he sidereal day_, which is the
interval of time between successive passages of a fixed star over a
given meridian; _the apparent solar day_, which is the interval between
two passages of the sun's centre over a given meridian, or the interval
between two successive noons on a sundial; and _the mean solar day_,
which is the interval between the successive passages of a fictitious
sun moving uniformly eastward in the celestial equator, and completing
its annual course in exactly the same time as that in which the actual
sun makes the circuit of the ecliptic. The mean solar days are all
exactly the same length; they are equal to the length of the average
apparent solar day; and they are each four minutes longer than a
sidereal day. We divide our days into 24 hours; each hour into 60
minutes; each minute into 60 seconds. This subdivision of the day
requires some mechanical means of continually registering time, and for
this purpose we use clocks and watches.

The sidereal day and the mean solar day necessitate some means of
registering time, such as clocks; therefore the original day in use must
have been the apparent solar day. It must then have been reckoned either
from sunset to sunset, or from sunrise to sunrise. Later it might have
been possible to reckon it from noon to noon, when some method of fixing
the moment of noon had been invented; some method, that is to say, of
fixing the true north and south, and of noting that the sun was due
south, or the shadow due north. Our own reckoning from midnight to
midnight is a late method. Midnight is not marked by the peculiar
position of any visible heavenly body; it has, in general, to be
registered by some mechanical time-measurer.

In the Old Testament Scriptures the ecclesiastical reckoning was always
from one setting of the sun to the next. In the first chapter of Genesis
the expressions for the days run, "The evening and the morning," as if
the evening took precedence of the morning. When the Passover was
instituted as a memorial feast, the command ran--

"In the first month, on the fourteenth day of the month at
even, ye shall eat unleavened bread, until the one and
twentieth day of the month at even. Seven days shall there be
no leaven."

And again, for the sabbath of rest in the seventh month--

"In the ninth day of the month at even, from even unto even,
shall ye celebrate your sabbath."

The ecclesiastical "day" of the Jews, the