March 2007 Sky from the Keeble Observatory
We often speak of the motion of planets and moons as an “orbital ballet,” calling
to mind images of the grace and elegance. Indeed, our mathematical understanding
of the motions is simplest when cast in terms of a two-body calculation, much like
the choreography of something like the pas de deux from Tchaikovsky’s Swan Lake.
The image becomes even more appropriate when thinking about the motion of robotic
probes launched from Earth to explore the other planets. Of course, the dancers
are moving very fast, with only the immense scale of the solar system making them
appear to move relatively slowly.
Consider that Earth orbits the Sun once every year, making an approximate circle
150 million kilometers (93 million miles) in radius. That means that we are moving
about the Sun at 30 km per second – that’s about 67,500 miles per hour! By contrast,
Jupiter is over 5 times our distance from the Sun, and takes almost twelve of our
years to complete one orbit – it’s moving at a leisurely 13 km/s – just under 30,000
NASA launched a probe called New Horizons on 2006 January 19. The Atlas V booster
and Centaur upper stage sent the probe away from the Earth at the fastest launch
speed ever attained – some 35,000 miles per hour, crossing the orbit of the Moon
in a mere 9 hours. Relative to the Sun, that speed gets added to Earth’s, so this
probe is moving out from Earth’s orbit with an initial speed over 100,000 mph. The
target is distant Pluto, almost 4 billion miles away. Pluto, recently demoted 9th
planet, is now considered by some to be just a “dwarf planet.” Left to its own ballet,
New Horizons would arrive at Pluto sometime in 2018. But, this speedy little probe
will get there about 3 years sooner, taking advantage of a game of cosmic “crack
the whip” played out on February 28th.
As the probe moves away from the Sun, gravity slows it down. At Jupiter’s distance
from the Sun, New Horizons had slowed to about 43,000 miles per hour. But, it was
aimed to pass close by Jupiter, and as it fell toward the giant planet, it will
gain speed again. This “gravitational slingshot” successfully added about 9,000
mph to New Horizon’s velocity, and also redirected it onto a swifter path to Pluto.
At closest approach to Jupiter, it was 1.4 million miles above the cloud tops. In
order for the slingshot maneuver to work, they had to pass through an imaginary
circle just 500 miles across – equivalent to a marksman in Washington hitting a
target in Baltimore. Arriving in July 2015, it will image the Pluto/Charon system,
and hopefully also give us our first close look at Pluto’s latest discovered moons,
Nix and Hydra.
Lunar phases for March: Full Moon at 6:17 pm EST, on the 3rd; (We
switch to Daylight Saving Time on the 11th, so the following times are EDT) Last
Quarter on the 11th, at 11:54 pm; New Moon on the 18th, at 10:43 pm, and First Quarter
on the 25th, at 2:16 pm.
This month’s Full Moon is accompanied by a total lunar eclipse. The Moon will be
in the darkest part of Earth’s shadow about 15 minutes before it starts to clear
the horizon at about 6:00 pm. Totality will last until nearly 7:00.
Twilight at both ends of the day provide good viewing for skywatchers. Early in
March, predawn finds Jupiter due south, about 30 degrees above the horizon, above
and a few degrees to the left of Antares. 16 degrees above the southeast horizon
is where we find Mars, with Mercury only 10 degrees from the ESE horizon at sunrise,
probably lost in ground clutter. Their relative positions don’t change much this
month, but as the Sun drifts slowly north, you’ll see them shifting to the west
as April approaches.
Evening skies begin with Saturn to the east, about 20 degrees above the horizon
at sunset, and very close to the Moon on the 1st. Venus will be brilliant to the
west, the first “star” to emerge from twilight, and setting about 2 hours after
sunset. Both planets will be higher as the month goes on.
Our overhead view at midmonth, about two hours after sunset, finds Castor and Pollux
in the constellation Gemini nearly at zenith. Both are binaries, which are resolved
in a small telescope. Below Gemini to the southwest is the now familiar shape of
Orion, trailed to the south by the brightest star visible from Earth, Sirius (also
known as the Dog Star) in Canis Major. Orion, the hunter, is being followed by his
faithful hunting dogs, as he pursues Taurus, the Bull, to the west-southwest. The
eye of the bull is bright Aldebaran. Riding the shoulder of the bull are the Seven
Sisters – the famous Pleiades cluster – which are a good binocular target. To the
northwest, Cassiopeia is tilted to look like a Greek ? (sigma), while the Big Dipper
(Ursa Major) is high to the northeast, with the bowl of the dipper inverted. Leo
lies to the east. Look for the familiar sickle asterism, above Regulus and below
Saturn. Binoculars will show you Saturn’s rings, and while you have them out, raise
your view to the Beehive Cluster, lying just above Saturn.
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.