Page:Popular Science Monthly Volume 37.djvu/75

Rh an uncertain and flaring aspect which appears to the naked eye as a strong scintillation. For this reason the ancients called it Στίλβων, 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 decided 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 because 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 satisfactory 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 observations 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 remark that the same spots generally occupy the central regions of its disk; but, if we study them and their distances from the eastern and western borders more minutely, we shall soon perceive, as Galileo first did about two hundred years ago, that they oscillate to a considerable degree, now toward the right and now toward the left—exemplifying the phenomenon called libration in longitude. This arises from the moon's directing one of its diameters 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