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 eight!
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 from Earth.
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 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.