Ancient day lengths?

[Mokele]

Ok, I might be totally off-base here, but I recall hearing something about the moon's rotation slowing the Earth. Or was it speeding up, or slowing down the moon? Something involving the two interacting and altering each other's rotation, anyway.

 

So, this got me thinking, could you possibly calculate, using what we know of this phenomenon, the day length a million years ago (assuming this was the only factor)? What about, say, 65 million years ago? Or 250 million?

 

I'm not really interested in the usual "will the moon fly off or hit the earth?" stuff, but more about day-length in general, and the possible ramifications for paleoecology.

 

Mokele

[starbug1]

I'm not sure, but I believe the only force the moon generates on earth is only of enough power to move the tides. To change the length of day by speeding or slowing the earth is beyond the moon's (gravational) capabilities.

[starbug1]

found some empirical evidence to disprove starbug1.

 

http://www.solstation.com/stars/earth.htm

 

and one religious site backing this observation:

 

http://www.religioustolerance.org/oldearth1.htm

[swansont]

Ok' date=' I might be totally off-base here, but I recall hearing something about the moon's rotation slowing the Earth. Or was it speeding up, or slowing down the moon? Something involving the two interacting and altering each other's rotation, anyway.

 

So, this got me thinking, could you possibly calculate, using what we know of this phenomenon, the day length a million years ago (assuming this was the only factor)? What about, say, 65 million years ago? Or 250 million?

 

I'm not really interested in the usual "will the moon fly off or hit the earth?" stuff, but more about day-length in general, and the possible ramifications for paleoecology.

 

Mokele[/quote']

 

The moon is slowing the earth by tidal friction, because the tidal bulge doesn't quite line up with the earth-moon center-of-mass. So they exert torques on each other, and explains why the moon is tidally locked to the earth, i.e. always shows us the same basic look, and rotates as often as it revolves.

 

It is the reason we add leap seconds from time to time, and will do so again this year. However, there are many things that can affect the rotation rate, so the slowing is not regular; in fact, we sped up a little in recent years, which is why there were no leap seconds for the last five or so years after having almost one a year for a while.

 

starbug's second link gives some good info on the rotation rates long ago.

[BobbyJoeCool]

I thought that it was the sun... that eventually we will be to the sun like the moon is to earth (rotating once with each revolution). ie: one side always facing the object it is orbiting...

 

I assume now that that is completly wrong. Anyone?

[mezarashi]

I don't see why the Earth-Moon rotation-orbit configuration should be preferred over any other. I think there needs to be an explanation as to why the Moon's "day" (one revolution around its own axis) is exactly equal to its "year" (one revolution around Earth). I find it very very odd for this to be the case. If the configuration is not preferred nor observed extensively in other orbital configurations, then the coincidence sets the ground for me to start believing in things like ID.

[J.C.MacSwell]

I don't see why the Earth-Moon rotation-orbit configuration should be preferred over any other. I think there needs to be an explanation as to why the Moon's "day" (one revolution around its own axis) is exactly equal to its "year" (one revolution around Earth). I find it very very odd for this to be the case. If the configuration is not preferred nor observed extensively in other orbital configurations, then the coincidence sets the ground for me to start believing in things like ID.

 

Mercury around the Sun is another example.

[Xyph]

I thought that it was the sun... that eventually we will be to the sun like the moon is to earth (rotating once with each revolution). ie: one side always facing the object it is orbiting...

 

I assume now that that is completly wrong. Anyone?

No' date=' that's correct. I don't know what has the bigger influence, although it's probably the moon, by far, but the sun is also having an effect on the Earth's rotation. Given enough time, anything will become tidally locked to what it's orbiting (same side always facing it), although for the Earth to become tidally locked to the sun would take far longer than the sun has to live. Eventually, the Earth would even become tidally locked to the moon, but once again, the solar system isn't going to last long enough for this to happen.

 

I don't see why the Earth-Moon rotation-orbit configuration should be preferred over any other. I think there needs to be an explanation as to why the Moon's "day" (one revolution around its own axis) is exactly equal to its "year" (one revolution around Earth). I find it very very odd for this to be the case. If the configuration is not preferred nor observed extensively in other orbital configurations, then the coincidence sets the ground for me to start believing in things like ID.

Again, it's because of tidal locking, which will occur eventually in any orbital system, as the gravitational pull of each orbiting body slows the rotation of the other. Pluto and Charon have had long enough to become tidally locked to each other, as have Mercury and the sun, as above. Extrasolar epistellar jovians are almost exclusively tidally locked to their star, as far as I'm aware, and there's been at least one example of a star tidally locked to it's planet. Any two orbiting bodies will eventually become tidally locked to each other, given enough time.

[mezarashi]

Any two orbiting bodies will eventually become tidally locked to each other, given enough time.

 

I see, so that's what it's called. Now that's an interesting phenomena I would say. Let's see if I can find more physics related to this tidal locking. Thanks for the pointer

[tony873004]

It is the reason we add leap seconds from time to time

A leap second due to the Moon slowing the Earth is only necessary about every 40,000 - 50,000 years.

 

 

Pluto and Charon have had long enough to become tidally locked to each other, as have Mercury and the sun.

The Sun is not tidally locked to Mercury. And Mercury is not tidally locked to the Sun. Mercury's day is 2/3 the length of its year. It would be more correct to call this a resonance.

 

 

If the configuration is not preferred nor observed extensively in other orbital configurations...

Many of our solar system's outer moons are tidally locked to their planet, including all 4 Galilean Moons of Jupiter.

 

 

Any two orbiting bodies will eventually become tidally locked to each other, given enough time.

As a general statement, yes. But there's lots of exceptions. The Earth can't tidally lock to the Sun and the Moon at the same time.

 

Bodies can spiral inward and collide with the planet. That is theorized to be the fate of Mars' moon, Phobos.

 

Bodies that spiral outward can exceed the Hill Sphere and get stripped away from their planets without ever tidally locking.

[BobbyJoeCool]

Is it not also true that unless near perfect conditions exist, any orbiting body will either escape orbit or crash into the body it is orbiting (be it in 5 seconds or 500 eons..)?

[swansont]

A leap second due to the Moon slowing the Earth is only necessary about every 40' date='000 - 50,000 years.

[/quote']

 

 

How do you figure this?

[tony873004]

How do you figure this?

With some basic math and some faulty logic

 

I should have said that in 40-50K years, the Earth's rotational period will be 1 second less. We'll need to add 1 leap second per day.

[swansont]

With some basic math and some faulty logic

 

I should have said that in 40-50K years' date=' the Earth's rotational period will be 1 second less. We'll need to add 1 leap second per day.[/quote']

 

It may be moot. There is discussion afoot to eliminate leap seconds. Astronomers hate the idea, generally, since they will have to do a lot of bookkeeping to keep observations straight with atomic time, but people who have to deal with inserting unpredictable time steps into their clock algorithms love it, and most people don't care one way or the other.

 

A positive thing is that it will reduce, however slightly, bad journalism. They won't have to botch the reports of why the leap second is inserted: not because the earth is slowing down, but because it has slowed down. (The earth is, generally, slowing down, but that just increases the rate of time accumulation. We'd still have leap seconds if the rotation rate stabilized.) They also won't have to mistakenly imply that the leap second is added at midnight, local time - it's added at midnight UTC (i.e. we all do it at once so "official" clocks can stay more or less in sync)