ass, the greater is this bending
found to be. Since the layer of air around us becomes denser and denser
towards the surface of the earth, it will readily be granted that the
rays of light reaching our eyes from a celestial object, will suffer the
greater bending the lower the object happens to be in the sky. Celestial
objects, unless situated directly overhead, are thus not seen in their
true places, and when nearest to the horizon are most out of place. The
bending alluded to is upwards. Thus the sun and the moon, for instance,
when we see them resting upon the horizon, are actually _entirely_
beneath it.

When the sun, too, is sinking towards the horizon, the lower edge of its
disc will, for the above reason, look somewhat more raised than the
upper. The result is a certain appearance of flattening; which may
plainly be seen by any one who watches the orb at setting.

In observations to determine the exact positions of celestial objects
correction has to be made for the effects of refraction, according to
the apparent elevation of these objects in the sky. Such effects are
least when the objects in question are directly overhead, for then the
rays of light, coming from them to the eye, enter the atmosphere
perpendicularly, and not at any slant.

A very curious effect, due to refraction, has occasionally been observed
during a total eclipse of the moon. To produce an eclipse of this kind,
_the earth must, of course, lie directly between the sun and the moon_.
Therefore, when we see the shadow creeping over the moon's surface, the
sun should actually be well below the horizon. But when a lunar eclipse
happens to come on just about sunset, the sun, although really sunk
below the horizon, appears still above it through refraction, and the
eclipsed moon, situated, of course, exactly opposite to it in the sky,
is also lifted up above the horizon by the same cause. Pliny, writing in
the first century of the Christian era, describes an eclipse of this
kind, and refers to it as a "prodigy." The phenomenon is known as a
"horizontal eclipse." It was, no doubt, partly owing to it that the
ancients took so long to decide that an eclipse of the moon was really
caused by the shadow cast by the earth. Plutarch, indeed, remarks that
it was easy enough to understand that a solar eclipse was caused by the
interposition of the moon, but that one could not imagine by the
interposition _of what body_ the moon itself could be eclipsed.