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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
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 ,0 in the direction OP; if it could be viewed from the centre E, 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 E. 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 E, 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 A 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 US 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.
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 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 J days; but, as the earth is constantly pass
Lug 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 real path of the moon is the result of its own proper motion and the onward movement of the earth. The two combined produce a wave-like curve that crosses the earth's path twice each month; this, owing to its small diameter compared with that of the ecliptic, is always concave toward the sun. As the moon constantly keeps the same side turned toward us, it follows that it must turn on its axis once each month.
Dimensions.—Its diameter is about 2,160 miles. It would require fifty globes the size of the moon to equal the earth. Its apparent size varies with its distance; the mean is, however, about one half a
degree, the same as that of the sun. It always appears larger than it really is, on account of its brightness. This is the effect of what is termed in optics Irradiation. To illustrate this principle, cut two circular pieces of the same size, one of black