 

 eters perpetually and almost exactly, not toward the center of the 

 earth, and not toward the center of the elliptical lunar orbit, but 

 toward the one of the two foci of its orbit which the earth does 

 not occupy. To the observer occupying this point, the moon 

 would consequently always present the same appearance. But to 

 us, who are at a mean distance of forty-two thousand kilometres 

 from that point, the moon presents somewhat different aspects 



VOL. XXXVII. — 5 



