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.
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.