The dilated size (generally) of the Sun or Moon, when seen near the
horizon, beyond what they appear to have when high up in the sky, has
nothing to do with refraction. It is an illusion of the judgment,
arising from the terrestrial objects interposed, or placed in close
comparison with them. In that situation we view and judge of them
as we do of terrestrial objects--in detail, and with an acquired
attention to parts. Aloft we have no association to guide us, and their
insulation in the expanse of the sky leads us rather to undervalue
than to over-rate their apparent magnitudes. Actual measurement with
a proper instrument corrects our error, without, however, dispelling
our illusion. By this we learn that the sun, when just on the horizon,
subtends at our eyes almost exactly the same, and the moon a materially
_less_, angle than when seen at a greater altitude in the sky, owing to
its greater distance from us in the former situation as compared with
the latter.--_Sir John Herschel’s Outlines._