He observed the moon when two days and a half old, in
the evening soon after sunset, before the dark part was visible, and
continued to watch it until it became visible. The two cusps
appeared tapering in a very sharp faint prolongation, each
exhibiting its farthest extremity faintly illuminated by the solar
rays, before any part of the dark hemisphere was visible. Soon
afterward, the whole dark limb became illuminated. This prolongation
of the cusps beyond the semicircle, I thought, must have arisen from
the refraction of the sun's rays by the moon's atmosphere. I computed,
also, the height of the atmosphere (which could refract light enough
into its dark hemisphere to produce a twilight more luminous than
the light reflected from the earth when the moon is about 32 degrees
from the new) to be 1,356 Paris feet; in this view, I supposed the
greatest height capable of refracting the solar ray, to be 5,376 feet.
My ideas on this topic had also received confirmation by a passage
in the eighty-second volume of the Philosophical Transactions, in
which it is stated that at an occultation of Jupiter's satellites, the
third disappeared after having been about 1" or 2" of time indistinct,
and the fourth became indiscernible near the limb.
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