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

THE GRAVITY OF THE SITUATION 65
was essentially wrong! What you’ll read here is now correct. It’s
funny, too—even people who get tides right rarely take the discussion
far enough. Tides have far-reaching consequences, from locking
together the Moon’s spin and orbital motion to the volcanoes
on Jupiter’s moon Io. Tidal forces can even cause entire galaxies to
be ripped apart, torn to shreds by even bigger galaxies.
When astronomers talk about tides, we usually don’t mean the
actual movement of water. We are using the term as a shorthand
for the tidal force. This is a force much like gravity, and in fact is
related to gravity. We’re all aware of gravity from the first time we
try to stand up and walk. As we age, we become increasingly aware
of it. For me, it seems harder to get out of bed every day, and easier
to drop things. Sometimes I wonder if the Earth is pulling harder
on me each day.
It doesn’t really, of course. Gravity doesn’t change with time.
The force of gravity, the amount that it pulls on an object, depends
on only two things: the mass of the object doing the pulling, and
how far away it is.
Anything with mass has gravity. You do, I do, planets do, a
feather does. I can exact a minute amount of revenge on Earth’s
gravity knowing that I am pulling back on the Earth as well. The
amount I am pulling is pretty small, sure, but it’s there. The more
massive the object, the more it pulls. The Earth has a lot more
mass than I do (something like 78,000,000,000,000,000,000,000
times as much, but who’s counting?), so it pulls on me a lot harder
than I do on it.
If I were to get farther away from the Earth, that force would
weaken. As a matter of fact, the force drops with the square of my
distance; that is, if I double my distance, the force drops by a factor
of 2 � 2 � 4. If I triple my distance, it drops by 3 � 3 � 9,
and so on.
That does not mean that I feel one-quarter of the gravity if I
climb a ladder to twice my height, though! We don’t measure distance
from the surface of the Earth, we measure it from its center.
A few hundred years ago, Sir Isaac Newton, the seventeenth-century
philosopher-scientist, showed mathematically that as far as distance