An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

32 THE EARTH.
of direction which light undergoes in passing through the
atmosphere. Let us conceive of the atmosphere as made up of
a great number of concentric strata, as AA, BB, CC, and DD
(Fig. 8), increasing rapidly in density (as is known to be the
Fig. 8.
D-I) D
fact) in approaching near to the surface of the earth. Let S
be a star, from which a ray of light Sa enters the atmosphere
at a, where, being turned towards the radius of the convex
surface, it would change its direction into the line ab, and
again into be, and cO, reaching the eye at 0. Now, since an
object always. appears in the direction in which the light
finally strikes the eye, the star would be seen in the direction
of the last ray cO, and the star would apparently change its
place, in consequence of refraction, from S to S', being
elevated out of its true position. Moreover, since on account
of the constant increase of density in descending through the
atmosphere, the light would be continually turned out of its
course more and more; it would therefore move, not in the
polygon represented in the figure, but in a corresponding
curve, whose curvature is rapidly increased near the surface of
the earth.
89. When a body is in the zenith, since a ray of light from
it enters the atmosphere at right angles to the refracting medium, it suffers no refraction. Consequently, the position of
the heavenly bodies, when in the zenith, is not changed by
refraction; while, near the horizon, where a ray of light strikes
the medium very obliquely, and traverses the atmosphere