Howard Wu wrote:
> As I recall from the discovery channel the moon had orbited alot
> closer to the earth and has been drifting further away overtime.
Elton responded:
> That is true Howard, in fact as you mathematically work backwards
> from where the moon is today-- you'll see that the moon gets closer
> and closer to the Earth.
A Once-Closer Relative
One factor common to all theories of lunar origin is that the Moon isn't where it used
to be. As shown below, the tidal bulges raised in our oceans do not lie directly on the
Earth-Moon line but actually precede it - a misalignment that affects both worlds. In
effect,
our planet rotates quickly enough to drag its tidal bulges along for some distance
before
they can respond to the Moon's changing location in the sky. Consequently, the Earth is
decelerated slightly (today will be about 0.3 billionth of a second longer than
yesterday).
The Moon is dragged forward, speeds up, and moves roughly 3 cm per year outward
from its present mean distance of 384,390 km.
If the Moon's orbital expansion is worked backward in time, our satellite should have
resided inside Earth's Roche limit (roughly 12,000 km distant for the Moon) between
1 and 3 billion years ago. There the Earth's gravity would have ripped it to pieces
- which obviously didn't happen. No evidence for such proximity exists on Earth either,
so the rate of separation must have been lower in ages past.
Most of the tidal-energy exchange occurs in shallow seas, which have changed in their
extent and location over geologic time. The Moon's rate of recession should have varied
accordingly. But since we can't calculate Earth's past land-sea boundaries exactly,
there
is no way to know precisely how far away the Moon was when it formed. However, K.S.
Hansen of the University of Chicago has performed calculations showing that the Moon
probably never came any closer than about 240,000 to 340,000 km, far outside the Roche
limit. Furthermore, in Hansen's models the Moon's orbit would always have been inclined
from 3� to 22� (but never 0�) to the Earth's equator.
Theories suggesting that the Moon fissioned from the Earth or was captured by it also
require our satellite to have been quite nearby soon after its formation. However,
Hansen's
results argue against both scenarios, and therefore he concludes that a double-planet
origin
seems the most plausible. (J. KELLY BEATTY, Sky and Telescope).
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