Aberration of Light — Chambers's Encyclopaedia

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 go through; but if the tube be carried forward horizontally, a drop entering the top will strike against the side before it goes far: 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

Diagram illustrating the aberration of light. It shows a vertical tube AB and a horizontal tube BC. A star is represented by a point C' above the tubes. A drop of rain is shown falling from C' towards the tubes. The path of the drop is shown as a dashed line from C' to B' (the bottom of the vertical tube) and then to d (a point on the horizontal tube). The diagram shows how the apparent position of the star shifts due to the motion of the observer (the tube) relative to the light source (the star).

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 B'C', 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 along with the earth in its orbit, and the light may be conceived 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 to the star, but at a slight angle in the direction of the earth's motion. Thus the place where we see the star is not its true place. As the earth's motion, however, is slow compared with the velocity of light, the angle of inclination is small—never exceeding about 20". The result is, that, if we conceive the true place of

a star as a fixed point, the apparent place of the star describes about this true place, in the course of a year, an ellipse whose greater axis is about 41". In the case of stars near the pole of the ecliptic, this ellipse approaches to a circle, while for stars in the plane of the ecliptic, it contracts to a line about 41" long. The aberration of light was discovered by the English astronomer Bradley in 1727, while seeking to determine the parallax of certain fixed stars.