Most of us appreciate the beauty of a full Moon sedately moving across our night skies. The Moon seems unchanging, as if for eons it has been there, controlling the tides, going through its phases every month and inspiring poets and lovers. It might surprise you to know that in the past, things were much different, and they are still changing.
A billion years ago, our relationship with the Moon was quite different. For one thing, the Moon was much closer and therefore appeared much larger: and we saw the entire Moon, not just one face as we do now. It took the Moon only twenty days to circle the Earth, and Earth's day was only eighteen hours long. Massive tides, over a kilometer in height, would ebb and flow every few hours. From the beginning though, things were changing, as the Moon's orbit was getting larger.
Every year, the Moon moves about four centimeters (about 1.6 inches,) farther out into space, away from Earth. Right now, the Moon's rotation rate, and the time it takes to orbit Earth are the same. Is this all just some cosmic coincidence? Would it surprise you to know that at one time, Earth didn't even have a moon? Where then, did our nearest neighbor in space come from, and what will the future bring? To answer these questions we must go very far back into time.
Four and a half billion years ago the Earth was about sixty percent formed, though it did have a differentiated core, crust and mantle. In these primal skies, there was no Moon. It was a very violent time in the Solar System with other planets forming from clumps of gas, dust and rock and impacts among these bodies were the norm. Orbits were not the sedate, orderly system they are today.
One planet, about the size of Mars, probably had an orbit which crosses Earth's, and eventually a collision occurred. The impacting planet made a hard, but glancing blow off Earth at just the right angle. It almost bounced off Earth, but was consumed instead. But the blow sent shock waves across Earth, spewing gas and debris into space, giving Earth, for a very short time, a ring around it.
The debris around Earth began to condense into clumps and quite rapidly, a blink in cosmic time, formed a large glowing ball: our primal Moon. The Moon would have looked about ten times larger than it does today, and the Earth was changed forever.
This story, about the birth of the Moon, is still a theory, one among several attempting to explain the source of our satellite. Currently, this is the most widely accepted of the theories.
Since that cosmic collision, the Earth and Moon have both have an effect on each other, and will continue to have an affect far into the future. Since the Earth is so much more massive than the Moon, the Earth has had the larger effect. Earth's gravity has caused the Moon to become tidally locked us as well as increasing the distance between the two worlds. By tidally locked, we mean that the moon's rotation rate is the same as the time it takes the moon to go once around Earth, which also results in us seeing only one side of the Moon.
The dynamics of this Earth/Moon relationship come basically down to Earth's tides so we need to take a closer look at them. We know that the Moon is the primary cause of Earth's ocean tides by the Moon “pulling” on the Earth as it goes around it. But gravity lessens with distance and this means that the pull from the Moon is stronger on the side of Earth that is facing the Moon.
Keep in mind that the Moon is not pulling on the Earth as a whole, but rather on the part of Earth beneath the Moon. So, as the Moon passes over a part of Earth, the Moon pulls on that area. If it is over water, the Moon actually pulls on the water, creating a void for more water to flow into that area and creating a high tide. Directly opposite that spot, the Moon's gravity is pulling on the Earth itself, pulling it away from the water and allowing more water to flow into that area creating another high tide. But because the distance from the Moon is greater, the pull from gravity on the far side of the Earth is about six percent less than on the side facing the Moon.
The Sun also has an effect on tides, but it only about forty six percent of the tidal effect from the Moon. That might sound confusing – seeing as how the Sun is so much bigger than the Moon, but remember – gravity decreases with distance – and the Sun is much farther away from Earth than the Moon.
When the Sun and Moon team up together (both on the same side of the Earth) we have even larger high tides. The point though, is that Earth is being tugged from several bodies, rotating under the shifting tides.
Also, remember that as the Moon is moving around the Earth, the Earth itself is spinning. The Earth spins much faster than the Moon moves around our planet. The effect this has, is that that the tidal bulge caused by the Moon is actually pulled ahead of the Moon by the Earth's faster rotation.
The Moon therefore is pulling back on the tidal bulge with the result that there is friction between the ocean floor and the water. The friction actually slows Earth down, therefore explaining why our days on Earth are getting longer.
These tidal bulges on Earth, have an affect on the Moon, in essence pulling the Moon and forcing it into a higher orbit. This is why the Moon is moving away from us.
We mentioned tides on Earth caused by the Moon – but Earth actually causes tides on the Moon's surface as well. Even though the Moon has a solid surface of rock, small tidal bulges actually occur. Earth tugging on the Moon's surface has slowed down the rotation of the Moon over time. Over time the Moon rotation slowed until it equaled the time it took the Moon to go around the Earth. At that point, those lunar tidal bulges lined up with Earth and the rotation ceased slowing down: the Moon was tidally locked with Earth and that's why we see only one side of the Moon.
Just because the Moon's rotation has ceased to slow down doesn't mean that the Earth/Moon system isn't changing in other ways. We already know that the Moon is moving farther away from Earth but other changes are happening as well.
We mentioned that Earth's rotation was being slowed by the friction between the oceans and the ocean floor. This will continue to happen until Earth's tidal bulges align with an imaginary line running through the center of the Earth/Moon system then Earth's rotation will cease slowing down. This will take a few billion years but when it does happen: Earth's day will be a month long (960 hours a day) and our month will be forth days long! By then the Moon will be twenty-five percent farther away. If we were on the Moon looking back at Earth – we would see the same face of Earth – just as now we see only one face of the Moon. And if someone were still on Earth: the Moon will have moved far enough away that it appears much smaller – and there are no more solar eclipses!
We have yet to answer that question about a cosmic coincidence. Do we “just happen” to be here on Earth when the Moon is close enough to give us tides, and slowed the Earth's rotation until we have a twenty-four hour day and thirty day month? Some scientists believe that it may be thanks to the Moon that we are here at all.
Although the Moon is slowing down Earth's rotation, it has stabilized it as well. If we had no moon, we would suffer movement of the Earth's poles, an axial tilt of up to ninety degrees, causing major climate swings. Some scientists believe that under those circumstances: intelligent life simply could not have evolved.
It is hard to imagine that our world as we know it, and all that is familiar to us, would likely not exist if not for the Moon. We may truly owe our lives to a lunar legacy.
Copyright © 1995 - 2007
Kathy Miles, Author, and Chuck Peters, Systems Administrator
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