The parallax of a body at _S'''_ is _Q'''S'''P_, which is seen to be greater than _QSP_; that is to say, the parallax of a heavenly body increases with its nearness to the earth. The distance and parallax of a body are so related, that, either being known, the other may be computed. 33. _Aberration._--_Aberration_ is a slight displacement of a star, owing to an apparent change in the direction of the rays of light which proceed from it, caused by the motion of the earth in its orbit. If we walk rapidly in any direction in the rain, when the drops are falling vertically, they will appear to come into our faces from the direction in which we are walking. Our own motion has apparently changed the direction in which the drops are falling. [Illustration: Fig. 46.] In Fig. 46 let _A_ be a gun of a battery, from which a shot is fired at a ship, _DE_, that is passing. Let _ABC_ be the course of the shot. The shot enters the ship's side at _B_, and passes out at the other side at _C_; but in the mean time the ship has moved from _E_ to _e_, and the part _B_, where the shot entered, has been carried to _b_. If a person on board the ship could see the ball as it crossed the ship, he would see it cross in the diagonal line _bC_; and he would at once say that the cannon was in the direction of _Cb_. If the ship were moving in the opposite direction, he would say that the cannon was just as far the other side of its true position. Now, we see a star in the direction in which the light coming from the star appears to be moving. When we examine a star with a telescope, we are in the same condition as the person who on shipboard saw the cannon-ball cross the ship. The telescope is carried along by the earth at the rate of eighteen miles a second: hence the light will appear to pass through the tube in a slightly different direction from that in which it is really moving: just as the cannon-ball appears to pass through the ship in a different direction from that in which it is really moving. Thus in Fig. 47, a ray of light coming in the direction _SOT_ would appear to traverse the tube _OT_ of a telescope, moving in the direction of the arrow, as if it were coming in the direction _S'O_. [Illustration: Fig. 47.] As light moves with enormous velocity, it passes through the tube so quickly, that it is apparently changed from its true direction only by a very slight angle: but it is sufficient to displace the star. This apparent change in the direction of light caused by the motion of the earth is called _aberration of light_.