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

66 FROM THE EARTH TO THE MOON
is concerned, you can imagine that all the mass of the Earth is condensed
into a tiny point at its center, so it’s from there that we
measure distance.
The Earth’s radius is about 6,400 kilometers (4,000 miles), so
for me to double my distance, I’d have to book a flight on a
rocket: I’d need to get an additional 6,400 kilometers off the
ground, nearly one-sixtieth of the way to the Moon. Only there
would I feel like I weigh a quarter of what I do now. It seems like
a rather drastic way to lose weight.
Because the Moon is smaller and less massive than the Earth,
you would feel a gravity about one-sixth that of the Earth’s if you
were standing on the lunar surface. That’s still a substantial pull.
Of course, the Moon is pretty far away, so its gravitational effect
here on Earth is much smaller. It orbits the Earth at an average distance
of about 384,000 kilometers (240,000 miles). From that distance
its gravity drops by a factor of nearly 50,000, so we can’t
feel it.
But it’s there. Gravity never goes away completely. Although
on the Earth the force of gravity from the Moon is terribly weak,
it still extends its invisible hand, grasping our planet, pulling on it.
That grasp weakens with distance, giving rise to an interesting
effect on the Earth. The part of Earth nearest the Moon feels a
stronger pull than the part of the Earth farthest from the Moon.
The difference in distance—the diameter of the Earth—means a
difference in gravity. The near side of the Earth feels a pull about
6 percent stronger than the far side. This difference in pull tends
to stretch the Earth a little bit. It’s because the gravity is different
from one side of the Earth to the other, so we call it differential
gravity.
Gravity always attracts, so the force of lunar gravity is always
a pull toward the Moon. So, you would think, since the near side
of the Earth feels a stronger pull, water would pile up there, giving
us a high tide. On the far side of the Earth there should be a low
tide, a flattening, perhaps, because even though the force is weaker,
it still points toward the Moon.
But we know that’s not right. There are two high and two low
tides a day. That means at any one time there must be a high tide