Page:Popular Astronomy - Airy - 1881.djvu/219

Rh battery, and that a shot is fired at the ship, and I remarked that the direction which the shot takes through the ship is not in a direction exactly corresponding to that in which it is fired, but has an inclined direction, which inclination depends upon this, that after the shot has entered the first side of the ship, and before it comes out at the second side, the ship has advanced sensibly. The magnitude of the inclination depends therefore on the proportion of the velocity of the ship to the velocity of the shot. From this it is plain that if we know the extent to which the apparent direction of that line of the motion of the shot was changed when passing through the ship, we shall have the means of computing the proportion of the velocity of the ship to the velocity of, the shot. Now this is a case strictly analogous to the motion of light. The earth is travelling along, and whilst it is so travelling along, light comes upon it from different objects, for instance from the stars. And the effect is the same as in the case of the ship; that in consequence of this motion of the earth, the light appears to come, not from the real place of the star, but from an ideal place of the star, which is in advance, as estimated by the direction of, the earth's motion. If we know in what direction the earth is moving, the light of the star appears to come from a point more in that direction than it should.

I then endeavoured to point out to you the influence which this would have on the apparent places of the stars. We have an earth revolving in an orbit round the sun. The place of the star then will not appear always the same, but will always be found in a circle, whose centre is the true place of the star, the line from the true place to the apparent place being always in the direction in which the earth is moving. If