August 2005 Sky from the Keeble Observatory
4th of July fireworks update: Deep Impact was a resounding success!
Striking the nucleus of the comet Tempel 1 precisely as programmed,
the impactor excavated a small crater about the size of a football
stadium. Data are still being analyzed to unravel the puzzle of
the comet’s makeup. To see images of the impact, and to further
follow the results, point your web browser to the Deep Impact site
If you’ve received an email about a spectacular close approach
to the planet Mars this month (and I’ve had several calls
from folks who did), please know that that information is about
two years out of date! Apparently nothing ever really goes away
on the internet. Mars will be fairly close in October, when it passes
through “opposition,” but it will not be as close as
it was in 2003. However, August does represent the current launch
window for sending probes to the Red Planet. Mars Reconnaissance
Explorer is scheduled for launch sometime starting August 10th.
It will provide the highest resolution images from orbit yet achieved,
and will also carry ground-penetrating radar to study the underlying
structure of the planet.
In May we started looking at some of the consequences of Albert
Einstein’s Special Theory of Relativity. In 1905 he produced
two papers on the subject, as well as a paper on “Brownian
motion” which provided the first convincing proof for the
existence of atoms as actual objects, and another paper explaining
the photoelectric effect (for which he later received the Nobel
Prize). Let’s look more closely at some results of Relativity
which we first mentioned in that column.
What are the key results? First, a clock in motion relative to
you as the observer will record time passing at a slower rate. For
example, you would hear your clock go “tick-tock-tick”
while a moving, identical clock would go “tick-tock.”
An even faster clock might only go “tick!” But, the
moving clock watcher would notice nothing unusual about her clock,
and would see your clock running slow! That’s the “relativity”
part. The effect is formally called time dilation.
Second, objects in motion relative to you will be shortened in
the direction of their motion by exactly the same factor that the
clocks are dilated. This effect is called length contraction, and
it also is relative to which observer is doing the measurement.
You will see her meter sticks contract, she will see yours contract!
How might we test for time dilation? Imagine that you have a pair
of very precise clocks, which you synchronize in your laboratory.
Place one of the clocks on a jet airliner, and fly it around the
world, bringing the two clocks back together at the end of the journey.
You will find the clocks are no longer synchronized – and
the discrepancy will be exactly what is calculated by Einstein’s
special theory! A further test comes if we fly the two clocks around
the earth in opposite directions, one to the east, the other two
the west. They will be even more out of synch, because their relative
speeds are even higher than the difference between the “stationary”
clock in the lab and the single aircraft.
We don’t need to do that experiment, though it has been done.
The Global Positioning System (GPS) uses a constellation of satellites
to precisely determine one’s location on the earth to within
a few meters. Those satellites, essentially an array of very precise
clocks, are all orbiting at a speed of 5 miles per second. Without
applying the effects of relativity, the GPS system would never work.
Your cell phone uses this system, if it was manufactured since 9/11.
If you have an on-board navigation system in your car, it uses GPS.
Thank you, Dr. Einstein.
Lunar phases for August: New Moon at 11:05 pm, on the 4th; First
Quarter on the 12th, at 10:38 pm; Full Moon on the 19th, at 1:53
pm; and Last Quarter on the 26th at 11:18 am.
Venus and Jupiter are both bright “stars” in the evening
twilight as the month begins, with Jupiter to the southwest, about
25 degrees off the horizon and Venus a little lower to the west.
By the end of the month they will be only a few degrees apart to
the southwest, and will actually pass within 1½ degrees on
the 2nd of September.
Predawn planet watchers will find Mars high above the southern
horizon at sunrise. Saturn rises less than an hour before the Sun
at the beginning of the month. By the end of August, you’ll
see it about 30 degrees above the horizon as the Sun comes up, nearly
two hours ahead of the Sun’s position on the sky. Mercury
will join Saturn in the predawn twilight, about halfway between
Saturn and the Sun at midmonth. It reaches “maximum western
elongation” on the 22nd, and then rapidly slips back closer
to the Sun.
Two hours after sunset, we look directly overhead to find Vega,
in the constellation Lyra, just to the east of zenith. Hercules,
which was overhead at this time in July, has moved west of zenith.
The faint luminous band of the Milky Way crosses the sky from NNE
to south. Below Vega, in the middle of the Milky Way, we see the
familiar cross of Cygnus, the Swan. Deneb marks the tail of the
swan, with the faint but beautiful binary Albireo as the head. This
constellation also marks the direction of the Sun’s orbit
about the center of the Galaxy. To the northeast, i.e. behind the
swan, we find the also familiar W shape of Cassiopeia. Below this
constellation lies the Andromeda galaxy, which will climb higher
into the evening skies as we approach fall. This galaxy, roughly
the same size and type as our home Galaxy, lies about two million
light years distant ... it is the farthest object visible without
a telescope. Its distance means that light takes two million years
to arrive here, traveling at 300,000 kilometers each second.
If we follow the Milky Way from Albireo to towards the south we
will find Altair (in Aquila) to the southwest, and then the “teapot”
shape known as Sagittarius to the south. Shrouded in opaque dust
clouds, about 25000 light years beyond the nearby stars which form
this constellation, lies the center of the Milky Way Galaxy. That
bright red star to the south southwest is Antares, in Scorpio.
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.