Page:Astronomy for Everybody.djvu/141

heavenly objects as we see them, and renders them somewhat dimmer, even in the clearest sky. It also refracts the light passing through it, causing it to describe a slightly curved line, concave toward the earth, instead of passing straight to the astronomer's eye. The result of this is that the stars appear slightly higher above the horizon than they actually are. The light coming directly down from a star in the zenith suffers no refraction. The latter increases as the star is farther from the zenith, but even forty-five degrees away it is only one minute of arc, about the smallest amount that the unaided eye can plainly perceive; yet this is a very important quantity to the astronomer. The nearer the object is to the horizon the greater the rate at which the refraction increases; twenty-eight degrees above the horizon it is about twice as great as at forty-five degrees; at the horizon it is more than one half a degree, that is more than the whole diameter of the sun or moon. The result is that when we see the sun just about to touch the horizon at sunset or sunrise its whole body is in reality below the horizon. We see it only in consequence of the refraction of its light. Another result of the rapid increase near the horizon is that, in this position, the sun looks decidedly flattened to the eye, its vertical diameter being shorter than the horizontal one. Anyone may notice this who has an opportunity to look at the sun as it is setting in the ocean. It arises from the fact that the lower edge of the sun is refracted more than the upper edge.

When the sun sets in the ocean in the clear air of the