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

ON THE DIVISIONS AND MEASURES OF TIME.

" Time of itself is nothing, but from thought
Receives its rise; by laborlring fancy wrought
From things considered, whilst we think on some
As present, some as past, or yet to come.
No thought can think on time, that's still confest,
But thinks on things in motion or at rest."-Lucretius.
THE following brief notices are intended to mark the natural
measures and divisions of time, as regulated by the apparent and
real motions of the sun and the moon, and the artificial adjustments
that have been made so as to secure their agreement with the orderly
returns of the seasons, and to regulate the division of time for the
purposes of ordinary life.
A measure has always a reference to some quantity which can be
measured; and any quantity can be measured by any assumed
measure of the same kind, Any length can be measured by any line
assumed as the unit of measurement, as a yard, a mile, &c. But with
respect to time, it is clear from its fleeting nature, being continuous,
no standard measures can exist, like the measures of space or other
quantities. There is, however, an analogy between the nature of a
geometrical line and time, and an identity of language is employed of
both by all nations. For instance, a point in a line and a point of
time; the beginning and end of a line and the beginning and end of
a period of time; the length or shortness of a line, and the length or
shortness of time; also the extension of a line backwards or forwards
exactly corresponds with the extension of a period of time into the
past or the future. This language implies that the ideas of time are
closely associated with the ideas of a line and motion; and that exact
measures of the motion of bodies in lines may be assumed and
employed as measures of time. It was by having recourse to the
apparent motions of the sun and the moon in the heavens, that
measures of time were at length ascertained.
The fact of the apparent daily motion of the sun round the,
heavens being observed to be constant and regular, led men to believe
that the sun did really move round the earth, and this belief lasted
for many ages. It had its origin in mistaking apparent motion for real
motion. When a body at rest is seen by an observer from another
body in motion, the former appears to move in a direction contrary to
that in which the latter body is ctually moving. As the daily revolution of the earth on its axis causes the sun to appear to move from
east to west; also while the earth is actually moving in its orbit round
the sun, in the direction from east to west, the sun appears to an
observer on the earth to move among the stars from west to east.
This distinction between real and apparent motions, not having been
observed, was the cause of the erroneous belief.1
1 This erroneous belief has led to singular consequences. In the course of time,
the bishops of Rome have decreed, that the belief of the motion of the earth on its