Page:Popular Astronomy - Airy - 1881.djvu/204

190 is the velocity of the shot, the smaller will be the space b B described by the ship while the shot is passing across her, and therefore the smaller will be the angle b CB between the apparent direction of the shot and its real direction.

The same thing happens with regard to the effect of the motion of the earth on the apparent path of light, and it will produce an apparent change in the places of the stars. And if we find that there is such an apparent change, it will be a certain proof that the earth is in motion; but if we find the change to be small, it will prove that the velocity of light is much greater than that of the earth.

Now I will point out to you the visible effect of the aberration of light upon the place of a star. The immediate interpretation of the consideration which I have mentioned is this. In whatever direction the earth is moving, the apparent position of any star which we are looking at, is displaced in the direction towards which the earth is moving. In Figure 53, let C be the sun, E',E",E"',E"", the earth in four successive positions of its orbit (viewed in perspective), its motion at each place being in the direction of the arrow drawn there; S the true place of a star. Then, in consequence of the aberration, when the earth is at E', as its motion is in the direction of the arrow drawn from E', the light coming from the star will enter the eye of a spectator or the tube of a telescope, not as if it came from S, but as if it came from s′, the line Ss’ being parallel to the arrow at E'; and therefore the observer, when the earth is at E', does not see the star at S but at s’. In like manner, when the earth is at E" he sees the star at s"; when at E"' he sees the star at s′″; and when at E"" he sees the star at s″″. Thus you will see that, in