December 2005 Sky from the Keeble Observatory
What were you doing a year ago? The answer is likely something to do with getting
ready for Christmas, Chanukah, or New Year’s Day. But, what if you were on Mars,
asking the same question? The Martian day (called a “sol”) is longer – 24 hours,
36 minutes – and so is the year. A Martian year is 686.67 days, or 677 sols.
A year ago, Mars time, we saw the landing on opposite hemispheres of two small robots
called Spirit and Opportunity. Spirit’s first Martian year passed on 20 November,
while Opportunity will celebrate on the 12th of December. These probes were designed
to operate for 90 sols – and they’re both still going strong! Each has rolled up
and down hills, into and out of craters, all the while taking pictures, testing
the chemistry of rock and soil, and even grinding holes into the rocks to test their
interiors. Opportunity got stuck in a sand dune for a few weeks, while Spirit climbed
several hundred feet to the top of a nearby hill and has now begun its descent to
explore the other side.
Electrical power provided by solar cells has varied with the changing distance to
the Sun as Mars follows its elliptical orbit, and by accumulated dust on the arrays.
But, what Mars gives Mars takes away. Passing “dust devils” – mini whirlwinds –
have also cleaned the solar arrays several times, restoring nearly optimum power
What have we learned? The most important findings have been to confirm that Mars
was once much warmer and wetter than we find it today. The rocks sampled by both
probes were formed in water – Opportunity has even found evidence for wave action
in the way the sediments are deposited, telling us that the water at that site was
at least several feet deep. Recall that we said last month that Earth was unique
in having liquid water. That’s true now, but Mars was clearly more like Earth in
the past – which gives some hope that, someday, we could find evidence for life
having evolves on the fourth planet from the Sun.
A computerized image of the Mars rover has been edited into a rover’s eye view of
the Martian surface. Courtesy of NASA, JPL.
Lunar phases for December: New Moon at 10:01 am, on the 1st; First Quarter on the
8th, at 4:36 am; Full Moon on the 15th, at 11:15 am; Last Quarter on the 23rd at
2:36 pm; and another New Moon at 10:12 pm on the 30th.
Mercury remains visible just before sunrise, about 12 degrees above the southeast
horizon, but it will settle lower (towards the Sun) as the month progresses. Jupiter
is much brighter, also to the southeast about 30 degrees above the horizon. It will
climb through the month to almost 40 degrees above the horizon. To the west, Saturn
is high and bright at about 50 degrees.
Sunset finds two bright planets emerging from the twilight. Mars is obvious and
bright red, beginning the month some 20 degrees from the eastern horizon and climbing
to almost 45 degrees by the end of December. It should remain visible into the early
morning, swinging high and to the south before setting towards the west at about
5:00 am. Venus is still spectacularly bright to the southwest. It will set a few
hours after sunset.
Andromeda (see finder chart in last month’s column) is almost directly overhead
two hours after sunset. The Milky Way divides the sky roughly from east to west,
bowing slightly towards the north. Just south of zenith, looking out of the plane
of the Galaxy, is a broad open patch of sky, marked by four widely spaced stars
forming the “great square” of the constellation Pegasus. To the north of zenith
is the familiar shape of Cassiopeia, which used to find Andromeda. To the west,
we can now see why Cygnus (the Swan) is also commonly known as the “northern cross.”
The bright star at the head of the cross (which is the tail of the swan!) is Deneb.
The foot of the cross is marked by Albireo (guess what? it’s the head of the swan!).
Toward the east we see rising the constellations which will mark prominently the
winter sky. As your eyes settle towards the horizon, you’ll first note the bright
star Aldebaran, marking the eye of the bull in the constellation Taurus. Just above
Aldebaran is the diffuse “Beehive Cluster” which looks spectacular in binoculars.
Below we see the rising shape of Orion, lying on its side from the more familiar
view of later in the evening. The bright red star at the “shoulder” is Betelgeuse,
the bright blue star at the foot is Rigel.
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.