Bad Astronomy. Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax'

that, then you do indeed get a force pointing away from the Moon on the far side of the Earth.

The force of the Moon’s gravity on the Earth always “points” toward the Moon. The force gets weaker with distance so that the near side of the Earth is pulled toward the Moon more strongly than the far side. When the force of the Moon’s gravity is calculated relative to the Earth/Moon center of mass, the far side of the Earth actually feels a force pointing away from the Moon, while on the near side the force is still toward the Moon. This results in a stretching of the Earth, which is why we have two high tides a day.

That is why we have two high tides. There is a net force toward the Moon on the near side, and a net force away from the Moon on the far side. The water follows those forces, piling up in a high tide on opposite sides of the Earth. In between the two high tides are the low tides, and of course there are two of them as well.

As a point on the Earth rotates under the high tide bulge, the water rises. A few hours later, when the Earth has rotated one quarter of the way around, that point is now under a low tide, and the water has receded. One- quarter of the way around again and 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 position; 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 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.

The Earth spins faster (once a day) than the Moon moves around the planet (once a month). A bulge caused by the lunar tide is swept ahead of the Moon by the Earth’s rotation. This in turn tugs on the Moon, pulling it faster in its orbit, and moving it away from the Earth by about 4 centimeters per year. It is also slowing the Earth’s spin at the same time.

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. This timing may seem like a miraculous coincidence, but it isn’t. Tides force this situation.

All the time the Moon’s gravity is exerting a tide on the Earth, the Earth is doing the same thing to the Moon. But the tides on the Moon are 80 times the force of the ones on the Earth, because the Earth is 80 times more massive than the Moon. All of the tidal effects on the Earth are also happening on the Moon, but even faster and