ABSOLUTE Equation, in Astronomy, is the aggregate
of the optic and eccentric equations. The apparent
inequality of a planet's motion arising from its not be-
ing equally distant from the earth at all times, is call-
ed its optic equation, and would subsist even if the plan-
et's real motion were uniform. The eccentric ineq-
uality is caused by the planet's motion being uniform.
To illustrate which, conceive the sun to move, or to
appear to move, in the circumference of a circle, in
whose centre the earth is placed. It is manifest, that
if the sun moves uniformly in this circle, it must appear
to move uniformly to a spectator on the earth, and in
this case there will be no optic nor eccentric equation;
but suppose the earth to be placed out of the centre of
the circle, and then, though the sun's motion should
be really uniform, it would not appear to be so, when
seen from the earth; and in this case there would be
an optic equation, without an eccentric one. Imagine
farther, the sun's orbit to be not circular, but elliptic,
and the earth in its focus; it will be as evident that
the sun cannot appear to have an uniform motion in
such ellipse: so that his motion will then be subject to
two equations, the optic and the eccentric.