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 be carried.
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
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
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
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.
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.