The earth travels through space at a rate of more than a million-and-a-half miles per day, besides the distance which each object upon it is carried in its diurnal rotation (24,000 miles). At first thought it would seem impossible that such a rapid motion should not be felt; but as the air and clouds, and every object belonging to the earth, moves with it, and this motion is perfectly smooth, it can only be perceived by looking at objects independent of the earth. If sailing on the water, when it is quite smooth and the land not very near, the ship seems to be stationary and the objects on shore appear to pass along in an opposite direction to that in which the ship is going. It is the same with the earth; the sun, which appears to move, is stationary, while the earth is going round it; and the result is, while the earth turns round on its axis the whole firmament appears to move, we lose sight of those stars which are to the westward, which are then said to "set," while those to the eastward, constantly coming into sight, are said to "rise," the whole appearing to pass from east to west, while in reality the earth's surface is passing from west to east. At the equator, or that part of the earth which lies midway between the poles, the sun and stars appear to rise and set perpendicularly to the horizon and at equal times above and below it, it is therefore 12 hours day and 12 hours night; in places not situated on the equator, they do not rise and set perpendicularly to the horizon, but form portions of circles at greater or less altitudes in the heavens, and the nearer the observer is to either of the poles, the smaller the circle, and the more of it is seen; so that, at the poles, any star situated exactly overhead does not appear to move at all. This place is called the north pole of the heavens, and the nearest star to it is called the "polestar," which neither rises nor sets, and, as it always keeps the same position, serves as a guide to mariners. All this is the effect of the diurnal motion of the earth, its annual motion not affecting the position of the stars, they being at such an immense distance that the circle which the earth forms in passing round the sun--although nearly 200,000,000 miles across--is as a mere point in proportion. But not so with the sun: as before stated, at the equator it is twelve hours above the horizon and the same time below it; now, as we approach towards the poles, it forms at each rotation an arc above the horizon, lower and lower, until at the poles its daily rising and setting is lost altogether, and there would never be daylight there, provided that the earth was only subject to the diurnal motion (fig. 9); but as the earth turns upon an oblique axis, the north and south poles share the blessing of daylight between them, so that through one-half of the earth's orbit the north pole is towards the sun, as shown in the figure, and the south pole during the other half.