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 at http://www.nasa.gov/mission_pages/deepimpact/main/index.html.
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information is about two years out of date!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
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 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.