March 2008 Sky from the Keeble Observatory
In 1974 and 1975, NASA’s Mariner 10 probe flew three times past the planet Mercury.
Because of the timing of its orbit, each pass photographed the same part of the
surface (less than half) at essentially the same sun angle. Until last January,
that was the only robot spacecraft to visit the closest planet to the Sun.
Launched in August 2004, NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry,
and Ranging) probe finally arrived for its first visit on 2008 January 14. (Some
claim that NASA actually means “National Acronym Synthesis Agency). In a complex
orbital ballet the probe flew past Earth a year later to use Earth’s gravity to
deflect its orbit in toward the Sun. Two flybys of Venus (one at only 338 km!) again
redirected its orbit toward Mercury. Seen from above the solar system’s “north pole”
the planets all orbit counter clockwise, but MESSENGER is going clockwise now. Each
pass by Mercury will reduce its speed (next encounters will be next October 6th,
then 29 September in 2009) before finally slowing enough to orbit Mercury on 2011
March 18. Each flyby and the final orbit are at altitudes of only 200 km.
We’ve already learned much that we didn’t know, just from the first pass. We see
Mercury has having been much more volcanically active than we surmised from Mariner
10, with evidence for vast lava flows. It still looks heavily cratered, much like
our own Moon, but there are indications that some of the craters are relatively
recent. In the middle of the Caloris Basin, a large lava-flooded impact feature
similar to the lunar maria, we even find a strange looking network of what may be
collapsed lava tubes spreading away from a crater. It has been dubbed the Spider,
but it remains to be determined whether the crater is related to the troughs are
associated with the crater, or if it was a chance impact after the lava tubes collapsed.
Image courtesy NASA, the Carnegie Institute for Science, and Johns Hopkins University
Applied Physics Lab.
Lunar phases for March: New Moon on the 7th, at 12:14 pm EST;
First Quarter on the 14th, at 6:46 am EDT; Full Moon on the 21st,
at 2:40 pm EDT; Last Quarter on the 29th, at 5:47 pm.
Don’t forget to set your clocks ahead on the 9th – Congress has moved
up the start of Daylight Saving Time to the second Sunday of March!
Pre-dawn sky watchers will still see Venus and Mercury rising about two degrees
apart before the Sun at the beginning of March, but they stay close to the horizon,
only reaching about 10 degrees above the southeast at sunrise. Venus will still
be very bright, so you may be able to pick it out of the horizon clutter. Jupiter
leads by about an hour; you’ll see it south southeast, about 24 degrees off the
horizon as the Sun brightens the sky. Saturn is just setting at sunrise. Later in
the month, Mercury and Venus are even lower as the Sun rises due east. Jupiter has
climbed higher, reaching about 30 degrees altitude due south.
Just after sunset, at the beginning of the month you’ll see Saturn near Regulus
in Leo due east. It should be visible all night. Mars is very high (about 70 degrees)
to the southeast. It continues to climb through the month, reaching nearly 80 degrees
above the horizon at sunset by the time we get to April. Saturn will be rising almost
2½ hours before sunset by month’s end, appearing about 40 degrees altitude above
the south southeast horizon.
About two hours after sunset, as you look toward the south, the first bright star
you are likely to notice is Sirius, in the constellation Canis Major (the big dog).
This is actually the brightest appearing star visible from Earth – but it is not
intrinsically the brightest, by far. It’s only about 25 times as bright as the Sun,
but it’s relatively close by – a mere 8.6 light years. (That is, light takes 8.6
years to reach us from Sirius, travelling at 300,000 kilometers per second!) Above
and to the right of Sirius, you will see the familiar shape of Orion. Look at the
bright red star on the upper left of the constellation. That’s Betelgeuse, a red
supergiant – and it’s almost 60,000 times as bright as the Sun! If placed at the
position of the Sun, Betelgeuse would reach past the orbit of Mars. However, since
it’s over 400 light years away, it appears not as bright as Sirius.
Now, look below the “belt” of Orion, at the grouping of stars which make up the
“sword.” With the naked eye, do you see anything unusual about the middle star?
Look with binoculars – what do you see? This is the Orion Nebula, a relatively nearby
star forming region. The distance to the nebula is about 1500 light years. The hot
blue stars that you see illuminating the nebula are less than a million years old.
Above Orion, almost at zenith, is the planet Mars. If you compare its appearance
with Betelgeuse, you’ll probably notice that it doesn’t “twinkle.” This is because
the planet actually shows a disk while the star is so far away that it appears as
a point. A bit toward the east from Mars, still not far from zenith, you’ll find
the bright pair Castor and Pollux, in the constellation Gemini. Castor is the upper
star, with Pollux below it. They’re sometimes called the twins, but they couldn’t
be more different. Castor is actually a complex system of six stars, but its two
brightest components are separated by only a tenth of a degree. It’s unlikely that
you will be able to resolve them, even with binoculars. We can barely resolve them
from the Keeble Observatory. Pollux is a single star, but it is known to have at
least one planet, about 2.5 times the mass of Jupiter.
Above and to the right of Orion you will find the asterism known as the Pleiades.
(In Japanese, this cluster is called Subaru, which is why you see seven stars on
the grill of that brand of car!) This cluster is about 400 light years away, and
only a few tens of millions of years old. The faint bluish haze surrounding the
cluster is dust, reflecting the starlight of the hot blue stars in the cluster.
To the northeast, you’ll eventually make out the familiar shape of the “Big Dipper,”
part of Ursa Major (the Big Bear). It will appear inverted, with the “handle” pointing
down. The two stars at the end of the bowl mark a line which should carry your eyes
up and to the right, encountering Polaris, the Pole Star. The middle star in the
handle of the dipper is actually a pair, known as Mizar and Alcor. Can you see the
pair with the naked eye? How about with binoculars?
Towards the east, you should be able to discern the familiar sickle shape of the
constellation Leo. The brightest star in the constellation is Regulus. It lies about
77 light years distant, and is 150 times brighter than the Sun. Below Regulus, you
will find the planet Saturn. With binoculars you may enable you to see its distinctive
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.