April 2006 Sky from the Keeble Observatory
Mars Reconnaissance Orbiter successfully made Mars orbit on the
10th of March. It will take about a year to circularize the orbit
and begin detailed observations. These maneuvers will be accomplished
by “aerobraking” – the probe will dip into the
outer atmosphere to successively lower the high point of its currently
elliptical orbit. It’s a risky trick, but one that has the
advantage of minimizing the amount of reaction fuel needed. Every
pound of fuel is a pound of science instruments that can’t
It’s been a good month for studying origins. Star Dust, the
NASA comet probe, has returned samples of dust from the tail of
Comet Wild 2. The “standard model” tells us that this
material has been essentially in the deep freezer of space since
the formation of the solar system, and should represent mostly pristine
and unprocessed leftovers from the formation of the sun and planets.
Comets are thought to have originated near the outer fringes of
the solar system. Accordingly, it was a great surprise to find that
some of the dust grains are formed of minerals that we usually think
of as forming in a high temperature environment. Olivine, for example,
is very common on Earth. It forms in volcanic vents, and is a component
of the familiar dark sands of Hawaii. The olivine grains found by
Star Dust are richer in magnesium than terrestrial samples, but
it’s still surprising to find them in a cold comet at all!
Teasing out the answer to the puzzle will help us refine our models
of the early solar system. One model suggests that high speed jets
of material flowing away from the plane of the early solar system
may have mixed materials formed in higher temperatures near the
Sun into the outer parts of the proto-solar nebula. Alternatively,
these grains may represent interstellar material brought from another
star system and incorporated into the solar system.
The other origin story takes us back even further – to the
beginning of the Universe itself. We find that the universe is filled
with residual electromagnetic radiation from its hot origins, an
“echo of creation.” This radiation originated when the
universe was perhaps 100,000 years old. Three years of continuous
observation and analysis of this cosmic microwave background have
given an unprecedented look into conditions when the universe was
less than a second old!. The Wilkinson Microwave Anisotropy Probe
(WMAP) had previously provided the best estimate for the age of
the universe, some 13.7 billion years. Additional observations have
now allowed more detailed analysis which has allowed a test of alternative
models for those beginnings.
The standard model includes a short but dramatic phase called “inflation”
when the universe was a tiny fraction of a second old, during which
its size expanded exponentially by a factor of 10 followed by 50
zeroes. One model for inflation says that the tiny fluctuations
measured by WMAP should have about the same amplitude, irrespective
of the angular size of those fluctuations. Another model says that
the smaller size features should be less bright – and this
model is the simpler of the two. WMAP’s results confirm the
latter prediction, seemingly ruling out the more complicated version
of inflation. Yes, it’s been a good season for origins.
Lunar phases for April: We switched to Daylight Saving Time on
the 2nd, so all times reported here are EDT. First Quarter on the
5th, at 8:01 am; Full Moon at 12:40 pm, on the 13th; Last Quarter
on the 20th, at 11:28 pm; New Moon on the 27th, at 3:44 pm.
Evening planet watchers can see Mars emerge from the evening twilight
high to the West, almost 70 degrees off the horizon. Saturn is also
high, about 65 degrees above the southeast horizon. As the month
advances, Saturn will appear closer and closer to Mars, and Mars
will settle closer to the horizon. By month’s end, look for
Mars to emerge only 53 degrees above due west, with Saturn now to
the southwest and still quite high. Jupiter will end the month nearly
at opposition, rising at sunset and setting at dawn.
In the predawn sky, you may see Mercury low to the east southeast,
but it’s only about 10 degrees above the true horizon. Trees
and buildings may block your view. Venus is a little higher, but
is still only about 20 degrees above the southeast. By the end of
the month, Venus will not change its height above the horizon, but
will drift towards the north, finishing April east southeast at
sunrise. Mercury will reach its greatest elongation on the 8th,
and will drift closer to the Sun for the rest of the month.
Two hours after sunset (about 9:30 EDT at midmonth) as you look
straight up toward zenith, you’ll see … not much! That
part of the sky is largely empty of bright stars, and the Milky
Way is hugging the western horizon. But, just to the south of zenith,
we encounter the familiar sickle of Leo, with bright Regulus at
the “heart of the lion.” Turning to the southwest, we
find Saturn in the otherwise faint constellation of Cancer. Binoculars
will show Saturn close to the open Beehive Cluster, #44 in Charles
Messier’s catalog of diffuse objects he didn’t want
to confuse with comets.
Turning further toward the west, Castor and Pollux are high in
the sky, above the red planet Mars. Below and a little to the right
we find the bright star Aldebaran, in the constellation Taurus.
Orion is low to the west southwest, and we prepare to bid this prominent
winter constellation adieu as we move into spring and summer. To
the north, Ursa Major (the Big Dipper) is inverted. By 10:30 the
“pointer stars” at the end of the bowl will be directly
above Polaris, the North Star. Further to the east, we find Arcturus,
the brightest star in Bootes, and then as we complete our turn,
we see Spica to the east southeast, in Virgo.
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