Posted to the UseNet Newsgroup "alt.astronomy"
Thousands of years ago in ancient Greece, the Moon was
called "Selene". And Selene was considered to be one of the
seven wanderers in the sky (in ancient Greece, "wanderer"
was pronounced "planet"). The wanderers were lights in the
sky that moved or "wandered" around against the backdrop
of fixed stars. We now know these seven wanderers/planets
by the following names...
So for thousands of years, even up into modern times, the
Moon was believed to be, not a satellite of Earth, but a full-
fledged planet. It was only when Galileo first saw the four
large moons of Jupiter through his nifty, new telescope that
our Moon's status as a planet began to come into question.
Since other planets had moons going around them, then the
Moon must also just be a satellite going around us. So
Selene/Luna was then seen as no longer a planet, but just a
natural satellite of Earth. And compared with the length of
time that the Moon was called a planet, it was not all that
While he was still alive, Isaac Asimov thought that it was high
time to restore the Moon's planet classification. And i agree
that Asimov was right! So i am dedicating these articles to
one of the greatest writers who ever lived, prolific author of
both fiction *and* non-fiction science books and essays, and
much, much more...
I S A A C A S I M O V
Please do keep in mind that there are several reasons for
reclassifying the Moon as a major planet, and that many of
these reasons will overlap and be intimately connected with
each other. While some of the reasons by themselves on their
own might seem weak (or strong), please wait until you've
seen all the reasons before you decide. The idea of our Moon
actually being a major planet was once proposed by Isaac
Asimov, and he makes some very strong arguments, as you
will see when all these reasons are added together...
1st Reason: The sizes of satellites relative to their primary
planets are normally very, very small. The size (specially the
diameter) of the Moon as compared with most other satellites,
and more particularly the Moon's size relative to the size of
Earth, is quite a bit larger than you would expect just a mere
satellite to be.
The Moon is huge. The only satellites bigger than the Moon
are Jupiter's Ganymede, Callisto, and Io, and Saturn's biggest
satellite, Titan. Even these four great satellites, however, are
very, very tiny when compared to the planets they orbit.
Our Moon's diameter is 3,475 kilometers (2,160 miles). That's
a wee bit bigger than 1/4 the diameter of planet Earth. If
Ganymede, Jupiter's largest-diameter satellite, wanted to
compare in size to Jupiter as our Moon compares to Earth,
Ganymede would have to grow to 7 1/2 times its present size!
You could argue that the Moon is only 1/4 the diameter of
Earth, has only 3/5 of Earth's density, only 1/50 of Earth's
volume, is only 1/80 the mass of the Earth, sports only 1/6 of
Earth's gravity, and has an escape velocity about 1/5 of Earth's.
One might think that the two members of a double-planet
system should be more like each other. And you just might be
Jeff Root's Earth/Moon Comparison Page
And yet size, both compared with Earth and with most of the
other companions of the planets, size isn't the *only* reason
for reclassifying the Moon to "major planet" status.
Another reason is because of where the "barycenter" is found.
A barycenter is a term used by astronomers to describe the
"center of gravity" of a system. And we'll touch on that next.
happy days and...
starry starry nights!
P.S. Thank YOU for reading!
P.P.S. Some secret sites (shh)...