Posted to the UseNet Newsgroup "alt.astronomy"
Where is your personal CG? What is a "Center of Gravity",
What we're talking about here is "balance". All physical
systems, whether it be a single pencil, a planet, a satellite
and its planet, or a binary planet system, all are balanced
systems. And each system has a center of gravity, a CG.
If you were to tie a rope around yourself, and then suspend
yourself in the air horizontally, parallel to the ground, you
will be balanced perfectly when the rope is positioned
exactly around your body's CG. You can try this with a
pencil, a screwdriver, or any object. Tie a string to the
object, and if the suspended object dips down on one end,
then the string should be moved slightly toward that end.
With a little patience, you can get the balance just right,
and then the string will be wrapped aroung the center of
gravity of the object.
When a physical system happens to be a star/planet, or
anything that's astronomy-related, scientists refer to the
center of gravity as the "barycenter".
Close your eyes for a moment and picture the Earth and
Moon. The Moon is moving and the Earth is moving, so in
your imagination, stop these movements. Freeze the Moon
and the Earth in time. Mentally draw a straight line that
goes through the centers of both Earth and Moon. The
barycenter of the Earth/Moon system will be on this line just
inside the surface of the Earth. The barycenter will be much
closer to the surface of Earth than it is to the center of Earth.
Of course, in your mind when you allow the action to start
up again, you will see that the barycenter of the Earth/Moon
system is constantly changing position. As the Earth rotates
(spins) on its axis, and as the Moon moves slowly around the
Earth in its monthly cycle, the barycenter moves, too. And
yet, the barycenter is always about a thousand miles below
the surface of the Earth.
Now, take the planets as they circle the Sun as an example.
They don't actually go around the Sun in a perfect circle,
you know. Each planet's orbit is actually an ellipse, which
is a circle that's been sttrrrretched just a bit, an "elongated"
circle. And an ellipse always has two "focal points". Now,
when a smaller object orbits a larger object, the barycenter
of the system will be one of the two focal points formed by
the smaller object's elliptical orbit around the larger object.
And it's the focal point that is nearest to the center of the
Here is a table of barycenters for each planet as it orbits
around the Sun. The "DISTANCE" is the number of miles
from the center of the Sun that the barycenter is located...
It may be interesting to note that, since the Sun's radius is
about 432,500 miles, The Sun/Jupiter barycenter is the only
one that lies *outside* the surface of the Sun. All the others
are inside the Sun. And some of them are very close to the
You might also note that the combination of all the planets'
different barycenters gives our Sun a bit of a "wobble". It
is this type of wobble in other stars that helps astronomers
find "exoplanets", planets that orbit around other stars.
Some night, when the Moon is pretty much right overhead,
look down at the ground below you. Try to imagine being
able to see through to the opposite surface of the Earth. If
you could do this, you would find the opposite surface to be
about 8,000 miles beneath you. The center of Earth is right
around 4,000 miles directly beneath your feet, and the CG,
the barycenter, of the Earth/Moon system is approximately
1,000 miles beneath the ground you're standing on.
Just a thousand or so miles! Every satellite of every major
planet in our Solar System has a barycenter that is either at
its planet's center or just a very short distance from the
planet's center. Our Earth/Moon barycenter is fully 3,000
miles out from Earth's center, about 3/4 of the way from
the center of Earth to the surface beneath your feet!
(NOTE: This drawing is not to scale. The Moon is much
larger and much, much farther away than shown.)
"b" is for barycenter...
, --- ,
( b ) O
`, ,` Moon
You could argue that, in a double-planet system, the
barycenter of the two planets ought to be *outside* the
surfaces of both planets. And once again, you might be
right. However, if you really think that's important, you
might also consider that Jupiter could easily be thought
of as a small star simply because the barycenter of the
Sun/Jupiter system lies outside the surface of the Sun!
But there are other tests that Jupiter would have to
pass to be thought of as a star. So the location of the
barycenter probably isn't all-important, is it? For now,
let's just remember that the location of the Earth/Moon
center of gravity, the barycenter, is very different from
the barycenters of all the satellites of major planets in
our Solar System.
The Earth/Moon barycenter lies very close to the surface
of Earth. Its distance from Earth's surface is only about
1/8th of Earth's diameter. All the satellites in our Solar
System have planet/satellite barycenters that are at or
near the very centers of the planets.
There are still other reasons to consider, too, if we are
to think of Earth's Moon as a "sister planet" to Earth.
For instance, whether the Moon goes fully around planet
Earth, or if instead it does not fully circle the planet.
What if the Moon revolves around the Sun exactly as if it
were a major planet in its own right? It does!
All the satellites in our Solar System revolve fully around
their planets. But one little-known Moon fact is that our
Moon does *not* go fully, totally, and completely around
our planet! I'll touch on this in more detail in the next
Oh! Some of you maybe didn't "get" the title of this here
article. I titled it to honor a fella who reads alt.astronomy,
who used to post here a lot, but who at present only posts
occasionally. His name is Barry, and when it comes to the
subject of astronomy, he is all business and extremely
knowledgable. So Barry, if you happen to read this, then
please know that you are missed here.
A long time ago, when i first started posting these Asimov
ideas about the Moon, Barry was able to diminish one of
(what i thought was) the strongest arguments that our
Moon is a full-fledged major planet. I'll talk about this a
little more in the next post, too.
happy days and...
starry starry nights!
P.S. Thank YOU for reading!
P.P.S. Some secret sites (shh)...