Or, one could argue, none of this is coincidence at all. If not for the Moon, some say, love as we know it would never have happened and we wouldn't be here to contemplate Earth's orbiting treasure.
The Moon has had dramatic effects on our planet and the life that inhabits it, researchers believe. The Moon stabilizes Earth's rotation, for example, preventing otherwise dramatic movements of the poles that would fuel climate swings that some scientists figure might have doomed any chance for life to form, let alone evolve.
And biologists speculate that tides, generated mostly by the Moon, would have been a logical place for life to originate. Sea creatures might have then used tidal regions as experimental sites for testing the habitability of land, and therefore as an excuse to develop lungs. Put short, your gilled ancestors might have used the Moon like a gravitational guiding light to the first non-aquatic procreation.
In that sense, the only coincidence in all this is the fact that the Moon ever came to exist in the first place. For there was a brief time in the early history of our planet, likely 100 million years or less, when there was no Moon in the sky.
4.5 billion years ago
The Earth has recently been forged out of the detritus of star formation, assembled from dust that became rock, then boulders that collided and grew. Other planetary hopefuls roam the solar system. Impacts are frequent. The scene is hectic.
A large rock, about the size of Mars, is doomed. It's heading toward Earth, destined for a slightly off-center impact that will set everything that isn't already rotating into a frenzy of spin.
There are other theories for how the Moon was born, but this one is widely accepted as the most plausible.
Earth may or may not have been rotating before the impact, but it certainly was afterward. Importantly, the orbital and rotational mechanics of this new Earth-Moon system were then planned out for all time. The impact imparted angular moment on the system, a spin that could never be destroyed, the laws of physics tell us. Curiously, the specific relationships would change over time -- dramatically -- and the shifts continue today.
The face of change
During the past 4.5 billion years, Earth's overwhelming gravity has slowed the Moon's rotation down and pushed the satellite away. The cause is complex, involving tides, which we'll discuss below. One amazing result, for now, is a readily observable set of very interesting facts: It takes the Moon 29.5 days to make one revolution about its axis. All the while, of course, the Moon is also going around the Earth. This orbit also takes 29.5 days.
Because the Moon's orbit and rotation times are the same, the satellite always shows the same face to Earth. We see that face because sunlight reflects off it (the Moon does not make its own light).
On the Moon, all this means that the Sun rises every four weeks, roughly. It also means there is no "dark side" of the Moon, at least not to someone living in any hypothetical Lunaville. The side of the Moon we cannot see from Earth gets its full share of sunshine periodically, when the Moon is between Earth and the Sun. In this configuration, the Moon is said to be new, and it reflects no sunlight our way.
There was a time, however, when the timing was much different.
Gravity is said to be the weakest of all the fundamental forces. But one aspect of it is very consequential: Gravity never goes away. It weakens with distance, but it is always at work. This fact is the primary driver of tides. The side of Earth nearest the Moon always gets tugged more than the other side, by about 6 percent.
Hey, you might say, there are two high tides on this planet at any given moment. True. And another far more complex set of phenomena explains this.
The Moon does not just go around the Earth. In reality, the two objects orbit about a common gravitational midpoint, called a barycenter. The mass of each object and the distance between them dictates that this barycenter is inside Earth, about three-fourths of the way out from the center.
So picture this: The center of the Earth actually orbits around this barycenter, once a month. The effect of this is very important. Think, for a second, of a spacecraft orbiting Earth. Its astronauts experience zero gravity. That's not because there's no gravity up there. It's because the ship and its occupants are constantly falling toward Earth while also moving sideways around the planet. This sets up a perpetual freefall, or zero-g.
Like the orbiting spaceship, the center of the Earth is in free-fall around the barycenter of the Earth-Moon system.
Here's the kicker: On the side of Earth opposite the Moon, the force of the Moon's gravity is less than at the center of the Earth, because of the greater distance. It can actually be thought of as a negative force, in essence, pulling water away from the Moon and away from Earth's surface -- a second high tide.
Our planet rotates under these constantly shifting tides, which is why high and low tides are always moving about, rolling in and rolling out as far as observers on the shore are concerned.
The Sun, too, has a tidal effect on Earth, but because of its great distance it is responsible for only about one-third of the range in tides. When the Earth, Moon and Sun are aligned (at full or new Moon), tides can be unusually dramatic, on both the high and low ends. When the Moon is at a 90-degree angle to the Sun in our sky (at first quarter or last quarter) tides tend to be mellower.
Effect on orbits
Earlier, we said tides are at the root of alterations in the entire Earth-Moon orbital system. Here's how: Earth spins once a day, while the Moon goes around the planet at a more plodding pace, once a month. So the planet is always trying to drag tides along, and it succeeds a bit.
The high-tide bulges are pulled just ahead of an imaginary line connecting the centers of Earth and the Moon. It might seem rather amazing, but a terrestrial bulge of water has enough mass to tug at the Moon from yet another angle. The effect is to constantly prod the Moon into a higher orbit, which explains why it is moving away from us.
The Moon, meanwhile, is yanking back on the tidal bulges. So the water, down where it meets the ocean floor, rubs against Earth. This slows the planet down, explaining why there are 24 hours in a day instead of the mere 18 of a billion years ago.
Finally, we need to bring up another factor that helped all these opposing dynamics reach an agreement of sorts:
More than just water is pulled up by tides. Earth's solid self actually stretches, too. And Earth's gravity lifts tides on the Moon, raising relatively small bulges in the seemingly solid satellite. (Similarly, Jupiter's gravity raises tides on its icy moons in the frigid outer region of the solar system, stretching some so dramatically that the action generates enough heat to maintain liquid oceans under their frozen shells, scientists believe.)