CHAPTER II

Summary

Aberration. — The aberration of light is another important phenomenon which requires to be taken into consideration in the reduction of rough astronomical observations. Although light travels with the enormous velocity of 192,000 miles per second—a speed so great, that for all practical terrestrial purposes we may consider it to be propagated instantaneously; yet the astronomer, who has to deal with distances of millions of miles, is obliged to be more particular. A simple illustration will show this: the mean distance of our globe from the Sun is 95,000,000 of miles, and since light travels at the rate of 192,000 miles, we can ascertain by a simple arithmetical process that the time occupied by a ray of light reaching us from the Sun is 8m. 18s., so that in point of fact, in looking at the Sun at a given moment, we do not see it shining as it is, but as it did 8m. 18s. previously. If the Earth were at rest, this would be all very well; but since the Earth is in motion, when the solar ray enters the eye of a person on its surface, he will be some way removed from the point in space at which he was situated when the ray left the Sun; he will consequently see that luminary behind the true place it actually occupies when the ray enters his eye.