Keeble Observatory
March 2003 Sky from the Keeble Observatory
NASA has bid farewell to another spacecraft, but this time it's
no tragedy. Rather, the 31-year voyage of Pioneer 10 is cause for
celebration, and perhaps a touch of wistful they don't make them
like that, anymore! Launched in March of 1972, the robot space
explorer was the first human-built object to pass through the asteroid
belt, first to fly by the giant planet Jupiter, and the first to
pass beyond the orbital distance of Pluto. The science mission officially
ended in March of 1997. Since then, its weak radio signal has been
tracked by the Deep Space Network to refine techniques for future
application. Last contact was January 22, though no telemetry was
returned. The last telemetry was received on 2002 April 27.
The last signal was received over a distance of 7.6 billion miles
- some 82 times the distance from Earth to the Sun, and nearly twice
as distant as Pluto. It took more than 11 hours 20 minutes for that
signal to reach us. The silent spacecraft is drifting outward in
the general direction of bright red Aldebaran, 68 light years away
in the constellation Taurus (above and to the right of Orion). It
should get there in two million years, bearing a gold plaque describing
humankind and our home.
A reader (sorry, I didn't catch your name from the voice message)
asked a timely question about the change of seasons, and how come
the times for sunrise and sunset don't shift as expected. He noticed
that when we passed December's Winter Solstice (shortest day of
the year) the lengthening of the day does not accompany earlier
sunrise and later sunsets. Rather, sunrise times continue to get
later every day until mid-January, and sunset times started to get
later several weeks before the solstice. Intuition might suggest
that the Sun should rise earlier for the days to get longer. Let's
try to figure out what's happening.
First, imagine an idealized situation, letting the Earth spin on
its axis with the Sun always above the equator, and our orbit about
the Sun a perfect circle at constant speed. In this imaginary world,
there would be no seasons, only climate variations with latitude.
Day and night would always be exactly 12 hours long everywhere except
at the poles, where the Sun would circle right at the horizon.
Now, we adjust our model by tilting the axis of rotation 23� degrees,
to match the real orientation of our planet. The Sun will now appear
to move higher and lower in the sky as the seasons change. From
Ashland, the Sun would be about 76 degrees above the horizon at
noon on the Summer Solstice, 53 degrees on Spring and Autumnal Equinoxes,
and a little less than 30 degrees on Winter Solstice. This is close
to what we actually see, so this picture has more promise than our
first, simpler model. It's not quite right, because it also predicts
that shorter days should be accompanied by later sunrise and earlier
sunset, which is what our reader's intuition suggested. A series
of noonday pictures of the Sun taken over a full year in this model
would trace out a straight line perpendicular to the southern horizon.
The only thing left is to let the model orbit be the ellipse that
we know it is. When Earth is slightly closer to the Sun, it moves
faster along its trajectory. Conversely, it moves slower when more
distant. Our 24-hour day is actually a solar day - if we look
at distant stars we learn that the sidereal day is about 4 minutes
shorter. The extra time in the solar day is to allow for the change
in the apparent direction of the Sun as we move around our orbit.
If we're moving faster, it takes longer for the Earth to rotate
around to place the Sun over the same point on the southern horizon.
Similarly, when we're moving slower, it takes less than the nominal
4 minutes to get there. Our series of noonday pictures would trace
out a figure 8, with the small loop on top because we're actually
farther from the Sun in the summer (hence, the day-to-day changes
in the Sun's position are smaller). The shape is called an analemma,
and you may remember seeing one drawn on a globe in school, with
the months of the year printed along the figure. Check out the web
site http://antwrp.gsfc.nasa.gov/apod/ap020709.html
to see for yourself!
Lunar phases for March: New on the 2nd, at 9:36 pm; First Quarter
on the 11th, at 6:16 am; Full at 5:35 am on the 18th; Last quarter
on the 24th at 8:52 pm.
Jupiter and Saturn remain visible most of the night, with Saturn
above Orion in Taurus (wave goodbye to Pioneer 10 while you're
finding Saturn!), and Jupiter further to the east in Cancer. Mercury,
Venus, and Mars are all visible to the southeast before sunrise,
though Mercury is low on the horizon and will be harder to find.
Venus is lower than it was last month, but it's still the brightest
object you'll see. Mars is above and to the right, but don't confuse
it with the red giant Antares, which is also red and in the same
general area.
About two hours after sunset at mid-month, you'll see the bright
twins of Castor and Pollux almost directly overhead. Both stars
are actually multiple star systems, but so closely spaced that you'll
need a small telescope to resolve them. High to the southwest is
Orion, probably the most easily recognized of the winter constellations.
Below the belt is the wispy smudge of the Orion Nebula, which a
telescope will resolve into bright reflecting gas and dust clouds
surrounding the Trapezium cluster of young (only a few million years)
stars. Sirius, the bright star to the left and below Orion, is another
binary. But, it's companion is too faint to see without a very large
telescope. Sirius B is actually a white dwarf star, the hot remnant
core of an old star which has exhausted its nuclear fuel. It's approximately
the size of Earth, but about the mass of the Sun. High to the west
is the V of Taurus, with Aldebaran as the Eye of the Bull. That
faint cluster that looks like a Subaru logo is the Pleiades cluster
(Subaru is the Japanese name for this asterism). These stars are
not as young as Trapezium stars, having formed about 10-20 million
years ago. The faint wisp of light through Sirius, Orion, Taurus,
and on to the northwest horizon is the Milky Way, marking the plane
of our home Galaxy. Leo, marked by the bright star Regulus is high
to the east. To the northeast you'll see the familiar Big Dipper
in the constellation Ursa Major.
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.
Copyright 2003
George Spagna