August 2013 Sky from the Keeble Observatory
After the Voyager 1 flyby of Saturn in November 1980 completed the primary scientific
objectives for the probe’s camera, Carl Sagan proposed that it be used to capture
an image of Earth. It would not be of scientific use because of the distance (about
1.3 billion kilometers at the time), but might be of philosophical interest by placing
our home world in perspective. Ten years later the cameras were activated to generate
a “family portrait” of the Solar System. By that time the distance had grown to
6 billion kilometers (3.75 billion miles) and Earth was less than 15% of one pixel
in the image. But there we were, the famous “pale blue dot” which was eventually
the title of Sagan’s 1994 book describing the family portrait. All of humanity,
all of our history, indeed everything and everyone who ever lived on this planet
in one pixel. On July 23rd NASA did it again, twice, but from closer
range. Hope you were smiling!
This image (courtesy Astronomy Picture of the Day at
http://apod.nasa.gov/apod/ap130723.html) shows Earth and our Moon from two
probes. On the left you can see Earth below the rings of Saturn, a perspective obtained
by the Cassini probe currently orbiting there. The right portrait was made by MESSENGER,
orbiting Mercury, and you can see both Earth and Moon. Still an incredible, humbling
perspective. As a poster I once had when in college said, if we had more than one
planet, this one wouldn’t be so important. But we don’t, so it is.
Lunar phases for August: New Moon on the 6th, at 5:51 pm; First
Quarter on the 14th, at 6:56 am; Full Moon on the 20th, at
9:45 pm, and Last Quarter on the 28th, at 5:35 am.
Predawn planet watchers will find Jupiter, Mars, and Mercury rising to the east
as twilight begins to fade. Jupiter leads the way in both timing and brightness,
with Mars below and to the left. Mercury follows early in the month, brighter than
Mars but low on the horizon and disappearing into the Sun’s glare by the end of
August. Jupiter and Mars will climb as the month advances. As evening twilight darkens,
Venus will be brilliant but low to the west. You should be able to see it before
it sets if you have a clear, uncluttered view of the horizon. Saturn starts the
month at evening twilight about 35 degrees off the southwest horizon, setting a
few hours later. It will move lower and to the west as the month passes.
Our mid-month overhead view three hours after sunset finds zenith neatly surrounded
by the stars of the Summer Triangle. Vega, Altair, and Deneb are respectively the
brightest stars in the constellations Lyra, Aquila, and Cygnus. They are among the
brightest stars visible in the night sky, and all lie close to the plane of the
Milky Way galaxy. Their formal astronomical designations are Alpha Lyrae, Alpha
Aquilae, and Alpha Cygni, and to the naked eye they appear nearly identical. They
are all somewhat hotter than the Sun at about 10,000 K. However, even though both
Vega and Altair are main sequence stars about 50-100% more massive than the Sun.
Even though they are fusing hydrogen to helium in the stellar core, they are very
different. In particular, Altair rotates on its axis once every 9 hours (compare
to the Sun’s one month rotation period!) and is distinctly non-spherical. This also
makes the surface temperature different at the poles and equator. Deneb is not a
main sequence star, and probably derives its energy from the fusion of helium into
carbon. It is a super-giant star, about 5-10 times the mass of the Sun. Vega was
the “pole star” around 12,000 BCE, and will again be within a few degrees of due
north in the year 13,727.