302. The Moon goes round the Earth in an elliptic Orbit, and therefore
she approaches nearer to the Earth than her mean distance, and recedes
farther from it, in every Lunar Month. When she is nearest: she attracts
strongest, and so rises the Tides most; the contrary happens when she is
farthest, because of her weaker attraction. When both Luminaries are in
the Equator, and the Moon in _Perigeo_, or at her least distance from
the Earth, she raises the Tides highest of all, especially at her
Conjunction and opposition; both because the equatoreal parts have the
greatest centrifugal force from their describing the largest Circle, and
from the concurring actions of the Sun and Moon. At the Change, the
attractive forces of the Sun and Moon being united, they diminish the
gravity of the waters under the Moon, which is also diminished on the
other side, by means of a greater centrifugal force. At the full, whilst
the Moon raises the Tide under and opposite to her, the Sun acting in
the same line, raises the Tide under and opposite to him; whence their
conjoint effect is the same as at the Change; and in both cases,
occasion what we call _the Spring Tides_. But at the Quarters the Sun’s
action on the waters at _O_ and _H_ diminishes the Moon’s action on the
waters at _Z_ and _N_; so that they rise a little under and opposite to
the Sun at _O_ and _H_, and fall as much under and opposite to the Moon
at _Z_ and _N_; making what we call _the Neap Tides_, because the Sun
and Moon then act cross-wise to each other. But, strictly speaking,
these Tides happen not till some time after; because in this, as in
other cases, § 300, the actions do not produce the greatest effect when
they are at the strongest, but some time afterward.
[Sidenote: Not greatest at the Equinoxes, and why.]
303. The Sun being nearer the Earth in Winter than in Summer, § 205, is
of course nearer to it in _February_ and _October_ than in _March_ and
_September_: and therefore the greatest Tides happen not till some time
after the autumnal Equinox, and return a little before the vernal.
[Sidenote: The Tides would not immediately cease upon the annihilation
of the Sun and Moon.]
The Sea being thus put in motion, would continue to ebb and flow for
several times, even though the Sun and Moon were annihilated, or their
influence should cease: as if a bason of water were agitated, the water
would continue to move for some time after the bason was left to stand
still. Or like a Pendulum, which having been put in motion by the hand,
continues to make several vibrations without any new impulse.
[Sidenote: The lunar day, what.
The Tides rise to unequal heights in the same day, and why.