184 OF THE LENGTH OF DAYS AND NIGHTS,

degrees, are subject to an annual change of'theirseasons, and a great variety in the length of theirdays and nights.

As only one half of the globe can be en-lightened at a time, the circle which is theboundary of light and darkness may be calledthe terminator; and it is evident, from a flightconsideration of the subject, that if the axis ofthe earth was perpendicular to the plane of itsorbit, the terminator would pass through thetwo poles, and divide each of the small circles,which are drawn parallel to the equator, intotwo equal parts. And as the uniform rotationof the earth upon its axis, must; occasion everyplace to describe equal parts of one of theseparallel circles in equal times, the days and- nights would of course, be equal all over theglobe, except at the poles, where the fun wouldneither rife nor set, but remain continually inthe horizon.

But, on the contrary, if the axis of the earthbe inclined to the plane of its orbit, all theparallels, except the equator, will be dividedby the terminator, into two unequal parts, hav-ing a greater or less portion of their circum-ferences in the enlightened, than in the darkhemisphere, according as their situation istowards the north or south poles of the world.So that all those places which lie under thenorthern parallels, will have their days longer

than