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

70 FROM THE EARTH TO THE MOON
you’ve got a high tide. On and on it goes, with high and low tides
alternating roughly every six hours.
But not exactly six hours. If we could hold the Moon stationary
for a while, you would indeed feel two high (and two low)
tides a day, separated by 12 hours. But as we saw in the last chapter,
the Moon rises about an hour later every day, because as the
Earth spins, the Moon is also orbiting the Earth. The Moon moves
during that day, so we have to spin a little bit extra every day to
catch up to it. So instead of there being 24 hours between successive
moonrises, there are actually about 25. That means there is a
little extra time between high tides; half of that 25 hours, or 12.5
hours. The time of high and low tides changes every day by about
a half hour.
An aside: Most people think that only water responds to these
tidal forces. That’s not true; the ground does, too. The solid Earth
isn’t really all that solid. It can bend and flex (ask anyone who’s
ever been in an earthquake). The forces from the Moon actually
move the Earth, shifting the ground up and down about 30 centimeters
(12 inches) every day. You can’t feel it because it happens
slowly, but it does happen. There are even atmospheric tides. Air
flows better than water, resulting in even more movement. So, the
next time someone asks you if the Earth moved, say yes, about a
third of a meter.
Incidentally, this puts to rest a common misconception about
tides. Some people think that tides affect humans directly. The idea
I usually hear is that humans are mostly water, and water responds
to the tidal force. But we can see that idea is a bit silly. For one
thing, air and solid ground respond to tides as well. But more importantly,
humans are too small to be affected noticeably by the
tides. The Earth has tides because it’s big, thousands of kilometers
across. This gives the gravity from the Moon room to weaken.
Even a person two meters tall (6 feet 6 inches) feels a maximum
difference in gravity of only about 0.000004% from head to foot.
The tidal force across the Earth is over a million times stronger
than that, so needless to say the tidal force across a human is way
too small to be measured. Actually, it’s completely overwhelmed by
the natural compression of the human body in the standing posi-