stronger.
The Earth raises a big tide on the Moon, stretching it out. There are two high tide bulges on the Moon, right in its solid rock. When the Moon formed, it was closer to the Earth and rotated much faster. The tidal bulge raised by the Earth on the Moon started to slow the Moon’s rotation, just as the Earth’s high-tide bulge does here. As the Moon slipped farther from the Earth, its rotation slowed, until the rotation period was the same as its revolution period (in other words, its day equaled a month). When that happened, its bulges lined up with the Earth, and the rotation of the Moon became constant; it stopped slowing.
That’s why the Moon always shows one face. It’s rotating, but the tidal force, well,
Remember, too, the Earth’s rotation is slowing down. Just as the Moon did eons ago, eventually the Earth’s rotation will slow down so much that the tidal bulge on the Earth will line up exactly between the centers of the Earth and the Moon. When this happens, the Moon will no longer be pulling the bulge back, and the Earth’s spin will stop slowing. The Earth’s day will be a month long (and by then the Moon’s recession will mean that the month will be longer too, about 40 days). In this time, far in the future, if you were to stand on the Moon and look at the Earth, you would always see the same face of the Earth, just as we see one face of the Moon from the Earth.
This kind of change due to tides is called tidal evolution, and it has affected the Earth and the Moon profoundly. When they were young, the Earth and the Moon were closer together and they both spun much more quickly. But over the billions of intervening years, things have changed drastically.
Once the Earth is rotationally locked with the Moon, there will be no more evolution of the Earth/Moon system from mutual tides. However, there will still be tides from the Sun. They would affect the system, too, but by the time all this happens, the Sun will be well on its way to turning into a red giant, frying the Earth and the Moon. We’ll have bigger problems than tides on our hands at that point.
Of course, we aren’t the only planet with a moon. Jupiter, for example, has dozens. The tides that Jupiter raises on its moons are hellish; the planet is over 300 times the mass of the Earth. Little Io, a moon of Jupiter, orbits its planet at the same distance the Moon orbits the Earth, so it feels tides 300 times stronger than does our Moon. Io is also tidally locked to Jupiter, so it spins once an orbit. If you could stand on Jupiter, you’d always see the same face of Io.
But Jupiter has lots of moons, and some of them are big. Ganymede, for example, is bigger than the planet Mercury! These moons all affect each other tidally, too. When one moon passes another, the differential gravity squeezes and stretches the moons, flexing them.
Have you ever taken a metal coat hanger and bent it back and forth really quickly? The metal heats up, possibly enough to burn you. The same thing happens when these moons flex. The change in pressure heats their interiors. It heats Io enough to actually melt its interior. Like the Earth, Io’s molten insides break out of the surface in huge volcanoes. The first was discovered when the Voyager I probe cruised past the blighted moon in 1979. Many more have been found since then, and it looks like there are always volcanoes erupting on the poor moon.
The tidal friction also warms the other moons. Europa shows evidence of a liquid-water ocean buried under its frozen surface. That water may be heated by tides from passing moons.
If we look even farther out, we see more tides. Sometimes stars orbit each other in binary pairs. If the stars are very close together, tides can stretch them into egg shapes. If they are even closer, the stars can exchange material, passing streams of gas from one to the other. This changes the stars’ evolution, affecting the way they age. Sometimes, if one of the stars is a dense, compact star called a white dwarf, the gas from the more normal star can pile up on the surface of the dwarf. When enough gas accumulates, it can suddenly explode in a cosmic version of a nuclear bomb. The explosion can rip the star to pieces, creating a titanic
And we can take one more step out, to a truly grand scale. Whole galaxies are affected by tides, too. Galaxies, collections of billions of stars held together by their own gravity, sometimes pass close to each other. The differential gravity of one passing galaxy can not only stretch and distort but actually tear apart another galaxy. Sometimes, as with the binary stars, the more massive galaxy actually takes material — stars, gas, and dust — from the less massive one in an event called galactic cannibalism. This is hardly a rare event. There’s evidence our own Galaxy has done this before, and as a matter of fact, we are currently colliding with a tiny galaxy called the Sagittarius Dwarf. It is passing through the Milky Way near the center, and as it does it loses stars to our much larger and more massive galaxy.
So the next time you’re at the beach, think for a moment about what you’re seeing. The force of tides may take the water in and out from the shoreline, but it also lengthens our day, pushes the Moon farther away, creates volcanoes, eats stars, and viciously tears apart whole galaxies. Of course, the tides also make it easier to find pretty shells on the coastline. Sometimes it’s awesome to think about the universe as a whole, but other times it’s okay just to wiggle your toes in the wet sand.
8.
The Moon Hits Your Eye Like a Big Pizza Pie: The Big Moon Illusion
On a warm spring evening when my daughter was still an infant, my wife and I put her in a stroller and set off for a walk through our neighborhood. Heading south, we turned onto a street that put us facing almost due west. The Sun was setting directly in front of us and looked swollen and flaming red as it sank to the horizon. It was spellbinding.
Remembering that the Moon was full that night, I turned around and faced east. There on the opposite horizon the Moon was rising, looking just as fat — though not as red — as the Sun, still setting 180 degrees behind us.
I gawked at the Moon. It looked positively
I knew better, of course. I also knew something more. Later that evening, around 11:00 or so, I went outside. It was still clear, and I quickly found the Moon in the sky. After so many hours, the rotation of the Earth had carried it far from the horizon, and now the full Moon was bright and white, shining on me from high in the sky. Smiling wryly, I noted that the Moon appeared to have shrunk. From the vast disk glowering at me on the horizon earlier that evening, the Moon had visibly deflated to the almost tiny circle I saw hanging well over my head.
I was yet another victim of what’s called the Moon Illusion.
There is no doubt that the vast majority of people who see the Moon rising (or setting) near the horizon think it looks far larger than it does when overhead. Tests indicate that the Moon appears about two to three times larger when on the horizon versus overhead.
This effect has been known for thousands of years. Aristotle wrote of it in about 350 b.c., and a description was found on a clay tablet from the royal library of Nineveh that was written more than 300 years earlier than that date.
In modern popular culture there are many explanations offered for this effect. Here are three very common ones: The Moon is physically nearer to the viewer on the horizon, making it look bigger; the Earth’s atmosphere acts like a lens, magnifying the disk of the Moon, making it appear larger; and when we view the horizon Moon we mentally compare it to objects like trees and houses on the horizon, making it look bigger.
Need I say it? These explanations are wrong.
The first one — the Moon is nearer when on the horizon — is spectacularly wrong. For the Moon to look twice as big, its distance would have to be half as far. However, we know that the Moon’s orbit isn’t nearly this elliptical. In fact, the difference between the perigee (closest approach to the Earth) and apogee (farthest point from the Earth) of the Moon’s orbit is about 40,000 kilometers. The Moon is an average of 400,000 kilometers away, so this is only a 10 percent effect, nowhere near the factor of two needed for the illusion. Also, the Moon takes two weeks to go from perigee to apogee, so you wouldn’t see this effect over the course of a single evening.
Ironically, the Moon is actually a bit
