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 mph.
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
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
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.