would have been carried into the position BQ, it is evident that the ball throughout its whole descent would be found in the tube; and a spectator referring to the tube the motion of the ball, and carried

Fig. 38.
P

ABERRATION OF LIGHT.

along with the former, unconscious of its motion, would fancy that the ball had been moving in an inclined direction, and had come from Q. A very common illustration may be seen almost any rainy day. Choose a time when the air is still and the drops large. Then, if you stand still, you will see that the drops fall vertically; but if you walk forward, you will see the drops fall as if they were meeting you. If, however, you walk backward, you will observe that the drops fall as if they were coming from behind you. We thus see that the drops have an apparent as well as real motion.

The general effect of aberration of light is to cause each star to apparently describe a minute ellipse in the course of a year, the central point of which is the place the star would actually occupy were our globe at rest.

PARALLAX.-This is the difference in the direction of an object as seen from two different places. For a simple illustration of it, hold your finger before you

Fig. 39.

R

PARALLAX.

in front of the window. Upon looking at it with the left eye only, you will locate your finger at some point on the window; on looking with the right eye only, you will locate it at an entirely different point. Use your eyes alternately and quickly, and you will

be astonished at the rate with which your finger will seem to change its place. Now, the difference in the direction of your finger as seen from the two eyes is parallax.

In astronomical calculations, the position of a body as seen from the earth's surface is called its apparent place, while that in which it would be seen from the centre of the earth is called its true place. Thus, in the cut, a star is seen by the observer at O in the direction OP; if it could be viewed from the centre R, its direction would be in the line RQ. It is therefore seen from O at a point in the heavens below its position in reference to R. From looking at the cut, we can see (1), that the parallax of a star near the horizon is greatest, while it decreases gradually until it disappears altogether at the zenith, since an observer at O, as well as one at R, would see the star Z directly overhead; and (2), that the nearer a body is to the earth the greater its parallax becomes. It has been agreed by astronomers, for the sake of uniformity in their calculations, to correct all observations so as to refer them to their true places as seen from the centre of the earth. Tables of parallax are constructed for this purpose. The question of parallax is also one of very great importance, because as soon as the parallax of a body is once accurately known, its distance, diameter, etc., can be readily determined. (See Celestial Measurements.)

Horizontal Parallax.-This is the parallax of

a body when at the horizon. It is, in fact, the earth's semi-diameter as seen from the body. In the figure, the parallax of the star S is the angle OSR, which is measured by the line OR-the semi-diameter of the earth. The sun's horizontal parallax (8.94") is the angle subtended (measured) by the earth's semi-diameter as seen from that luminary. As the moon is nearest the earth, its horizontal parallax is the greatest of any of the heavenly bodies.

Annual Parallax.-The fixed stars are so distant from the earth that they exhibit no change of place when seen from different parts of the earth. The lines OS and RS are so long that they are apparently parallel, and it becomes impossible to discover the slightest inclination. Astronomers, therefore, instead of taking the earth's semi-diameter, or 4,000 miles, as the measuring tape, have adopted the plan of observing the position of the fixed stars at opposite points in the earth's orbit. This gives a change in place of 183,000,000 miles. The variation of position which the stars undergo at these remote points is called their annual parallax.

THE MOON.

New Moon, . First Quarter, . Full Moon, . Last Quarter, ●.

ITS MOTION IN SPACE.-The orbit of the moon, considering the earth as fixed, is an ellipse of which our planet occupies one of the foci. Its distance from

ing on in its own orbit around the sun, it requires over two days longer before it comes into the same position with respect to the sun and earth, thus completing its synodic revolution.

the earth therefore, varies incessantly. At perigee it is 26,000 miles nearer than in apogee: the mean distance is about 238,000 miles. It would require a chain of thirty globes equal in size to the earth to reach the moon. An express-train would take about a year to accomplish the journey. The moon completes its revolution (sidereal) around the earth in about 27 days; but, as the earth is constantly pass