Re: Pull of the Moon

[Phil Bigelow <bigelowp@juno.com>]

On Tue, 18 Apr 2006 11:48:55 +0100 Steve White
<steve_d_white@hotmail.com> writes:
> I have a question. I read sometime back that the moon has been slowly 
> moving 
> further out of orbit and that during the Mesozoic


Indeed it was. The Moon is constantly "stealing" rotational energy from
the Earth (by causing tidal drag on the Earth's angular momentum). 
Earth's days are getting longer.  This "stolen" energy is then used to
push the moon into a higher orbit.   Voila, conservation of energy.



> it was actually 
> something 
> like a quarter closer to Earth (but don't quote me on that).


I asked a similar question a while ago on the vrtpaleo list, the only
difference was that I was seeking an algorithm to calculate a rate of
separation vs. time curve.  I didn't get any answers then, but I have
since learned that such an algorithm cannot be created, because the
"true" curve probably doesn't follow a strict mathematical formula.  To
be sure, in a "perfect" 2-body system, the curve of the changing rate of
separation (in cm/year) would start out high near the Y-axis (the moon is
hypothesized to have formed at a distance of only 14,000 miles from Earth
when it was formed ~4.4 Gya) and then the rate of separation decreases
with geologic time (today: at 238,000 miles, with a rate of separation of
1.5 inches per year).  But the E-M system isn't a perfect system, and
that real curve is probably bumpy and jagged.

So, failing to find a nice smooth curve to explain everything, the only
fall-back methodology is to rely solely on the empirical data.  This data
is obtained primarily from two sources:  1) rythmites and 2) growth
layers on certain corals.  These data will give you the length of a day
and the length of a lunar month.  From that information, you can
calculate the E-M separation at that point in time.

I believe there are both rythmite and coral data from the Mesozoic.


 > I was 
> interested in this from an artistic view (should we make the moon 
> larger in 
> our Mesozoic skies?)


By all means.   The Mesozoic moon would only be modestly larger in the
sky than it is today. (If you *really* want to shock your viewers, paint
a scene from 4 Gya, when the moon was only 14,000 miles away!).  So if
you already know the E-M distance for a point in time, and if you are a
stickler for scientific accuracy in your artwork, you can calculate the
"angular" size of the Moon in the Mesozoic sky from its diameter (2160
miles).  It's just simple trigonometry.

As an aside, I recently saw a piece of paleo artwork (the painter's name
escapes me, but it isn't anyone on this mailing list).  It was of a
Precambrian shoreline scene at dusk, with stromatolite mats sticking out
of a tidal pool.  The author had drawn the moon at today's angular size! 
Worse, he also had the Big Dipper in the scene!

He demonstrated good artistic technique (the painting was beautiful), but
his contextual knowledge was in desperate need of improvement.  :-(


> but was also curious to the effects a closer 
> moon would 
> have had on Earth - would the gravitational pull have been stronger, 
> 
> affecting tides more radically in the likes of the Tethys Ocean and 
> 
> Cretaceous Inland Seaway?


Tethys yes.  WIS not as much.

Tidal magnitude not only depends on the E-M distance, it also depends on
the geometry of the land masses and the size of the body of water
involved.  Scattered land masses block global tidal movement.  N-S
trending land masses also block global tidal movement.   At points in
time when there was only one huge landmass and mostly ocean, the ocean
tides would be freer to move, unhindered by N-S trending continents. 
This would cause huge tides like nothing currently seen (larger single
bodies of water = larger tides).

Tethys shorelines, being E-W trending and bordering a huge sea, would
have had large tidal currents, whereas WIS shorelines, being N-S trending
and bordering a smaller sea, would have had considerably lower tidal
currents. [Although ironically, the WIS shorelines, being perpendicular
to the tidal movements, may have experienced higher tidal *amplitudes*
than did the Tethys shorelines which were parallel to the tidal
movements].

Gondwana and Pangea, being huge lumpy land masses surrounded by a single
Megasea (I coined a new term), probably gave the ocean a lot of room to
slosh around unhindered.  In contrast, some of the late Mesozoic land
masses, like N. and S. America and Africa (post break-up), were
dominantly N-S trending.

<pb>
--
"We recognize, however dimly, that greater efficiency, ease, and security
may come at a substantial price in freedom, that law and order can be a
doublethink version of oppression, that individual liberties surrendered,
for whatever good reason, are freedoms lost." - Walter Cronkite, preface
to the 1984 edition of George Orwell's _1984_.