#1vicmineAnswered at 2013-02-09 00:11:13
The full moon and new moon phases relate to the umbra / penumbra of Earth's shadow on the moon ( which is related to sunlight ) , no gravitational effect . The moon will remain at ~ 238,900 miles at all times , therefore , the gravitational effect is constant , without hesitation . Gravity is always instantaneous , no reason lag.The two high tides per day is due to the near side (evening ) retreated to , at the same time, there is a high tide just off the bulge in the side of the moon . As the moon rotates every 24 hours, every 12 hours we will have a high tide. Two lumps , the side nearest the far side .
#2AnonymousAnswered at 2013-08-15 14:28:29
Nearly everybody knows the tides are caused by the gravitational attraction between the earth and the moon (and to a lesser extent; between the earth and the sun); but nearly everybody gets it wrong HOW the attraction causes the tides; and especially why the tides are highest at full and new moon.
Gravitational attraction between astronomical bodies acts between the centres of gravities of the bodies (in our case; the moon is attracted to the centre of the earth).
On the side of the earth facing the moon, the gravitational attraction is stronger than at the centre (and the SURFACE is attracted TOWARDS the moon, relative to the earth's CENTRE).
On the side of the earth facing away from the moon, the gravitational attraction is weaker than at the centre, (and the CENTRE is attractedd TOWARDS th moon, relative to the SURFACE.)
In effect, the earth is STRETCHED because of the difference in gravitational attraction TOWARDS the moon on the near side and AWAY from the moon on the far side.
This effect is called tidal stretching.
The same effect occurs with respect to the sun; but because the sun is a lot further away, the effect is only about 1/3 that of the moon.
When the sun and the moon are on the same side of the earth (new moon), the earth on that side is attracted TOWARDS both the sun and the moon, and the opposite side attracted AWAY from both the sun and the moon.
BUT: When the sun and the moon are on opposite sides of the earth (full moon), the side facing away from the moon is attracted TOWARDS the sun and AWAY from the moon. The side facing the moon is attracted TOWARDS the moon and AWAY from the sun. Hence, rather strangely, the effect of tidal stretching during a full moon is very similar to that during a new moon. ('Spring tides')
During a half moon, the attractions towards the sun and the moon are at right angles, and tend to cancel each other out., Hence the tides are weakest ('neap tides')
One of these days, I'll write a more straightforward answer to this common question.