Posted to the UseNet Newsgroup "alt.astronomy"


"Lawrence D'Oliveiro" wrote...
> Brad Guth wrote:
>> Our moon is a planet that just so happens to be captured
>> by Earth.
> In what sense is it "captured by" the Earth? The strongest
> gravitational attraction on it is from the Sun, not the Earth.

If i may field this question?...

You are probably aware of the widely followed mainstream
depiction of the Moon's origin, the "GIH"...

In contrast to this is an idea i call the "GCH", or the "gentle
capture hypothesis". Keep in mind that any theory of the
Moon's origin MUST explain the present-day observations of
astronomers, and anyone who doesn't like the GIH should
endeavor to design a replacement for it.

So, here is my GCH...

First of all, the main problem with previous "capture" ideas
is that the Moon is too large and massive to have been both
captured by the Earth AND have an almost circular "quasi-
orbit" around the Earth (the "quasi-orbit" is the result of the
"open" orbit of the Moon around the Earth caused by the fact
that you brought up -- at New Moon, the Sun's gravitational
force on the Moon is more than twice Earth's effect).

In accordance with the gentle capture hypothesis, both the
Earth and Moon of the early Solar system began absorbing/
accreting more matter in almost exactly the same orbit
around the Sun. Earth was both in front of the Moon and
just a tiny bit nearer to the Sun, so Earth not only orbited
the Sun just a tiny bit faster than the Moon, its position out
in front of the Moon meant that Earth got most of the stuff
out in front of it, while the Moon only got the "dregs".

At some point, hundreds of thousands of years later, Earth
moved far enough ahead of the Moon so that the Moon was
able to accrete more material, but the Earth had already
"scooped up" most of the iron and heavier materials, so
the Moon got very little of these.

Then, perhaps millions of years later, Earth finally "caught
up" to the Moon by virtue of Earth's slightly faster orbital
speed around the Sun. By this time, both Earth and Moon
were fairly fully formed as we see them today. At some
point when the Earth and Moon came together in close
enough proximity, they "gently captured" each other.

Since the wide arc of the Moon's Solar orbit, as well as
the increasing gravitational effect of the Earth, placed the
Moon in a position to move closer to the Sun, the Moon
began to do so and began to orbit the Sun in a "scalloped-
shaped" fashion. Soon the Earth caught up to the Moon,
but by that time the Moon was between the Earth and Sun
(New Moon), and perhaps a distance from Earth that was
a little more than Earth's Roche limit (much closer to Earth
than it is today).

As the Earth slowly passed the Moon, the Moon moved to
a position directly behind the Earth (First-Quarter Moon).
Then soon, the Moon moved to a position in league with
Earth, and farther from the Sun (full moon).

The Earth and Moon then continued a well-synchronized
and dual orbit around the Sun, just as we see them do
today. Both Earth and Moon orbit the Sun in scalloped
patterns; the Moon's scalloped pattern is more distinct
than Earth's due to Earth's much larger mass.

happy days and...
    starry starry nights!

Indelibly yours,

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