Planet Selene (The Moon) - #6. on far, far distant shores

Planet Selene (The Moon) - #6. on far, far distant shores

#1. The size of the Moon relative to most natural satellites
and, more importantly, the Moon's size relative to our
planet Earth.

#2. The "barycenter", also called "center of gravity" (CG)
of the Earth-Moon system is about 3,000 miles out from
Earth's center toward the surface, or only about 1,000
miles below Earth's surface.

#3. All major planets "fall" toward the Sun. Satellites don't.
Our Moon always falls toward the Sun. The Moon never
goes all the way "around" the Earth.

#4. The size of the Moon relative to the other solid planets.
Selene "fits in" with the major planets very well!

#5. Natural satellites tend to orbit their primary planets
directly above each planet's equator. Major planets orbit
the Sun on or near the ecliptic. Selene's orbit around the
Sun (and the Earth) is off the ecliptic by only 5 degrees.
Selene is fully 18-23 degrees off the Earth's equatorial
plane.
____________________________________

_Asimov on Astronomy_
http://tinyurl.com/bxdgs

In the above reference you'll find one of several hundred
books by Isaac Asimov. This particular book contains his
essay titled "Just Mooning Around". This is where you will
find several of these reasons explained by him. And it's also
where you can find more detail on the following reason...

#6. The Moon's orbit is far too distant from Earth to be a
satellite.

(Copyright law allows us to reprint brief excerpts of works
without infringing upon the rights of the author. This post is
designed more to "wet your whistle" and get you even more
interested and in love with astronomy. So let me suggest
that you read "Just Mooning Around" so you can get all the
Asimovian details.)

In this essay, Asimov came up with a fascinating idea. He
reasoned that since the Sun is much more massive than the
rest of our Solar System (everything... all the planets, and
satellites, planetoids, comets, meteoric matter, all of it put
together is only 1/750th the mass of the Sun), then the Sun
must be in a "tug of war" with each planet that has satellites.
The Sun must pull considerably on those moons, but just
*how much* is "considerably"?

To find out, Asimov came up with what he called the "Tug-
of-War value". He used the mass of the Sun and the mass of
each planet calculated with the distance of each satellite from
its planet, and from the Sun. It is obvious that the Sun would
win the tug of war when only mass is figured, but since the
satellites are so much closer to their primary planets than they
are to the Sun, we can expect the Sun to ultimately lose out in
the tug of war. So... let's check it out.

We'll look at satellites of Neptune first, Triton and Nereid.
Here is a table of Tug-of-War values...

NEPTUNE
SATELLITE TUG-OF-WAR VALUE
------------- --------------------------
Triton 8,400
Nereid 34

So Neptune's hold on Triton is very strong, but it pulls on
Nereid only 34 times as strongly as the Sun does. Keep in
mind that these are average figures. Nereid's orbit about
Neptune is highly eccentric. It comes as close as 800,000
miles at one end of its orbit, but at the other end it gets as
far away from Neptune as 6 million miles! So when Nereid
is that far away from its primary, the Tug-of-War value is
only 11.

Astronomers feel that Nereid may only be a temporary
moon of Neptune. Perhaps the Sun's influence might help
snatch it away one day.

Asimov figured the Tug-of-War value for several other
satellites, and i've listed a few here...

URANUS
SATELLITE TUG-OF-WAR VALUE
------------- --------------------------
Miranda 24,600
Ariel 9,850
Umbriel 4,750
Titania 1,750
Oberon 1,050

We can see that all of Uranus' moons are securely tied to
their orbits.

SATURN
SATELLITE TUG-OF-WAR VALUE
------------- --------------------------
Janus 23,000
Rhea 2,000
Titan 380
Iapetus 45
Phoebe 3.5

As massive as Saturn is, and as far away from the Sun as
Phoebe is, we can see that the Sun still doesn't lose this tug
of war by much. Saturn's pull on Phoebe is only 3½ times
stronger than the Sun's.

JUPITER
SATELLITE TUG-OF-WAR VALUE
------------- --------------------------
Amaltheia 18,200
Europa 1,260
Callisto 160
X 4.3
XII 1.3
VIII 1.03

Jupiter's grip on its outer satellites is feeble indeed.

MARS
SATELLITE TUG-OF-WAR VALUE
------------- --------------------------
Phobos 195
Deimos 32

Even Mars, much smaller than Earth, holds on to its
moons and wins the tug-of-war.

From here, Asimov talks about two types of satellites...
"true" satellites and "captured" satellites. The true ones
are believed to have formed along with their primary
planets way back when the Solar System was very
young. And the captured ones were, well, captured.
At some point in the history of our Solar System, they
were passing near a planet and were caught in the huge
gravitational field. And they became part of the planet's
satellite system.

Up to now, i've left out the Tug-of-War value where our
own Moon is concerned. You'll see why in a few
moments. And now i'd like to show you an excerpt from
Asimov's "Just Mooning Around"...

Among the true satellites the lowest Tug-of-War value is
that of Deimos, 32. On the other hand, among the
satellites listed as captured, the highest Tug-of-War value
is that of Nereid with an average of 34.

Let us accept this state of affairs and assume that the
Tug-of-War figure 30 is a reasonable minimum for a true
satellite and that any satellite with a lower figure is, in all
likelihood, a captured and probably temporary member
of the planet's family.

