May 2005 Sky from the Keeble Observatory
Albert Einstein is probably the most recognizable scientist of
the 20th century. Indeed, he stands for most as the ultimate icon
of science itself. Just 100 years ago he wrote four fundamental
papers while working as a patent clerk, upending much of classical
physics and laying the foundations for quantum mechanics. Perhaps
most famous for his formulations of special and general relativity,
in that year he also proved the existence of atoms and explained
how light interacts with matter.
To commemorate that “miracle year,” we’ll spend
this and a few following columns exploring some of those revolutionary
ideas. First, let’s look at relativity.
Imagine riding through Ashland on a northbound train, and playing
a game of catch with the person just ahead of you on the train.
When you toss the ball forward, an observer in the train station
will see the ball traveling north just a bit faster than the train.
When your partner tosses it back, the same observer sees it traveling
just a bit slower. From the train’s point of view, the ball
is simply moving back and forth, and the station appears to be moving
backward. We say that the velocities add, and that the measured
motions are relative to which frame of reference is doing the measuring.
Intuition suggests that the same addition of velocities should apply
to light, which travels much faster than the train or ball. (The
speed of light is almost exactly 300,000 kilometers per second -
that’s 186,282 mi/hour – equivalent to more than seven
trips around Earth’s equator each second.) So we expect a
northbound flash of light emitted on the train to travel at the
speed of light plus that of the train, and a southbound flash at
the speed of light minus that of the train.
Experiments in the 1860s and since have shown that not to be the
case. The speed of light is a constant, irrespective of the relative
motion of source and observer. Einstein describes a “thought
experiment” in which he imagines running along with a beam
of light, matching its speed. What would that look like? He concluded
that it would look like nothing in nature, and therefore must be
Speed is a measure of distance traveled divided by the elapsed
time. If that ratio is fixed for light, then relative motion must
distort the measurements of space and time. Einstein’s Special
Theory of Relativity describes precisely those distortions. We don’t
notice them in everyday life because we don’t travel at speeds
anywhere close to that of light. The size of effect depends on the
square of the ratio of speed to the speed of light, which is normally
a very small number – other than saying that, I’ll spare
you the equations!
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!
Counter-intuitive though these may seem, the predictions of special
relativity are well-confirmed by precise experiments. We’ll
say more next month.
Lunar phases for May: Last Quarter on the 1st, at 2:24 am, EST;
New Moon at 4:45 am, on the 8th, EDT; First Quarter on the 16th,
at 4:57 am; Full Moon on the 23rd, at 4:18 pm; and a second Last
Quarter on the 30th at 7:47 am.
Jupiter is already up as the Sun sets this month. Look for it about
30 degrees above the southeast horizon early in the month, climbing
to about 40 degrees by month’s end. Saturn is still lingering
near Castor and Pollux in Gemini. You’ll find it at about
60 degrees altitude to the southwest, but getting lower and closer
to the Sun. It’s about 40 degrees above the western horizon
by the time the calendar changes to June. Venus is about 10 degrees
off the western horizon early in the month at twilight. On a clear
evening you’ll see it as the first “star” to emerge
from twilight. It will be a few degrees higher later in the month,
and still quite bright.
Mars rises about 3 hours before the Sun, and is about 30 degrees
above the southeast horizon at dawn. It’s moving away from
the Sun’s position on the sky, so look for it to rise earlier
and appear a little higher each day.
At mid-month, an overhead view about 2 hours after sunset still
shows a mostly empty patch of sky! Nothing dramatic has changed
since last month, but all the constellations are about 30 degrees
further west. At this time the Milky Way rings the horizon, so the
vast majority of stars in our Galaxy are too low to see. Castor
and Pollux are keeping company with Saturn, about 30 degrees above
the western horizon. Turning to the southwest we see Leo, marked
by its brightest star Regulus about 60 degrees above the horizon.
To the south we see the constellation Virgo, with Jupiter a bit
brighter than Spica, that constellation’s brightest star.
Turning to the north, we see the familiar inverted shape of the
“Big Dipper” high above the horizon. Following the curve
of the handle towards the east will bring us to Arcturus, the brightest
star in Bootes.
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.