MERCURY VENUS THE EARTH. 73 



tion. Placed at the distance of ninety-five millions of miles from the sun, the earth has 

 an orbit of near six hundred millions of miles to travel over. This circuit is performed 

 at the mean rate of sixty-eight thousand miles an hour. Supposing the impulse that 

 appoints this path to be withdrawn, in sixty-four days and a half, the terrestrial would 

 crash with the solar surface, and sink into its mass like a millstone in the sea, owing to 

 its greater density. That diversity of seasons which marks the year of our globe is the 

 joint result of its movements, and of the inclination of its axis to the plane of its orbit. 

 Supposing one of the poles to be always pointed towards the sun, as in jig. 1, then, not 

 withstanding the daily rotation and the motion in space, the 

 regions from the poles to the equator would be one half in 



constant light and the other half in constant darkness. Or 

 . n 



supposing the equator always pointed directly towards the sun, 



as in jig. 2, light and darkness would alternate for equal times 

 on all parts of the globe ; and there would be different seasons 

 at different places, but no change of seasons in every place as 

 at present. The axis is inclined 23^ from a line perpendicular 

 to the plane of the orbit, and remains constantly parallel to itself during the annual 

 revolution. This arrangement causes the same part of the globe to experience days and 

 nights of unequal length, and to receive the solar influence in a more or less oblique 

 direction. The annexed view represents the earth in four different parts of its orbit. 



It illustrates the varying direction of the sun's rays to the same regions of the globe, 

 and the varying duration of the diurnal exposure to their action, and withdrawment from 

 it. Thus, at the vernal and autumnal equinoxes, it will be seen, that the sun's light 

 reaches equally from pole to pole, so that each part of the surface is carried by the daily 

 rotation into light and darkness of the same duration. At the summer solstice, as the 

 earth rotates, the whole arctic circle enjoys continued day, while the north temperate zone 

 has not only long days and short nights, but receives the sun's rays less obliquely than 

 at the former periods, and consequently experiences an increase of temperature. A 

 precisely opposite effect takes place in the same region at the winter solstice. Hence arises 

 seasonal vicissitude with its resulting phenomena, the repose of vegetation in winter, 

 the renewal of the face of nature in spring, the manhood of its productions in summer, 

 and their autumnal ripeness and garnering. 



There is a third motion to which the earth is subject, occasioning what is called the 

 precession of the equinoxes. Wherever, in spring and autumn, the sun, in his apparent 

 annual course, crosses the equinoctial, or the circle of the earth's equator extended to 

 the heavens, there are the vernal and autumnal equinoxes. These points are found to 



