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Moon Mechanics: What Really Makes Our World Go 'Round

By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
18 March 2003

Moon_mechanics - Sci Tues for March 17: Moon Mechanics In advance of the March 18 Full Moon, we the Moon's orbit, why it rises earlier each night, why months are not neatly divided into 30 days

 

A billion years ago, the Moon was much closer to Earth than it will be tonight. Its tighter orbit meant it needed just 20 days to go around us, to make a lunar month. Other things were noticeably different, too. A day on Earth back then was only 18 hours long. People were probably wishing, "If only I had 24 hours in a day"

Okay, there were no people then, but the critters of the time eventually got their wish. In the intervening eons, the Moon has been drifting away. Each year, it moves about 1.6 inches (4 centimeters) farther into space.

It is a coincidence of orbital and species evolution that we humans are on this planet during an era when we can work 24/7, should that be demanded.

Also by coincidence, we're here when the Moon's apparent size in the sky is equal to that of the Sun, so that a total solar eclipse is possible. Furthermore, we arrived comfortably after the pockmarked satellite began showing just one face to Earth, providing that immutable and unchanging beacon we call a full Moon, cosmic governor of terrestrial love and a lot of loony ideas.able -->


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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.

Upon impact, material from the incoming object and from the new Earth is cast into space. A ring of debris orbits the planet, and in an amazingly short amount of time -- about one day -- it begins to coalesce into a satellite. It takes somewhere between 1 and 100 years for the Moon to gather most of the stuff into a ball.

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.

Shifting tides

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.)

Back to our Moon: 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.

A moment in time

Things continue to change, of course.

Earth's rotation rate is still slowing down -- our days are getting longer and longer. Eventually, our planet's tidal bulges will be assemble along that 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 happens, billions of years from now, the terrestrial month will be longer -- about 40 of our current days -- because during all this time the Moon will continue moving away.

In this future picture, any lunar colonists would then henceforth see just one face of Earth. You can imagine this setup by stretching your arm out and looking at your palm. Now twirl around. You're face and your palm stare at each other 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.

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The upshot: One day your descendants, if they survive a swelling Sun and other 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 seems like days and days. Imagine the loony things they'll have time to imagine, the strange lore they might conjure.

A note about this article: The complex dynamics of tides are frequently explained incorrectly. This article owes to several conversations with astronomers over the years and many scientific papers and books. In particular, one book proved invaluable: "Bad Astronomy" (Wiley & Sons, 2002). Its author, Philip Plait, delves more accurately into the science of this, while explaining it in a sensible and simple manner, than any other writer I'm aware of. -- RRB

 

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