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ON THE VARIOUS KINDS OF THE DAY OR NYCHTHE.MERON,
AND ON DAY AND NIGHT IN PARTICULAR.
According to the general usage of Muslims, Hindus, Definition
and others, a day or nychthemeron means the dnra- nighn"
tion of one revolution of the sun in a rotation of the
universe, in which he starts from the one half of a
great circle and returns to the same. Apparently it is
divided into two halves : the clcty (i.e. the time of the
sun's being visible to the inhabitants of a certain place
on earth), and the night (i.e. the time of his being in¬
visible to them). His being visible and being invisible
are relative facts, which differ as the horizons differ.
It is well known that the horizon of the equator, which
the Hindus call the country without latitude, cuts the
circles parallel to the meridian in two halves. In con¬
sequence, day and night are always equal there. How¬
ever, the horizons which cut the parallel circles without
passing through their pole divide them into two un¬
equal halves, the more so the smaller the parallel circles
are. In consequence, there day and night are unequal,
except at the times of the two equinoxes, when on the
whole earth, except Mem and Vadavamukha, day and
night are equal. Then all the places north and south
of the line share in this peculiarity of the line, but only
at this time, not at any other.
The beginning of the day is the sun's rising above Manushya-
the horizon, the beginning of the night his disappearing ^°''^*'^'*-
below it. The Hindus consider the day as the first, the