ABERRATION OF LIGHT is an apparent alteration in the place of a star, arising from the motion of the earth in its orbit combined with the progressive passage of light. When rain is falling perpendicularly, a drop entering at the top of an upright tube at rest, will Fo through; but if the tube be carried forward horizontally, a drop entering the top will strike against the side before it goes far; and to make the drop go through the tube in motion, we must incline the top of it forward in the direction of the motion. The amount of this inclination will be the greater the more rapid the motion of the tube is compared with that of the falling drops. If in the time that a drop takes to fall through the height AB of the parallelogram in the annexed cut, the inclined tube BC is moved horizontally over a space equal to its breadth, AC, a drop entering the top of the tube will descend without touching the sides. For in half the time the tube will be in the position NC', and the drop in the position d; and so for any other portion of the time. This exactly illustrates the astronomical phenomenon in question. The tube is a telescope directed to receive the light of a star; this tube, and the person looking through it, are moving alono. with the earth in its orbit, and the light may be con- ceived as particles coming from the star like drops of rain, moving much faster, no doubt, still requiring time. That a particle or ray of light from .
the star may pass through the tube, it must be directed, not straight a to the star, but at a slight angle in the direction of the earth's motion. A 1 C• e Thus the place where we see the star is not its true place. As the r 0 earth's motion, however, is slow compared with the velocity of light, II A I I ? • the angle of inclination is small—never exceeding about 20'. The . ' i i I / •result is, that, if we conceive the true place of a star as a fixed point, .1 ..' ill the apparent place of the star describes about this true place, in the i il 3 4. ? course of a year, an ellipse whose greater axis is about 40% The , d aberration of light was discovered by the English astronomer Brad- /,4 ley, in 1727, while seeking to determine the parallax of certain fixed ; • fl stars. ',- i/ a AB'ERT, JOHN JAMES, 1787-1863; an American military engineer. / 1 1 He was educated at 'West Point, and long employed in the war . ','' department. He participated in the battle of Bladensburg, Aug. B 4, 1814. In 1829, he was lt.-col. in command of the engineers and head of the topo.. graphical bureau. In 1832-33 he was commissioner for Indian affairs, and in 1838 made col. of the corps of engineers, having charge of the topographical service of the government until his retirement, Sept. 9, 1861.