Moon moving away very slowly

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?

Regards
Wal

"Wal" wrote in message ...

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?


The Earth's rotation propels the tidal bulges raised
by the Moon so that they're slightly ahead of the
Earth-Moon line. The gravity of the nearest bulge
pulls the Moon ahead slightly more than the further
one retards it. So it's the tides that do the
dirty work.

"Wal" wrote in message
...

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?

Regards
Wal





Hello Wal!
You have chosen a neat name!

Your natural satellite, the Moon, is always in a downward plunge to Earth.
Because it is also Going Around the Earth, it has a velocity component that
would normally keep it always at about the same distance from the Earth.
Also, because your Moon's orbit is not a perfect circle, it sometimes moves
a little toward, and then a little away from the Earth.

Finally, because your Moon interacts with the Earth in a manner you call
"tidal action," there is a friction between the two orbs.
This friction has caused the Moon to lose rotation speed and to always show
the same "face" to the Earth.
And this friction is also behind the transfer of "angular momentum."
That is a term that you might want to look up.
When two bodies rotate around a common center of gravity, the larger body
loses angular momentum.
This momentum must be conserved, so it must transfer somewhere.
It transfers to the smaller body, in your case, it transfers to your Moon.

This transfer of angular momentum from your Earth to the Moon results in a
very slow increase in the diameter of the Moon's orbit around the Earth.
So, many millions of years ago the Moon rotated much faster, your Earth
rotated much faster, and the Moon orbited much closer to the Earth.

Can you imagine the beauty in seeing a much larger Moon in the sky with a
face that changes from night to night?

Nish

"Nician" wrote in message
...
"Wal" wrote in message
...

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?

Regards
Wal





Hello Wal!
You have chosen a neat name!

Your natural satellite, the Moon, is always in a downward plunge to Earth.
Because it is also Going Around the Earth, it has a velocity component
that
would normally keep it always at about the same distance from the Earth.
Also, because your Moon's orbit is not a perfect circle, it sometimes
moves
a little toward, and then a little away from the Earth.

Finally, because your Moon interacts with the Earth in a manner you call
"tidal action," there is a friction between the two orbs.
This friction has caused the Moon to lose rotation speed and to always
show
the same "face" to the Earth.
And this friction is also behind the transfer of "angular momentum."
That is a term that you might want to look up.
When two bodies rotate around a common center of gravity, the larger body
loses angular momentum.
This momentum must be conserved, so it must transfer somewhere.
It transfers to the smaller body, in your case, it transfers to your Moon.

This transfer of angular momentum from your Earth to the Moon results in a
very slow increase in the diameter of the Moon's orbit around the Earth.
So, many millions of years ago the Moon rotated much faster, your Earth
rotated much faster, and the Moon orbited much closer to the Earth.

Can you imagine the beauty in seeing a much larger Moon in the sky with a
face that changes from night to night?

Nish


=================================

Thank you for the explanation Nish.

Regards
Wal

=================================

Wal posted:

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?

Yes, the moon is moving around the Earth, but its orbital path is very slowly
expanding, increasing the moon's average distance very slightly each year.
The reason for this is tidal forces. The moon raises water tides on the
Earth, since its gravitational pull is slightly higher on the side of the
Earth which is pointed towards the moon and slightly lower on the part of the
Earth which is away from the moon. The action of these water tides is to
eventually (and very gradually) slow the rotation of the Earth almost like a
friction brake. However, the mass of these tides also acts to very slowly
accelerate the moon in its orbit (conservation of angular momentum). As the
Earth slows, some of the angular momentum of the Earth is transfered to the
moon. Thus, over the years, the moon's orbit expands. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

On Sat, 28 Feb 2004 17:48:27 +0000 (UTC), Wal wrote:

According to a TV documentary last week the

moon is very slowly moving away from us.

Does it need to be accelerating to do that?

If that is the case were is acceleration coming from?

Yes, it is accelerated by tidal forces.

Gravity from the moon causes the facing earth's surface to bulge (also the
surface on the other side of the earth from the moon, but that's not
terribly important here). Because the earth rotates faster than the moon
revolves, that bulge is rotated out from below the moon.

Draw an imaginary line between the centers of the earth and moon. The
bulge will be continually carried to one side of the line, thereby causing
a slightly larger gravitional pull from that side. This pulls the moon
laterally a bit instead of directly towards the earth's center. This
accelerates the moon in a line tangential to its orbit. The energy
required to produce this acceleration comes from the earth's angular
momentum. So the earth slows down, and the moon moves faster (and as a
consequence of the latter, the moon's orbit recedes).

This is a vast oversimplification, of course. A bulging ocean has much
more of an effect than a bulging landmass, as it's fluid and is more
easily held back from rotation by the moon, thereby robbing angular
momentum via friction (it's much as if the crust of the earth is trying to
move through a stationary ocean). So, the rate of recession at any given
time will be affected greatly by the nature of the earth's surface below
the moon. At the present, the rate of recession is much larger than it
was in the past, despite the tidal forces being lower (since the moon is
further away).

The recession will continue until the period of rotation of the earth
slows down to match the period of revolution of the moon. Just as the
moon shows the same face to the earth, the earth will show the same face
to the moon. The tidal bulge then would not be rotated ahead of the moon,
and therefore no longer pull it laterally.

I believe the best calculations show that this state will be reached in
several billion years.

In the much more near future (though still well beyond our lifetime),
however, the moon will recede to the point where total solar eclipses will
never occur again. All eclipses from that point on will be annular.


--
- Mike

Remove 'spambegone.net' and reverse to send e-mail.

I didn't see the documentary, but the thread brought up a few
questions in my mind.
Is the rate of separation staying the same?
Would the periodic ice ages make a measurable change in the rate of
separation? By that I mean, if large amounts of the earths water was
locked up in ice, and from what I read the sea levels dropped
considerably, there would have been less water to affect a "tidal
breaking" effect.
If the earth is around 6 billion years old or so, would the tidal
slowdown of the earths rotation put a high limit on how old the earth
/ moon combination could be?
For example, if the moon was moving away from the earth a mile a year
(gross exaggeration I know) and the moon were a million miles from the
earth, one could say that the earth moon system could not be a million
years old, because at that point they would have been together.
If the moon were a result of an impact by a large body into an early
earth, would the rate of separation indicate how long ago the impact
was?
Thanks,
Frank

Frank,
Tidal forces aren't just contained to the oceans. Land masses are also
affected by tidal forces. Land masses actually move up & down with tidal
forces, about 2 feet (if memory serves me). Basically put...Earth's crust
is floating on a molten liquid center, and it has high and low "tides" too.
During the ice age, the rate of recession was probably not much different,
if any different at all.

Here's an interesting link:
http://csep10.phys.utk.edu/astr161/lect/moon/tidal.html

--

Don Baker
www.geocities.com/thebugbomber


wrote in message
...
I didn't see the documentary, but the thread brought up a few
questions in my mind.
Is the rate of separation staying the same?
Would the periodic ice ages make a measurable change in the rate of
separation? By that I mean, if large amounts of the earths water was
locked up in ice, and from what I read the sea levels dropped
considerably, there would have been less water to affect a "tidal
breaking" effect.
If the earth is around 6 billion years old or so, would the tidal
slowdown of the earths rotation put a high limit on how old the earth
/ moon combination could be?
For example, if the moon was moving away from the earth a mile a year
(gross exaggeration I know) and the moon were a million miles from the
earth, one could say that the earth moon system could not be a million
years old, because at that point they would have been together.
If the moon were a result of an impact by a large body into an early
earth, would the rate of separation indicate how long ago the impact
was?
Thanks,
Frank