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 levels.
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.
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