Message-Id: <l03102801b0f71f06fc13@[156.1.196.12]>
Date: Fri, 30 Jan 1998 00:25:51 -0700
To: pinhole@exploratorium.edu
From: Ron Wong <ronwong@inreach.com>
Subject: re rotation of Earth
Suzi said:
>I had an argument with a friend
>during dinner about why the moon has almost stopped spinning over the
>years.
Actually, the moon has never stopped spinning. It's period of rotation just
happens to be equal to the period of its orbit about the earth, about 27.3
days. This state of affairs is called synchronous rotation.
It isn't by accident. At an earlier period of time one would have been able
to see the far side of the moon just by waiting for it to rotate into view
(this was long before mankind appeared on this planet). At that time, so
the theory goes, the moon's rocky material was more fluid. Just as the moon
causes tidal effects here on earth, earth caused tidal affects on the moon.
The tidal effects on the liquid rock must have been enormous due the moon's
closer proximity to the earth at that time. The tidal friction produced by
the moon as it rotated under the tidal humps of its fluid rock produced
thermal energy that was radiated away into space. This energy came at the
expense of the moon's rotational energy and the result was a decrease in
it's rotation till it reached the same rate as the rate the moon moved
about the earth. At this point, there would be no more tidal friction and
thus no further change in the moon's rotation.
>I also explained that, eventually, the Earth will stop spinning
>because of the Sun's gravitational friction with the Earth. Is this
>true? He says that the earth will stop spinning because of the friction
>of the atmosphere with the mountains on earth.
Newton's third law says that what is good for the goose is good for the
gander.
The moon produces tides on our atmosphere, our oceans, and our earth. As
far as tidal friction is concerned, the atmospheric tides are negligible.
The other tides are more significant as far as tidal friction is concerned
(although we are more conscious of the ocean's tides due to the several
feet of change in water level during the course of the day, there is a
measurable rise and fall of the earth's surface during that same time -
about a quarter of a meter). As with the moon, the heat produced comes at
the expense of the earth's rotation. It amounts to an increase of about 2
thousandths of a second/century. So, in time, only one side of the earth
will face the moon.
The reason we have not entered into synchronous rotation with our moon at
the time it did is simply because, with our far greater mass, we had more
rotational energy to begin with and will take a lot longer time to get to
this state.
Regarding the sun: The sun's ability to produce tidal effects on the
earth's oceans is pretty small. Not negligible, but pretty small compared
to that of the moon. So, when we talk about the earth slowing down due to
tidal effects, it's in terms of the moon's influence and not that of the
sun.
An aside:
In his response to your question, Neil mentioned that the earth's
atmosphere rotates at the same rate about the earth as the earth itself and
thus is carried along with the earth.
As he pointed out, if it didn't there would be this constant wind out of
the East due to the earth rotation from west to east (needed to explain why
the sun rises in the east and sets in the west). At the equator, this wind
would have a speed of 1000 mi/hour (about 600 mi/hr here in SF)! Our
weather would come out of the East (instead of the west as it does)! Birds
taking off would suddenly be swept to the west at speed measured in
hundreds of mi/hr! All of this is quite contrary to our common sense of
things.
Interestingly enough, for many hundreds of years, these were just the
arguments used to "prove" that the earth did NOT rotate. After all, being
as tenuous as it was, it would be very difficult if not impossible for the
earth to set all of it moving at the same time as the earth - if indeed the
earth was rotating. With a rotating earth and a non-rotating atmosphere
comes all the silly consequences mentioned above. Thus, the earth doesn't
rotate.
Oh well, now we know better ;-).
ron