October 2006 Sky from the Keeble Observatory
From the announcement of its discovery in 1930, it has been clear
that Pluto is not of the same cloth as the rest of the planets.
Compared to the inner, rocky planets, Pluto is a puny ice-ball.
Compared to the outer gas giants … it’s still a puny
ice-ball, more like an escaped moon than a planet in its own right.
And, unlike the rest of the planets, its elliptical orbit is so
eccentric that it slips in closer to the Sun than Neptune for 20
years out of its 248 year orbit. That orbit is inclined almost 18
degrees to the plane of the solar system’s other planetary
orbits. And, it’s not even alone in most or all of its unusual
parameters. Eris (what was informally called Xena until last month’s
IAU meeting) is a little larger, and has a larger, more elliptical
orbit even more inclined to the ecliptic. Sedna and Quauar have
similar orbital periods. These are denizens of the Kuiper Belt –
a reservoir of planetesimals and comets, leftovers from the formation
of the solar system 4.5 billion years ago.
So, of course, the IAU convention voted to demote Pluto to “dwarf
planet” status, and included Eris and Ceres (the largest asteroid
between the orbits of Mars and Jupiter) and Sedna, etc. in that
category. What could be simpler or more logical? Of course, all
astronomers in the world can agree.
But, of course, they do not. A proposal was carried to the IAU
in Prague, from a committee which had been asked to look at the
question of actually defining what is meant when calling an object
a planet. They recommended a set of physical criteria to be applied:
An object must be massive enough for gravity to crush it into a
sphere. (This eliminates virtually all the asteroids and comet nuclei,
because they are small enough to be irregular in shape). That object
must be orbiting a star. (This eliminates free-floating planet-sized
object and also the satellites of the known planets.) By extension,
a planet so defined could not be massive enough for fusion to take
place at the core, eliminating “brown dwarfs” and other
failed stars. The definition would have increased the number of
planets officially in the solar system by including Ceres, Eris,
Sedna, Quauar, Pluto, and even Charon. Charon is Pluto’s largest
moon, but since it is so close to Pluto in mass, their common center
of mass is not inside either body, so neither can be said to orbit
This proposal was roundly defeated in favor of a set of criteria
which creates the new “dwarf planet” category, but which
does not otherwise appear to me as well thought through. Planets
are supposed to orbit the Sun, with no criteria for classifying
the 150+ planets known to orbit other stars. Planets would have
to “dominate” their orbital vicinity. This is particularly
curious, as even Earth clearly doesn’t dominate its orbit
… you may have noticed a little object called the Moon, nearly
¼ the size of Earth, and only 30 times Earth’s diameter
I suspect the debate is not over – there’s a petition
circulating among some astronomers, refusing to acknowledge or apply
the IAU criteria. In any case, Pluto cares not a whit what the IAU
Lunar phases for September: Full Moon at 11:13
pm, on the 6th; Last Quarter on the 13th, at 8:26 pm; New Moon on
the 22nd, at 1:14 am, and First Quarter on the 29th, at 4:25 pm.
This last time is Eastern Standard, the other three are Eastern
Daylight. Remember to turn your clocks back an hour overnight on
Saturday the 28th.
Saturn continues its climb into the predawn skies, rising 3 hours
before sunrise at the beginning of October, almost 4 hours by month’s
end. Look for it high and bright to the east. As planet displays
go … that’s about it for this month! Jupiter is low
to the southeast at sunset, probably too low to see through horizon
clutter and haze. Mars and Venus are both slipping behind the Sun
– Mars is at conjunction on the 23rd, Venus is at superior
conjunction on the 27th. (Mars can never be at inferior conjunction
– between Earth and Sun – while Venus can be either
at inferior or superior conjunction.)
Our overhead look at mid-month, about two hours after sunset, finds
the bright star Vega, which last month was near zenith at this time,
has now slipped about 30 degrees down the sky towards the west.
The Milky Way is now dividing the sky roughly NE to SW. Deneb in
the constellation Cygnus, is now just bpast zenith. About 60 degrees
above the southwest horizon, we find the bright Altair in Aquila.
If you have a clear view of the northern horizon, you should be
able to pick out several of the “circumpolar” constellations.
Because of our place in Ashland, some 37 ½ degrees north
of the equator, any star within that angle from the north celestial
pole will never be below the horizon. Low to the northwest you should
see the familiar “big dipper” of Ursa Major. Follow
the “pointer stars” at the end of the bowl to find Polaris,
the “pole star.” Contrary to popular misconception,
this is not a terribly bright star. Also, despite once being used
as a standard for brightness, Polaris is actually a variable star!
Polaris is actually about a degree away from the true north celestial
pole, and moving slowly away. The wobble of Earth’s axis will
bring Polaris back to the pole in another 26000 years. The “little
dipper,” more formally Ursa Minor, is a faint asterism extending
upwards from Polaris.
Above and to the right of Polaris lies Cepheus, which looks like
a child’s upside down line drawing of a house. A bit more
to the northeast lies the familiar sideways W shape of Cassiopeia.
Draco winds through the circumpolar constellations, roughly wrapping
Ursa Minor from Cepheus around towards the Big Dipper.
For your own monthly star chart, you can direct your web browser
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.