September 2005 Sky from the Keeble Observatory
Mars Reconnaissance Explorer was launched on August 12th. Scheduled
to arrive at Mars in about 6 months, it will eventually provide
the highest resolution images from orbit yet achieved, and will
also carry ground-penetrating radar to study the underlying structure
of the planet. First it will undergo a series of passes through
Mars’ outer atmosphere to slow it into a circular orbit. Watch
for more details in the spring.
Another bit of astronomy news was the announced discovery of a
“tenth planet.” Dr. Mike Brown (California Institute
of Technology) announced his discovery on July 29th, saying that
it is definitely bigger than Pluto. He was using an automated telescope
at Palomar Observatory, the 48” Samuel Oschin Telescope. It’s
in a highly elliptical orbit, ranging from nearly 100 AU (1 Astronomical
Unit is the average distance from Earth to Sun) to less than 40
AU. Its orbital plane is also highly inclined, about 45 degrees
to the plane of the ecliptic (where the rest of the planets’
orbits lie). The discovery images were obtained on 2003 October
21. Because of the sheer volume of survey images to be analyzed,
it took until early 2005 to actually identify this new object. It
currently has a catalog designation of 2003 UB313 – a name
has been submitted to the International Astronomical Union, but
will not be announced until a decision is formalized.
How do we discover such an object? The automated survey telescope
captured three images of the same region of the sky at 90 minute
intervals. Stars are so distant that they do not shift in the field
of view over that time, but a relatively nearby object (like a planet
or asteroid) would change its position relative to the stars. See
below, and note that the circled image of the planet has moved slightly
from frame to frame.
(Image credit: Samuel Oschin Telescope, Palomar Observatory)
Once identified, the discoverers were able to search archive data
and find previous images which allowed them to calculate its orbit.
A bit of possible controversy still applies here. Some have argued
that Pluto should not be considered a planet, rather just a large
representative of what are called Kuiper Belt Objects. If this object
is declared by the IAU not to be a planet, then Pluto could also
be formally demoted. That would make the discovery of a tenth planet
grounds to reduce the acknowledged number of planets from nine to
Lunar phases for September: New Moon at 2:45 pm, on the 3rd; First
Quarter on the 11th, at 7:37 am; Full Moon on the 17th, at 10:01
pm; and Last Quarter on the 25th at 2:41 am.
Venus and Jupiter are both bright “stars” in the evening
twilight as the month begins, to the west-southwest, about 15 degrees
off the horizon. Venus is the brighter of the two, just to the left
of Jupiter a little lower to the west. They’ll start the month
about a degree apart (twice the width of a Full Moon), with Venus
drifting further to the southwest as the month progresses and Jupiter
moving closer to alignment with the Sun. Mercury will reappear in
the evening twilight towards the end of the month, but will be very
low on the horizon.
Predawn planet watchers will find Mars high above the southwest
horizon at sunrise, about 60 degrees from the horizon at the beginning
of the month and only 40 degrees by the end. Saturn rises about
two hours before the Sun at the beginning of the month. By the end
of September, you’ll see it about 60 degrees above the horizon
as the Sun comes up, nearly four hours ahead of the Sun’s
position on the sky. Mercury starts the month in the predawn sky,
rising about an hour before sunup, but it will disappear into the
Sun’s glare by midmonth, to reappear in the evening twilight.
On the 17th it will be at “superior conjunction,” roughly
in the same direction as the Sun but not directly behind it as seen
Two hours after sunset, we look directly overhead to find Cygnus
just to the east of zenith. Deneb lies just to the northeast at
the “tail of the swan;” Albireo marks the head to the
southwest. This constellation also marks the plane of our home Milky
Way Galaxy. About 15 degrees from zenith, towards the west we find
the bright star Vega in Lyra. Binoculars will allow you to resolve
the “double-double” near Vega, a system of four stars
in two closely spaced binaries. On a clear, moonless night you may
also find the faint smoke ring of the Ring Nebula. Below Vega lies
the smallish irregular “square” of Hercules. Your binoculars
should pick up the globular cluster M13, near the lower right hand
corner. Below Hercules we find the bright star Arcturus.
From zenith to the east we encounter a largely empty patch of sky,
marked primarily by the relatively faint stars of the Great Square
of Pegasus. Of more interest is what we can see to the northeast.
Locate the irregular W shape of Cassiopeia – follow the line
of the left side of the W back towards Pegasus (to the right). On
a clear night you should be able to discern the light smudge of
the Andromeda Galaxy, even without binoculars. This system of perhaps
100 billion stars is about 2 million light years away, making it
the most distant object discernable without a telescope.
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.