according to the time when we look at it, sometimes showing us a little more of its eastern, sometimes a little more of its western, regions. Mercury presents itself to the sun in different phases of its cycle in a similar manner. It constantly directs one of its diameters, not toward the focus of its elliptical orbit which is occupied by the sun, but toward the second focus. These two foci being distant from one another not less than a fifth of the whole diameter of the orbit of Mercury, the libration of the planet is enormous. The point that receives the rays of the sun vertically changes its place on the surface of the planet, and performs an oscillatory movement along the equator forty-seven degrees in amplitude, or through more than one eighth of the equatorial circumference. The whole duration of this oscillation, including the going and returning, is equal to the time employed by Mercury in traversing its orbit, or about eighty-eight terrestrial days. Thus Mercury stands oriented toward the sun like a magnet toward a mass of iron; but this orientation is not constant to the point of excluding a movement of oscillation of the planet to the east and to the west, like that which the moon performs toward us.
This oscillation is of great importance for the physical condition of the planet. Suppose, for instance, that it did not exist, and that Mercury always turned the same hemisphere to the light and heat of the sun, the other hemisphere remaining plunged in perpetual night. The point of the surface situated at the central pole of the illuminated hemisphere would have the sun eternally in the zenith; the other points of the planet accessible to the solar rays would have the sun always at the same point in their horizon, at the same height, without any apparent movement, without any perceptible change; consequently, no alternation of night and day, no variety of season; the stars eternally invisible because of the perpetual presence of the sun; and, Mercury having no moon, we can hardly imagine how the inhabitants of those regions, condemned to an endless day, could find a means of regularly computing time.
Such are, in fact, nearly the conditions that prevail in Mercury, but only approximately. The oscillating movement of the Mercurial globe as toward the sun would be attributed by an observer on the surface of the planet to the sun, as we attribute to the sun the diurnal movement which really appertains to the earth. To us the sun seems to circle regularly from east to west, defining in twenty-four hours the period of day and night; to the observer on Mercury, the sun will describe a back-and-forth movement through an arc of forty-seven degrees in the celestial vault, while the position of the arc as toward the horizon will always be the same. The complete period of the double oscillation will com-