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mous differences in the distances of objects on the earth and the heavens, that the real difficulty of forming a mental picture of them in their true relation arises. I shall ask the reader's careful attention in an attempt to present these relations in the simplest way, so as to connect things as they are with things as we see them.

Let us suppose the earth taken away from under our feet, leaving us hanging in mid space. We should then see the heavenly bodies —sun, moon, planets, and stars— surrounding us in every direction, up and down, east and west, north and south. The eye would rest on nothing else. As we have just explained, all these objects would seem to us to be at the same distance.

A great collection of points scattered in every direction at an equal distance from one central point, must all lie upon the inner surface of a hollow sphere. It follows that, in the case supposed, the heavenly bodies will appear to us as if set in a sphere in the centre of which we appear to be placed. Since one of the final objects of astronomy is to learn the directions of the heavenly bodies from us, this apparent sphere is talked about in astronomy as if it were a reality. It is called the celestial sphere. In the case we have supposed, with the earth out of the way, all the heavenly bodies on this sphere would at any moment seem at rest. The stars would remain apparently at rest day after day and week after week. It is true that, by watching the planets, we should in a few days or weeks, as the case might be, see their slow motion around the sun, but this would not be perceptible at once. Our first impression would be that the