September 2004 Sky from the Keeble Observatory
Cassini has been orbiting Saturn for two months, and has already begun to return
impressive data on the ringed planet's many satellites. Phoebe, the most distant
of Saturn's "major" moons, was imaged while the probe was still inbound.
Long-range images of Titan have begun to penetrate its smoggy atmosphere. Two previously
unknown moons have been discovered, too small to be seen by earth-based telescopes.
Titan is the only satellite in the solar system with a substantial atmosphere. Two
Voyager probes in the 1980s managed no better than to confirm that the atmosphere
was dense and hazy ... nothing could be seen of the surface. The atmosphere is mostly
nitrogen, which is also true of Earth's atmosphere, but there's no free oxygen on
Titan. It is also extremely cold, fortunately so, since the moon is not massive
enough to hold a warm atmosphere. The haze in the atmosphere seems to be hydrocarbon
"smog" ... and radar reflected from the surface tells us that there may
be pools of liquid hydrocarbon on Titan. Think lakes of gasoline! Initial images
of Titan at wavelengths not absorbed by the smog have given a preliminary view of
the surface, which will be much improved when Cassini passes within a few thousand
miles of the surface in December. A separate probe, Huygens, will penetrate that
atmosphere and parachute to the surface. See for yourself at
The newly-discovered moons, as yet unnamed, are a mere 3 and 4 kilometers (2 and
2 ½ miles) in diameter, making them substantially smaller than any previously
identified satellites. The smallest previously known were about 20 kilometers in
diameter. The existence of these small satellites has been used to argue that there
are fewer small comets in the outer solar system than many had believed. The reasoning
is that over the 4.5 billion year age of the solar system, house-sized comets would
have blasted small satellites into rubble, which would have become part of the rings.
Since these moons are still there, perhaps there aren't as many small comets. Cassini
researchers will continue to look for evidence of more small moons, particularly
in the several gaps in the ring plane.
Lunar phases for September: Last Quarter on the 5th; New Moon on the 14th; First
Quarter on the 21st; Full Moon on the 28th.
Evening planet watching will continue to disappoint! Mars and Jupiter are essentially
lost in the Sun's glare, Uranus and Neptune can only be seen with a modest telescope.
Morning sky-watchers will have a better array of planets. Mercury, Venus, and Saturn
all grace the eastern sky at sunrise. Saturn rises about 3 hours before the Sun,
followed an hour later by Venus. These will both be very bright ... Venus will probably
be the brightest object in that direction. Mercury lies below Venus, and should
also be bright. Look for all three to gain height above the horizon as the month
Two hours after sunset finds Cygnus, the Swan, directly overhead. Its brightest
star, Deneb, is just to the north of zenith. Cygnus marks the northeast to southwest
plane of our Milky Way Galaxy, and also lies in the direction towards which our
entire solar system is moving as we orbit the distant galactic center.
To the west of zenith lies Vega, in Lyra. Binoculars should enable you, on a clear
moonless night, to pick out the faint Ring Nebula, also in Lyra. Also, seeing doesn't
have to be perfect for binoculars or a small telescope to resolve the "double-double"
- the modestly bright star next to Vega - as a pair of close binaries. Hercules,
lies about 30 further degrees to the west. You should still be able to see the globular
cluster M13 in Hercules - it will look like a faint dandelion puff, composed of
perhaps a million stars.
From zenith towards the east, your line of sight moves out of the plane of the galaxy.
The largely open sky is marked with the large but rather faint "great square"
of Pegasus. Low to the northeast you'll see the familiar, irregular W shape of Cassiopeia.
To the right, at about the same angle from the horizon, you should be able to see
the faint "smudge" of the Andromeda galaxy, even without a telescope.
It's best seen in a clear, moonless sky. Light from that galaxy has taken about
2 million years to reach your eyes, making it the most distant "naked eye"
For your own monthly star chart, you can direct your web browser to
http://www.skymaps.com. You will find extensive descriptions of what's worth
looking for, and you can download and print a single copy for your personal use.