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

THE GRAVITY OF THE SITUATION 71
tion; you shrink from gravity more than you are stretched by tides.
Even large lakes can barely feel tides; the Great Lakes, for example,
have a change in height of only four or five centimeters due to
tides. Smaller lakes would have an even smaller change.
As complicated as all that sounds, amazingly, we aren’t done
yet. Tides due to the Moon are only half the issue. Well, actually,
they’re two-thirds of the issue. The other third comes from the Sun.
The Sun is vastly more massive than the Moon, so its gravity
is far stronger. However, the Sun is a lot farther away. The Earth
orbits the Sun in the same way the Moon orbits the Earth, so the
same idea applies. The Earth feels a gravitational pull toward the
Sun and a centrifugal force away from it. If you do the math, you
find out that tides due to the Sun are roughly half the strength of
the lunar tides. In the tidal game mass is important but distance
even more so. The nearby, low-mass Moon produces more tidal
force on the Earth than the much more massive but much farther
away Sun. Of the total tidal force exerted on the Earth, two-thirds
is from the Moon and one-third is from the Sun.
The Earth is in a constant, complicated tug of war between the
Sun and the Moon. There are times when the two objects’ forces
are in a line. As we saw in the last chapter, “Phase the Nation,”
when the Moon is new it is near the Sun in the sky, and when it’s
full it’s opposite the Sun. In either case the tidal forces from the
Moon and the Sun line up (because, remember, high tides occur
simultaneously on opposite sides of the Earth, so it doesn’t really
matter which side of the Earth you are on), and we get extra-high
high tides. It also means the low tides line up, so we get extra-low
low tides. These are called spring tides.
When the Sun and Moon are 90 degrees apart in the sky, their
forces cancel each other out a bit, and we get tides that aren’t quite
as low or as high (it’s like a lower high tide and higher low tide).
These are called neap tides.
Even worse, the Moon orbits the Earth in an ellipse, so sometimes
it’s closer to us than other times, and the forces are that much
greater. The Earth orbits the Sun in an ellipse, too, so we get more
exaggerated tides during the time of closest approach to the Sun
as well (around January 4 each year). If these two events—closest