Earth's Days are Getting Longer

Renaissance Man · 25th August 2004, 09:36

Tidal coupling will one day render the earth's rotational period the same as the length of its month. This is due to tidal friction caused by the differential forces of gravity of the moon on the earth. Once upon a time, around 400 million years ago, the length of an earth day was 22 hours. Currently, a day is shortening at a rate of 0.00002 seconds per year. Billions of years from now, the length of a day will equal roughly 47 of today's earth days. This effect is the same reason that the moons rotational period is the same as its revolutionary period, and the reason why the dark side of the moon exists.

In addition to increasing the time it takes the earth to complete one rotation, the distance to the moon will also increase. Right now, this distance is increasing at a rate of 4cm per year. When complete tidal lock occurs this distance will have increased to over 100% of its current value.

All this is really an effort to conserve the angular momentum of the earth moon system. It is a manifestation of the conservation of energy. Does anyone have any thoughts on this? Do you think the moon's position when a child is born has an effect on that childs personality? Does anyone know why these tidal forces cause the oceans tides? How does the sun's gravity compare? What are its effects on the earth's tides? Did you know the moon has a tide?

**DaCuBaN** · 25th August 2004, 23:01

the reason why the dark side of the moon exists.

I'll assume either you're speaking crap here, or you meant to say that there's a side of the moon we never get to see from earth - The same face always stares down at us, but the 'Dark Side of the Moon' doesn't exist...

Does anyone know why these tidal forces cause the oceans tides?

As I understand it, the moon's gravity essentially 'sucks' the ocean up as it passes over it, causing it to rise and fall as it passes over.

Renaissance Man · 26th August 2004, 09:12

Actually, there is a Dark Side of the moon, just as there is a dark side of the earth. This dark side is dependent on the moon's position relative to the sun, and does not correspond to what is known as the far side of the moon, except during a full moon.

I apologize for using a common colloquialism, but I happen to be a Pink Floyd fan and like to use the expression "dark side of the moon" to refer to the far side of the moon. Pink Floyd's Dark Side of the Moon is the 11th top selling album of all time. At the end of the album they actually address this same question, in a sense. There is a voice over that states; "There is no dark side of the moon. As a matter of fact, it's all dark. The only thing that makes it look alight is the sun", which is correct. In fact, the moon is a very dark astronomical body. It only reflects about 7% of the sun's electromagnetic radiation, so its albedo rating is 0.07. Imagine how bright the night sky would be if its albedo were say 0.9.

At any rate, sorry about using a commonly misconstrued concept. I just happen to like the phrase, because there is a certain mystery to the far side of the moon, since we never get to see that side.

As far as the cause of the tides goes, these are caused by the same differential forces that are slowing down the Earth's rotation, and pulling the moon into a more distant orbit. What you say is pretty much correct. However, the effect of these differential forces on one side of the planet is not the same as the other side of the planet. The behavior of the system is for water to bulge on either side of the planet along the line of the barycenter for the earth moon system. On the side closest to the moon, this bulge is caused by the gravitational force of the moon on the earth's oceans. The bulge on the side opposite of the moon is due to the fact that the attractive forces on the earth are stronger, since the earth is closer to the moon. Gravity is inversely proportional to the square of the distance, so the earth gets pulled away from the water on that side, resulting in the other bulge. When the earth rotates under these bulges, any point will see two high tides and two low tides for each rotation.

One must also consider the sun's effect on earth's tides. When the moon is in opposition (180deg elongation), the bulges are maximized. This is known as spring tide. When the moon is in quadrature (90deg elongation), the bulges are minimized. This is called neap tide.

At the same time, and to a lesser extent, the Earth's crust, mantle, and core also experience these tidal effects, which cause it to bulge in a similar fashion. The bulk modulus for solid earth is greater than the bulk modulus for water, therefore, distortion occurs to a lesser extent. The tidal force of the earth on the moon has caused its center of mass to shift. One side of the moon is actually more dense than the other, largely due to these tidal forces. Did you know that tides caused the moon to become lopsided over time?

**DaCuBaN** · 26th August 2004, 21:41

I apologize for using a common colloquialism, but I happen to be a Pink Floyd fan and like to use the expression "dark side of the moon" to refer to the far side of the moon

Similarly, I apologise for 'jumping' on this fact: Given that you had the volume up loud enough at the end of DSotM to hear that bit, I'm sure you've heard the other 'secret' info in that album. Anyway, I'm rambling.

It only reflects about 7% of the sun's electromagnetic radiation, so its albedo rating is 0.07. Imagine how bright the night sky would be if its albedo were say 0.9.

Indeed - I was under the impression that despite this, we only perceive on earth approximately 5% of the sun's reflected radiation, the stratosphere absorbing the rest, although I'm by no means certain.

The behavior of the system is for water to bulge on either side of the planet along the line of the barycenter for the earth moon system

Was it not this effect - the electromagnetic 'tunnel' that was the catalyst for Hawking's thoughts on Black Holes, and parallel universe theory? If I remember correctly, the inverse gravitational force has a cummulative effect on the far side of the planet.

At the same time, and to a lesser extent, the Earth's crust, mantle, and core also experience these tidal effects, which cause it to bulge in a similar fashion

Again, I believe this is tied to the theory of tectonic shifting - essentially the moon has 'pulled' sections of the earth around in the millions of years she has orbited us.

One place I feel more stufy should be made: What effect does this have on the force exerted upon us by gravity? I would assume that it would be marginally easier for us to 'jump' during moon

Did you know that tides caused the moon to become lopsided over time?

Now that's a new one on me

All in all, this is probably the best thread I've seen on here in a while. Keep it up

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