Bad Astronomy: Misconceptions and Misuses Revealed, from ...

(a.)
(b.)
center of the Earth
THE GRAVITY OF THE SITUATION 69
Earth/Moon system barycenter
force of gravity
Moon
forces relative to the center of the Earth
The force of the Moon’s gravity on the Earth always “points” toward the
Moon. The force gets weaker with distance so that the near side of the
Earth is pulled toward the Moon more strongly than the far side. When
the force of the Moon’s gravity is calculated relative to the Earth/Moon
center of mass, the far side of the Earth actually feels a force pointing
away from the Moon, while on the near side the force is still toward the
Moon. This results in a stretching of the Earth, which is why we have
two high tides a day.
it’s because we are measuring that force relative to the center of
the Earth. When you do that, then you do indeed get a force pointing
away from the Moon on the far side of the Earth.
That is why we have two high tides. There is a net force
toward the Moon on the near side, and a net force away from the
Moon on the far side. The water follows those forces, piling up in
a high tide on opposite sides of the Earth. In between the two high
tides are the low tides, and of course there are two of them as well.
As a point on the Earth rotates under the high tide bulge, the
water rises. A few hours later, when the Earth has rotated onequarter
of the way around, that point is now under a low tide, and
the water has receded. One-quarter of the way around again and