Posted to the UseNet Newsgroup "alt.astronomy"


We've already talked about size... and we've seen that the
planet Selene's size relative to the Earth as compared with
the satellites of our Solar System relative to their planets
is very large. This 4th reason also concerns Selene's size,
and this time, relative to all the "rocky" planets and Pluto.
So here's a table (remember, when we refer to "Selene",
we're talking about Earth's "sister planet", the Moon)...

    Planet        Diam (miles)     Diam (Earth=1)
    --------         -----------        --------------
    Earth               7913                 1.00
    Venus              7520                 0.95
    Mars                4223                 0.53
    Mercury           3029                 0.38
    Selene             2160                 0.27
    Pluto                1423                 0.18

(I included Pluto to show that, while it is quite a bit
smaller than Selene, Pluto was still classified as a major
planet in its own right since a time that was long before
it was discovered in 1930! And astronomers knew Pluto
was smaller than the Moon for a good, long time before
Pluto was finally reclassified as a minor, or "dwarf",
planet in 2006!)

So we see that, even though the smaller Pluto is more of
an icy body and possibly a potential short-term comet,
and even though Pluto has been reclassified as a dwarf
planet, which actually makes Selene, the Moon, the
smallest major planet in the Solar System, Selene still
appears to fit in fairly well with the rock planets. The
Moon is nearly 3/4 the size of planet Mercury, and has
a diameter that is 1.5 times that of Pluto!

This reason doesn't stand alone, of course. It cannot be
used by itself as the lone reason to see our Moon as a
full-fledged major planet. This can be seen more clearly
when you look at the next table...

    Body             Diam (miles)        Diam (Moon=1)
    --------            -----------            --------------
    Ganymede           3240                     1.50
    Titan                    3200                     1.48
    Callisto                3040                     1.41
    Io                        2261                     1.05
    Selene                 2160                     1.00

Planet Saturn's big moon, Titan, and three Galilean moons
of Jupiter--Ganymede, Callisto and Io--are all larger than
our Moon. And we might also remember that Ganymede,
Titan and Callisto are even larger than planet Mercury!
(Again, Mercury's diameter is 3029 miles.)

So even though this reason, Selene's size compared with
other solid planets, will not stand alone to get acceptance
that our Moon is a major planet, both Isaac Asimov and
i believe that it still stands up well in backup support of
the other reasons listed and described in this series.

The next reason, the 5th, stands alone much better. And
when it is put together with the other reasons, it really
strengthens the whole idea that Selene, the Moon, is a
full-fledged major planet in its own right.

You'll remember that the flat plane made by Earth's orbit
around the Sun has a name. It's called the "ecliptic". And
all of the other planets in our Solar System also go around
the Sun on this ecliptic plane (or very near it). If our Solar
System were a whole lot smaller, it would be flat enough
to fit inside a pizza box!

Another distinction between a planet and a satellite would
have to include the plane of the orbit. Does the object go
around the equator of the planet? Or does the object go
around on or near the ecliptic plane? The plane in which
our beautiful and awesome sister planet, Selene, orbits
around (uhm, well, almost around) the Earth, and for that
matter around the Sun as well, will be the subject of my
next article.

happy days and...
    starry starry nights!

Indelibly yours,

 P.S. Thank YOU for reading!

    P.P.S. Some secret sites (shh)...