August 2007 Sky from the Keeble Observatory
Last month we looked at Newton’s “Law of Universal Gravitation,” and promised to
write this month about our best current formulation of gravity, Einstein’s General
First, however, I’d like to inject a note about the twin rovers currently on Mars.
Both of these robotic explorers are well past their designed lifetime of 90 days
– they’ve been actively returning data for three years. They are currently in some
stress because of a huge dust storm affecting much of the southern hemisphere of
the red planet. Both rely on solar energy, and the dust storm has reduced the intensity
of sunlight reaching the probes to dangerous levels. They typically require about
200 watt hours of energy per day for minimal survival, but are receiving less than
150. Controllers are running the probes at minimal power, and have not moved them
for several weeks in order to conserve energy for the overnight heaters required
to keep the electronics healthy. More about this next month.
Newtonian gravitation has been successfully used to calculate the orbits of planets
and moons, and is the computational basis for navigating between planets in our
solar system. We can also apply it to determine the mass and orbits of stars and
galaxies. In spite of the seeming hubris of calling it universal, it works! However,
if we push Newtonian theory to extremes of large mass, it begins to break down.
For example, if we observe the orbit of the planet Mercury (closest to the Sun)
and compare it with that calculated from Newtonian gravitation, it’s not quite right.
Mercury misses its calculated position by about half the diameter of the planet
on each orbit.
Einstein began his exploration of gravity by asking simple questions. What would
be the difference between standing on the Earth’s surface, where objects fall at
a constant acceleration, and riding in an elevator way out in space which was accelerating
“upward” at the same rate? He reasoned that there would be no observable difference,
and hence decided that these two scenarios were exactly the same. From this “principal
of equivalence” he was able to construct his General Theory of Relativity – essentially
a new formulation for gravity itself.
General relativity tells us that the presence of mass warps the geometry of space
and time, causing objects to move on curved paths rather than in straight lines.
What Newton attributed to a “force” Einstein attributes to geometry. The mathematics
is more sophisticated than Newton’s, but on the scale of the solar system and beyond,
Relativity reduces to Newtonian gravitation. However, on the scale of the orbit
of Mercury orbiting the mass of the Sun, Relativity gives us the correct orbit.
One consequence of this picture is that even light will follow curved paths due
to the distortion of space-time. The deflection of starlight passing the Sun was
measured in 1919 by British astronomer Sir Arthur Eddington, and was found to be
as predicted by Einstein. It was this successful measurement that transformed Einstein
from a rather obscure scientist into a cultural icon.
Relativity predicts exotic behavior under extreme conditions. For example, if enough
mass is concentrated in a small enough volume, the curvature of space-time makes
it impossible for light to escape. This is the so called “black hole” – once believed
to be simply a theoretical curiosity, now thought to occupy the central regions
of most galaxies. If a mass is rotating, it will “drag” the local space-time distortion
around with it. This, too, has been observationally confirmed, most recently by
a satellite called Gravity Probe B, but also in studies of pulsars in binary systems.
Lunar phases for August: Last Quarter on the 5th, at 5:20 pm; New
Moon on the 12th, at 7:03 pm; First Quarter on the 20th, at 7:54 pm; Full Moon on
the 28th, at 6:35 am.
Full Moon will be accompanied by a total lunar eclipse – the Moon will be entirely
within Earth’s shadow. The penumbral eclipse begins at 3:52 am, but you’ll not likely
notice this piece. The Moon slips into the central, darkest part of Earth’s shadow
beginning at 4:51, with totality beginning at 5:52. Though totality lasts ninety
minutes, you’ll not see it from here – moonset and sunrise occur at 6:35. For the
best view, take your vacation to Hawaii, where the Moon will be high in the sky
for the entire show.
Early in the month you will see Mercury in the pre-dawn skies, rising to
the east-northeast about an hour before the Sun. By midmonth it disappears into
the solar glare, reappearing by the end of August emerging low on the western horizon
at sunset. Mars rises about 1:00 am early this month, and finds itself
high to the south-southeast at sunrise. It will clear the horizon a bit earlier
each day, moving high to the south at sunrise by month’s end. Venus replaces Mercury
by the last week of the month, with Saturn also coming out of the Sun’s glare by
the end of August.
Saturn begins the month with Venus, very low to the west at sunset, but
moves behind the Sun at midmonth. Venus actually moves in front of the Sun, but
disappears in its glare, anyway. Jupiter is near Antares, in the
constellation Scorpio, all month, bright to the south at sunset. Antares will be
red – don’t confuse it with Mars – while Jupiter will be bright and white.
Looking overhead at mid-month, about two hours after sunset, you will see the bright
star Vega in Lyra nearly at zenith. The “summer triangle,” with Vega, Deneb,
and Altair will be among the first bright stars to emerge from twilight.
Deneb is to the east of Vega, in the constellation Cygnus. Altair is to the southeast,
in Aquila. In the 20 July issue of the journal Science, researchers report
that they have reconstructed an image of Altair using multiple telescopes as an
“interferometer.” As a general rule, stars are so far away that the light we receive
comes effectively from a single point. The star is rapidly rotating, an oblate spheroid
rather than a nearly perfect sphere like the Sun.
At the other end of Cygnus from Deneb we find the binary Albireo, resolved
with binoculars, impressive with two distinctly colored stars in a modest telescope.
Cygnus lies parallel to the Milky Way, which runs north-northeast to south-southwest.
Below Cygnus, towards the eastern horizon we are looking out of the plane of our
Galaxy. This relatively open patch of sky is marked by the broad “Great Square”
of Pegasus. To the west of zenith, a smaller irregular rectangle marks the constellation
Hercules. Closer to the horizon, the bright star Arcturus is found in Bootes.
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.