Knowing the mass of a planet and its distance from the
Sun, we can calculate the distance from the planet's
center at which this Tug-of-War value will be found. That
will be the maximum distance at which we can expect to
find a true satellite.

(Note that Asimov left Pluto out of all this, because little
detail was known about Pluto at the time.)

We can also set a minimum distance at which we can
expect a true satellite; or, at least, a true satellite in the
usual form. It has been calculated that if a true satellite is
closer to its primary than a certain distance, tidal forces
will break it up into fragments. Conversely, if fragments
already exist at such a distance, they will not coalesce
into a single body. This limit of distance is called the
"Roche limit" and is named for the astronomer E. Roche,
who worked it out in 1849. The Roche limit is a distance
from a planetary center equal to 2.44 times the planet's
radius.

So, sparing you the actual calculations, here are the results
for the four outer planets:

DISTANCE OF TRUE SATELLITE
(MILES FROM THE CENTER OF THE PRIMARY)

MAXIMUM MINIMUM
PLANET (TUG-OF-WAR=30) (ROCHE LIMIT)
---------- ---------------------- ------------------
Neptune 3,700,000 38,000
Uranus 2,200,000 39,000
Saturn 2,700,000 87,000
Jupiter 2,700,000 106,000

As you see, each of these outer planets, with huge masses
and far distant from the competing Sun, has ample room for
large and complicated satellite systems within these generous
limits, and the 22 true satellites all fall within them.

Next we can try to do the same thing for the inner planets.
Since the inner planets are, one and all, much less massive
than the outer ones and much closer to the competing Sun,
we might guess that the range of distances open to true
satellite formation would be more limited, and we would be
right. Here are the actual figures as I have calculated them:

DISTANCE OF TRUE SATELLITE
(MILES FROM THE CENTER OF THE PRIMARY)

MAXIMUM MINIMUM
PLANET (TUG-OF-WAR=30) (ROCHE LIMIT)
---------- ---------------------- ------------------
Mars 15,000 5,150
Earth 29,000 9,600
Venus 19,000 9,200
Mercury 1,300 3,800

Thus, you see, where each of the outer planets has a range of
two million miles or more within which true satellites could form,
the situation is far more restricted for the inner planets. Mars
and Venus have a permissible range of but 10,000 miles. Earth
does a little better, with 20,000 miles.

Mercury is the most interesting case. The maximum distance at
which it can expect to form a natural satellite against the
overwhelming competition of the nearby Sun is well within the
Roche limit. It follows from that, if my reasoning is correct, that
Mercury CANNOT have a true satellite, and that anything more
than a possible spattering of gravel is not to be expected.

Venus, Earth, and Mars are better off than Mercury and do have
a little room for true satellites beyond the Roche limit. It is not
much room, however, and the chances of gathering enough
material over so small a volume of space to make anything but a
very tiny satellite is minute.

And, as it happens, neither Venus nor Earth has any satellite at
all (barring possible minute chunks of gravel) within the indicated
limits, and Mars has two small satellites, each less than 20 miles
across, which scarcely deserve the name.

It is amazing, and very gratifying to me, to note how all this
makes such delightful sense, and how well I can reason out the
details of the satellite systems of the various planets. It is such a
shame that one small thing remains unaccounted for; one trifling
thing I have ignored so far, but--

WHAT IN BLAZES IS OUR OWN MOON DOING WAY
OUT THERE?

(Note... Selene's mean distance from Earth is 239,000 miles. This
is fully 210,000 miles more distant than Asimov's maximum.)

It's too far out to be a true satellite of the Earth, if we go by my
beautiful chain of reasoning--which is too beautiful for me to
abandon. It's too big to have been captured by the Earth. The
chances of such a capture having been effected and the Moon
then having taken up a nearly circular orbit about the Earth are
too small to make such an eventuality credible.

There are theories, of course, to the effect that the Moon was
once much closer to the Earth (within my permitted limits for a
true satellite) and then gradually moved away as a result of tidal
action. Well, I have an objection to that. If the Moon were a
true satellite that originally had circled Earth at a distance of,
say, 20,000 miles, it would almost certainly be orbiting in the
plane of Earth's equator and it isn't.

But, then, if the Moon is neither a true satellite of the Earth nor
a captured one, what is it? This may surprise you, but I have
an answer; and to explain what that answer is, let's get back to
my Tug-of-War determinations. There is, after all, one satellite
for which I have not calculated it, and that is our Moon. We'll
do that now:

SATELLITE TUG-OF-WAR VALUE
------------- --------------------------
Moon 0.46

The Moon, in other words, is unique among the satellites of
the Solar System in that its primary (us) LOSES the tug of war
with the Sun. The Sun attracts the Moon twice as strongly as
the Earth does.

We might look upon the Moon, then, as neither a true satellite
of the Earth nor a captured one, but as a planet in its own right,
moving about the Sun in careful step with the Earth. To be sure,
from within the Earth-Moon system, the simplest way of
picturing the situation is to have the Moon revolve about the
Earth; but if you were to draw a picture of the orbits of the
Earth and Moon about the Sun exactly to scale, you would see
that the Moon's orbit is everywhere concave toward the Sun.
It is always "falling" toward the Sun. All the other satellites,
without exception, "fall" away from the Sun through part of their
orbits, caught as they are by the superior pull of their primary--
but not the Moon.

Nope... not our Moon, not Selene, Earth's sister planet?

--
Indelibly yours,
Paine http://www.savethechildren.org/
http://www.painellsworth.net