A View of Sir Isaac Newton's PhilosophyPemberton, Henry
Science
A View of Sir Isaac Newton's Philosophy
Pemberton, Henry
Newton, Isaac, 1642-1727. Principia
the supposition, that the action of the sun upon the moon and earth
were equal; whereas the moon being acted upon more than the earth,
did the sun’s action draw the moon in the line L O parallel to A S,
it would draw it down so far as to make L P greater than I M; whereby
the angle under P M N will be rendred less, than that under L I N. But
moreover, as the sun draws the earth in a direction oblique to I N, the
earth will be found in its orbit somewhat short of the point M; however
the moon is attracted by the sun still more out of the line L O, than
the earth is out of the line I N; therefore this obliquity of the sun’s
action will yet farther diminish the angle under P M N.
13. THUS the moon at the point G receives an impulse from the sun,
whereby her motion is accelerated. And the sun producing this effect in
every place between the quarter and the conjunction, the moon will move
from the quarter with a motion continually more and more accelerated;
and therefore by acquiring from time to time additional degrees of
velocity in its orbit, the spaces, which are described in equal times
by the line drawn from the earth to the moon, will not be every where
equal, but those toward the conjunction will be greater, than those
toward the quarter. But now in the moon’s passage from the conjunction
D to the next quarter the sun’s action will again retard the moon, till
at the next quarter in E it be restored to the first velocity, which it
had in C.
14. AGAIN as the moon moves from E to the full or opposition to the
sun in B, it is again accelerated, the deficiency of the sun’s action
upon the moon, from what it has upon the earth, producing here the same
effect as before the excess of its action. Consider the moon in Q,
moving from E towards B. Here if the moon were attracted by the sun in
a direction parallel to A S, yet being acted on less than the earth,
as the earth descends toward the sun, the moon will in some measure be
left behind. Therefore Q F being drawn parallel to S B, a spectator
on the earth would see the moon move, as if attracted from the point
Q in the direction Q F with a degree of force equal to that, whereby
the sun’s action on the moon falls short of its action on the earth.
But the obliquity of the sun’s action has also here an effect. In the
time the earth would have moved from A to I without the influence of
the sun, let the moon have moved in its orbit from Q to R. Drawing
therefore R T parallel to A I, and equal to the same, for the like
reason as before, the moon by the motion of its orbit, if not at all
attracted by the sun, must be found in T; and therefore, if attracted
in a direction parallel to S A, would be in the line T V parallel to
A S; suppose in W. But the moon in Q being farther off the sun than
the earth, it will be less attracted, that is, T W will be less than
I M, and if the line S M be prolonged toward X, the angle under X M
W will be less than that under X I T. Thus by the sun’s action the
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