Suspend a cannon-ball by a fine wire at the equator; set it vibrating north and south, and it swings all day in precisely the same direction.  But suspend it directly over the north pole, and set it swinging toward Washington; in six hours after it is swinging toward Rome, in Italy; in twelve hours, toward Siam, in Asia; in nineteen hours, toward the Sandwich Islands; and in twenty-four, toward Washington again, not because it has changed the plane of its vibration, but because the earth has whirled beneath it, and the torsion of the wire has not been sufficient to compel the plane of the original direction to change with the turning of the earth.  The law of inertia keeps it moving in the same direction.  The same experimental proof of revolution is shown in a proportional degree at any point between the pole and the equator.

But the watchers on the Acropolis do not get turned over so as to see the moon at the same time every night. [Page 110] We turn down our eastern horizon, but we do not find fair Luna at the same moment we did the night before.  We are obliged to roll on for some thirty to fifty minutes longer before we find the moon.  It must be going in the same direction, and it takes us longer to get round to it than if if it were always in the same spot; so we notice a star near the moon one night—­it is 13 deg. west of the moon the next night.  The moon is going around the earth from west to east, and if it goes 13 deg. in one day, it will take a little more than twenty-seven days to go the entire circle of 360 deg..

[Illustration:  Fig. 42.—­Showing the Sun’s Movement among the Stars.]

[Page 111] In our outlook we soon observe that we do not by our revolution come to see the same stars rise at the same hour every night.  Orion and the Pleiades, our familiar friends in the winter heavens, are gone from the summer sky.  Have they fled, or are we turned from them?  This is easily understood from Fig. 42.

When the observer on the earth at A looks into the midnight sky he sees the stars at E; but as the earth passes on to B, he sees those stars at E three minutes sooner every night; and at midnight the stars at F are over his head.  Thus in a year, by going around the sun, we have every star of the celestial dome in our midnight sky.  We see also how the sun appears among the successive constellations.  When we are at A, we see the sun among the stars at G; but as we move toward B, the sun appears to move toward H. If we had observed the sun rise on the 20th of August, 1876, we should have seen it rise a little before Regulus, and a little south of it, in such a relation as circle 1 is to the star in Fig. 43.  By sunset the earth had moved enough to make the sun appear to be at circle 2, and by the next morning at circle 3, at which time Regulus would rise before the sun.  Thus the earth’s motion seems to make the sun traverse a regular circle among the stars once a year:  but it is not the sun that moves.