Elementary arithmetic, with brief notices of its history... by Robert Potts.

4

ON THE DIVISIONS AND MEASURES OF TIME.

It is unknown at what period the intercalation of five days was
adopted. It was in use 1322 B.c., when Amunoph I. was king of
Egypt, as an inscription of that age, preserved in the Museum of
Turin, records the year of 365 days.
After the discovery that the apparent imotion of the heavens and
of the sun round the earth was caused by a real motion of the earth,
it became necessary that the definitions of the day and the year should
contain some reference to the true causes, as well as the apparent
causes of these phenomena.
Instead of the sun daily moving round the earth at rest, it was
found that the earth, being nearly globular, was actually revolving
round its own axis, and that the natural day, the interval between two
successive risings or settings of the sun, is, in fact, the time in which
the earth makes one complete revolution on its axis; and that the
periods of light and darkness are caused as one half of the surface of
the earth in its rotation is turned towards the sun and receives its
light, or is turned from the sun and does not receive it; and instead
of the sun moving round the heavens, it was discovered that the earth
was actually moving in an orbit round the sun, the central luminary
of the system of worlds of which the earth is one; and it
was ascertained that the natural year, during which the sun appears
from the earth to move once round the circle of the heavens, is, in
fact, the period in which the earth completes one revolution in its
orbit round the sun. It is this revolution of the earth round the sun
that occasions the different lengths of the light and darkness, and the
change of the seasons by the different positions of the earth in its
orbit. The axis of the earth maintains very nearly the same inclination to the plane of the orbit in which its revolution takes place;
but more exact observations have shown that the obliquity of the
ecliptic is slowly diminishing.
Though the period of light and the period of darkness is different
at different seasons of the year, yet the natural day, or the whole
period of light and darkness, has always been of constant length. The
days are longer and the nights are shorter in summer, and the nights
are longer and the days are shorter in winter. And there is one day
in summer when the daylight is longest and the darkness shortest;
and one day in winter when the darkness is longest and the light
shortest. Also, twice in the course of the year, both the periods of
light and darkness of the day are equal. The two days on which
these events take place are called the vernal and the autumnal equinox,
one happening in the spring and the other in the autumn.
If the orbit of the earth were circular, there would be no difference
between the successive days of the year. But as the earth's orbit
is not a circle but an ellipse, having the sun in one of the foci, the
earth's daily motion in her orbit is not uniform, and the sun in its
apparent motion does not daily return to the same meridian of the
earth after equal intervals, and consequently its apparent daily motion
cannot be an exact measure of time.
From the time of Hipparchus, the intervals of the successive returns
every day of any place on the surface of the earth to the same star in
the heavens has been observed to be constant at all times of the year,
and this period of time has been named the sidereal day. On this
account the sidereal day has been adopted by astronomers as the
principal unit of time. It may appear paradoxical that at whatever