66 THE POPULAR SCIENCE MONTHLY. 



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 verti- 

 cally 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 Mer- 

 cury 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 con- 

 dition 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 hav- 

 ing no moon, we can hardly imagine how the inhabitants of those 

 regions, condemned to an endless day, could find a means of regu- 

 larly computing time. 



Such are, in fact, nearly the conditions that prevail in Mercury, 

 but only approximately. The oscillating movement of the Mer- 

 curial 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- 



