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

72 FROM THE EARTH TO THE MOON
Moon, and closest point to the Sun—happen at the same time, we
get the biggest possible tides. It’s not really as big an effect as all
that, though; it’s only a few percent more. But as you can see, tides
are complicated, and the force is never constant.
But there’s no reason to stop here. There is another effect. It’s
subtle, but the implications are quite profound.
As I mentioned, the Earth is spinning on its own axis while the
Moon orbits us. The water responds quickly to the tidal force, and
“piles up” under the Moon and on the side of the Earth opposite
the Moon. However, the Earth is spinning, and its spin is faster
(one spin a day) than the Moon’s motion around the Earth (one
orbit a month). The water wants to pile up under the Moon, but
friction with the spinning Earth actually sweeps it forward a bit,
ahead of the Moon. The tidal bulge, as it is called, does not point
directly to the Moon, but a little in front of it.
So picture this: the bulge nearest the Moon is actually a bit
ahead of the Earth-Moon line. That bulge has mass—not a lot, but
some. Since it has mass, it has gravity, and that pulls on the Moon.
It pulls the Moon forward a bit in its orbit. It acts like a small
rocket, pushing the Moon ahead a little. When you push an orbiting
object forward, it goes into a higher orbit, that is, one with a
larger radius. So, as the tidal bulge on the Earth pulls the Moon forward,
the Moon gets farther away from the Earth. This effect has
been measured quite accurately. The Moon is actually farther away
now than it was a year ago by about 4 centimeters (1.5 inches).
Next year it’ll be another 4 centimeters farther away, and so on.
Of course, the Moon is pulling on that tidal bulge as well. If
the bulge is ahead of the Moon, then the Moon is behind the bulge
(relative to the rotation of the Earth). That means it’s pulling the
bulge backwards, slowing it down. Because of friction with the rest
of the Earth, this slowing of the bulge is actually slowing the rotation
of the Earth! This is making the day get longer. Again, the
effect is small but measurable.
Besides the phase, the most obvious feature of the Moon is
that it always shows the same face to us (described in chapter 3,
“Idiom’s Delight”). This is because the Moon spins once on its
axis in the same amount of time it takes to orbit the Earth once.