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The Night Sky
Mars Meets Its Rival
Mars and Saturn are still present in the evening sky shining low in the southwest after sunset all through
the month of September. Mars will remain at nearly the same altitude all month an hour after sunset,
but Saturn descends a little lower into the sky with respect to the sunset each evening. Saturn sets 3
hours after the sun at the beginning of the month, however by month’s end, Saturn sets only 2 hours
after sunset. Both Mars and Saturn are nearly the same brightness at the beginning of the month, with
Saturn to the right of reddish Mars. As the month progresses, Mars continually moves away from Saturn
in the sky and slightly fades in brightness as the Earth pulls away from it in their respective orbits about
the sun.
Mars is named after the Roman god of war due to its red color being associated with blood. On the
nights of September 27th and 28th, Mars passes just to the north of the bright red supergiant star,
Antares, whose name literally means “rival of Ares,” the Greek name for their god of war. On these
nights, compare the colors of Mars and Antares and you will see why this star is considered the rival of
the planet Mars. These two celestial objects will be in the vicinity of each other during the last week of
September. I always get a kick out of watching Mars progress along the background stars from night to
night when it passes bright objects on the sky.
Planet Mercury makes an evening appearance during the last half of the month. Unfortunately it will
not get high enough in the western sky to easily be seen with the naked eye in the bright twilight before
it sets. The two farthest planets in the solar system, Uranus and Neptune, are visible all night; however
both are too faint to be seen without a telescope.
One needs to wait until the early morning before the other bright planets rise. Venus and Jupiter were
very close together in the sky on August 18th; however Jupiter has been slowing pulling away from Venus
since that date. In the beginning of September, Jupiter rises 2½ hours before the sun, and by month’s
end, it rises 4½ hours before the sun. Meanwhile, Venus rises only one hour before the sun as
September begins, and a half-hour before the sun as the month closes. This will be the last month to
catch Venus in the morning sky before it swings behind the sun in October. Venus will then make its
evening appearance in November as it comes out from behind the sun.
While you’re out before dawn from September 21st through October 6th, you might be able to spot a
huge pyramid of faint light extending from the horizon out to Jupiter a few hours before sunrise. This
glow is called the zodiacal light since the glow follows the zodiac constellations. It is caused by dust
particles in the inner solar system scattering sunlight. No special equipment is needed to view the
zodiacal light – one only needs to look towards the eastern horizon in a clear dark sky.
This month’s full moon will be the last of the supermoons occurring in 2014. “Supermoon” is a name
given to those full moons that occur when the moon is at or near perigee, the point in its orbit where
the moon is closest to Earth. Supermoons also are called perigean moons. Last month’s supermoon
occurred just a few minutes before perigee. This month, the moon is at perigee 22 hours before the
exact full phase. The full phase occurs at 9:38 p.m. EDT on September 8th, whereas the moon is at its
perigee point at 11:30 p.m. EDT on September 7th. This results in September’s full moon being 1/2–
percent smaller on the sky as compared to the August full moon. You may wonder why the full phase
and the perigee point move in time with respect to each other. This results from the moon’s perigee
point migrating around the celestial sphere due to gravitational perturbations by the sun. It takes about
8.85 years for the lunar perigee point to complete one cycle. As such, next year’s perigean moons will
begin about 1½ months later than they did this year.
The month of September sees the end of the summer season and the beginning of autumn. At 10:29
p.m. EDT on September 22nd, the sun will leave the northern sky crossing the celestial equator into the
southern sky. This marks the beginning of autumn in the northern hemisphere and the beginning of
spring in the southern hemisphere. The point on the sky where the sun crosses the celestial equator
moving south is called the autumnal equinox. It should be noted that the supermoon described above is
also this year’s Harvest Moon – the full moon that occurs closest to the autumnal equinox. This full
moon gets this name since crops can be harvested late into the evenings under its light this time of year.
The next free public astronomy open house at the ETSU Powell Observatory will occur on Saturday,
November 1st from 8 to 10 p.m. At these open houses, the public can view objects in the sky through
telescopes and hear talks by faculty of the Physics and Astronomy Department. At this November 1st
open house, I will be presenting the talk. Note that the open houses are cancelled if the sky is cloudy.
Further information about these open houses and directions to the observatory can be found on the
web at http://www.etsu.edu/cas/physics/observatory/default.aspx.
This month’s Night Sky was written by Dr. Donald G. Luttermoser, Chair of the Department of Physics
and Astronomy at ETSU. He can be reached at [email protected]. Astronomy-related information
for the public, including a link to the ETSU Powell Observatory, can be found at
http://www.etsu.edu/cas/physics/outreach/astronomy.aspx.