August 2004 Sky from the Keeble Observatory
Slipping just inside Saturn's weirdly braided F-ring, Cassini
successfully completed its orbital insertion maneuver on June 30th
and began a planned 4-year mission orbiting the second largest planet
in our solar system. The spacecraft is named for the 17th century
Italian-French astronomer who first observed an apparent gap in
Saturn's rings. We now understand that the gap, and others less
prominently seen from Earth, is not empty. Rather, it contains less
bright ice than the more prominent rings ... Cassini transited
the ring plane with its shielded antenna forward to protect the
probe from collisions with dust and small pebbles.
Early images of the rings are at much higher resolution that we
obtained in the 1970s from two Voyager flybys ... so spectacular
that some mission scientists thought that their colleagues were
fooling them with simulations rather than real data. (See for yourself
) Braiding, "scallops," and assorted "spiral density
waves" give the rings the appearance of an exotic CD (or phonograph
record, for those of my generation). These features represent the
consequences of gravity acting between the ring material and various
of Saturn's moons, some barely larger than the ring particles themselves.
The bright rings are a swarm of icy chunks, ranging in size from
snowflake to iceberg, and reflect sunlight efficiently. The darker
gaps, analyzed in infrared and ultraviolet light, now seem to consist
of smaller particles with more rocky dust. They've been described
as akin to frozen mud. Analysis also shows that the composition
of the rings is less ice and more mud as you get closer to the planet.
The puzzling exception is the "Cassini division" - the
apparent gap - which splits the bright icy rings with a band of
darker material. Good thing they didn't target the probe to pass
through that gap!
Next month we'll discuss Saturn's larger satellites, including
Titan. The only moon in the solar system with a substantial atmosphere,
Titan is one of the prime targets of the mission. It will receive
a Christmas present, when Cassini drops into its atmosphere a probe
named Huygens, after the early 17th century Dutch scientist who
first correctly identified the rings.
Lunar phases for August: Last Quarter on the 7th, at 6:01 pm EDT;
New Moon on the 15th, at 9:24 pm; First Quarter on the 23rd, at
6:12 am; Full Moon on the 29th, at 10:22 pm.
Evening planet watching will continue to disappoint! At sunset
early in August, you'll find Jupiter only 20 degrees above the western
horizon, setting about two hours later. Mars and Mercury are lower
still, approximately midway between the position of the Sun and
that of Jupiter. Unfortunately, summer in central Virginia usually
means haze and humidity, so you may not even see Jupiter. Mercury
will completely disappear into the Sun's glare, re-emerging into
the morning sky by the end of the month. Morning sky-watchers will
have a better array of planets. Saturn begins August about 17 degrees
above the ENE horizon as the Sun rises, but climbs to nearly 40
degrees by month's end, rising then more than three hours before
the Sun. Venus is already high in the predawn as the month begins,
about 35 degrees ahead of the Sun. By month's end, you'll find it
high and brilliant right next to Saturn.
At about 10:00 pm, at mid-month you will find the bright star Vega,
in the constellation Lyra - the lyre - at zenith. Binoculars should
enable you, on a clear moonless night, to pick out the faint Ring
Nebula. 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, directly overhead at this time last month, has shifted
about 30 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.
Cygnus, the Swan, is now to the east, about 56 degrees from the
horizon, with the long axis is parallel to the horizon, with bright
Deneb at the northern end of the cross (the tail of the swan). At
the other end is a modest star called Albireo. As mentioned last
month, binoculars or a small telescope will reveal this as a beautiful
binary, one star blue, the other orange. It's not an extremely bright
star, but it's nearly alone in that patch of sky.
Altair, to the southeast about 55 degrees above the horizon, marks
the downward apex of a triangle with Vega and Deneb. Low to the
southwest, that reddish star is Antares, in Scorpio. It's name means
"Mars' rival" - but with Mars lost in the evening twilight,
Antares has no competition to worry about.
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.