It is a fact that our Moon is drifting further away from Earth every day at a rate measured as 3.8 cm per year. Leading to tidal braking that lengthens everyday by 0.002 seconds every century as measured by what Neil and Buzz left on the moon.
The Moon has had dramatic effects on our planet and the life that inhabits it. The Moon stabilizes Earth's rotation preventing dramatic movements of the poles that contribute to climate swings that some scientists claim might have doomed any chance for life to form, let alone evolve, if not for the Moon. And biologists speculate that tides (generated primarily by the Moon and Sun) 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 providing an opportunity to develop lungs. It has been postulated that ancient gilled creatures might have used the Moon like a gravitational guiding light similar to modern migratory birds to the first non-aquatic procreation sites. 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 (Approximately 100 million years or less) when there was no Moon in the sky above Earth.
During the past 4.5 billion years, the Earth's gravity has slowed the Moon's rotation down resulting in the satellite's drifting away. The cause is complex, involving tides. (discussed below) The amazing result is now 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. Both are computed as sidereal periods of the moon, with respect to the stars.
Because the Moon's orbit and rotation times are the same, the satellite currently shows the same face to us on Earth. The area of the moon that is illuminated is visible to us when sunlight reflects off the Earth facing side. On the Moon, all this means that the Sun rises every four weeks, roughly 29.5 days. 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. Just as a lunar eclipse these are the often the darkest of nights on Earth.
There was a time when the timing of our system was much different.
So close your eyes and picture this with the largest portion of your mind, the visual cortex. 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 is 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 free-fall, or zero-g. Like the orbiting spaceship, the center of the Earth is in free-fall around the barycenter of the Earth-Moon system. And on he 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 forces, which is why high and low tides are always moving about, rolling in and rolling out as far from the perspective of observers on the shore.
The Sun, too, has a tidal effect on Earth, but because of its greater 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 less extreme.
Tides are at the root of alterations in the entire Earth-Moon orbital system. Tides affect the Earth as it spins once per day, while the Moon goes around the planet at a much slower monthly pace. The planet is always dragging the tides along. 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 (in part) why it is drifting away from our Earth. Meanwhile the moon is yanking back on the tidal bulges. Therefore 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 hours per rotation experienced a billion years ago.
The final factor that
helped all these opposing dynamics reach an agreement of sorts involves the deformation of solid rock. More
than just water is pulled up by tides. Earth's solid self actually
stretches under these astrological forces as well. The Earth's gravity lifts tides on the Moon, raising
relatively small bulges of solid material 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.) Continual tugging on the lunar
bulges reduced the Moon's rotation rate over time. When the rotation had
slowed to the point that it equaled the time it took for the Moon to go
around the Earth, the lunar bulges lined up with our planet, and the
slowdown stopped. At that moment, one face of the Moon became forever
locked in our direction. But it is not the only natural satellite. Earth's rotation rate is still decelerating as our days are getting longer and longer. Eventually, our planet's tidal bulges will be assembled along the, a fore mentioned, imaginary line running through the centers of both Earth and the Moon, and our planetary rotational change will pretty much cease. Earth's day will be a month long. When this equilibrium is attained, billions of years from now, the terrestrial month will be longer (approximately 40 of our current days) because during all this time the Moon will continue moving away.
In this future Earth-Moon any lunar colonists would then, henceforth, see just one face of Earth. We can imagine this setup by stretching out our arms, and looking at our palms. Now twirl around like a whirling dervish. Our face and palm are relatively parallel the whole time. If the United States happens to be on the back of your head, well, just think what people there do not see, and will never see again from there.
One day our descendants, if they survive our swelling Sun, cosmic, and human perils, will have at least 960 hours to work
with each day. On some nights, half the world will be able to stare up
at a full Moon for what (today) seems like days and days. Imagine the loony
things they will have time to imagine, the strange lore they might conjure, the problems they will discover and solve, the peace they will enjoy.
