Page:Popular Astronomy - Airy - 1881.djvu/205

Rh every position of the earth, the star's place is affected by the aberration of light; and from this cause every



star apparently describes a small circle every year parallel to the earth's orbit. It is a minute circle; its angular diameter, as seen without any foreshortening, is found to be about forty seconds. This quantity is, however, so serious that it cannot be omitted in the computation of any observation whatever.

From the measure of the apparent semi-diameter of the small circle described by the star, corresponding to the angle BCb in Figure 52, we are able to compute the proportion of the earth's velocity to the velocity of light; and we find that the velocity of light is about 10,000 times as great as the earth's velocity in its orbit, or about 200,000 miles in a second, In other words, light travels a distance equal to eight times the circumference of the earth between two beats of a