Dictionary of Physics or an Attempt at an Explanation of the Most Important Concepts and Technical Terms of Natural Philosophy

to traverse the diameter of the Earth's orbit. See Light.

He could now immediately imagine that we must perceive the stars standing in the ecliptic, when they are in conjunction with the sun and thus stand behind it and further from us, 16 minutes later than when they are in opposition, i.e., on the same side of the sun as we ourselves are, and thus stand closer to us by the diameter of the Earth's orbit, and that for this very reason we see them in the former case displaced by 40'' less than in the latter, from which the phenomena of aberration for the stars standing in the ecliptic, which seem to describe a straight line instead of an ellipse, are perfectly explained.

Regarding the stars standing outside the ecliptic, Bradley hit upon the happy idea of combining the movement of light with the movement of the Earth according to the laws of the composition of motions (see Composition of motions), and after he had found his explanation to be consistent with all observations, he rendered a public report thereof in the year 1728. (Philos. Transact. No. 406.)

Let E (Plate I. Fig. 2) be a star that sends the light ray EB to us, AB a small part of the Earth's orbit, and CB the path that the ray has traversed as the Earth went from A to B, such that CB and AB must behave as the velocities of light and the Earth, i.e., as 10313 : 1. If one draws the parallelogram ABCD, the movement of the light ray CB will be able to be decomposed into the two movements CD = BF and CA = DB (see Composition of motions). Of the part BF, the eye arriving at B can perceive nothing, because the movement BF goes in the same direction as the movement of the eye through AB; it therefore only perceives the part DB and sees the star E in the direction BD, thus removed from its true position E by the angle EBD, which is = ACB, and is called the aberration angle.