Skywatching September 2012


Venus and Jupiter Shine Brightly
in the eastern sky during early September
mornings in 2012.

Moving through September takes us into autumn.  At night the Milky Way with its myriad of stars arches from north to south.  During the month, the amount of sunlight rapidly diminishes.  At the beginning of the month, the Chicago area receives slightly over 13 hours of daylight.  By month’s end, it is less than 12 hours.  Each September morning, as seen along the horizon, the sun rapidly rises farther south.  Late in the month, it rises precisely east.  The equinox occurs on September 22 at 9:49 a.m.  From the equator, the sun appears directly overhead (zenith) at noon.  For us at mid-latitudes, the sun is a little more than halfway up in the southern skies at noon.

September’s Moon Phases
9/8 — Last Quarter
9/15 — New Moon
9/22 — First Quarter
9/29 — Full Moon

Mercury is not visible this  month.  It reaches its farthest point from Earth behind the sun (superior conjunction) on September 10.  This rapidly moving planet will not be visible again in the northern mid-latitudes until December, when it appears near Venus in the morning sky.

Morning Sky

Venus is the brightest starlike object in the morning sky.  It rises a little to the north of east about 3.5 hours before the sun throughout the month.  This brilliant Morning Star can be seen in the predawn sky well into morning twilight.    As compared to the bright background stars, Venus is farther east each morning.  Its rapid eastward movement compared to the stars is easily visible from morning to morning and week to week.

On September 1, Venus is closest to Pollux, one of the Gemini twins, in the eastern sky.  The accompanying chart shows Venus in its relative position compared to the star and its “twin” star Castor.  Venus is less than 9 degrees from Pollux.  The moon covers about 1/2 degree in the sky.  So Venus is about 18 full moon diameters from Pollux.

In less than a week (September 7), Venus lines up with Castor and Pollux.  Venus is about 11 degrees from Pollux.  Venus’ orbital motion during this week is very obvious.

Jupiter is the second bright Morning Star in the eastern sky.  It is higher than Venus and in front of the stars of Taurus.  On September 8, the moon joins Jupiter and the bright star Aldebaran nearby, as seen in the diagram above.  To see the relative positions of the two morning planets, here is a photo from August 25, 2012.  At the beginning of the month, Venus is 40 degrees below Jupiter.  In the image linked above, the pair was separated by 33 degrees.

After passing Jupiter and continuing eastward on this orbital path, the waning crescent moon is near Venus on the mornings of September 11 – September 13.  The best photography is on September 12 when the separation is about 4 degrees.

On the morning of September 13, Venus passes a distant star cluster, commonly known as the Beehive star cluster.  The diagram above shows the scene as if viewed through a binocular.  The cluster is fairly dim and likely washed out in areas with outdoor lights.   A binocular or telescope at a magnification of  20 times will show them nicely.

By month’s end, Venus is farther down the plane of the solar system (ecliptic), nearing Regulus, the bright star in Leo.  Venus passes very close to Regulus in early October.  It is 70 degrees from Jupiter on the last day of the month.

Evening Sky

Mars and Saturn start September low in the western sky just after sunset.  Saturn is about 5 degrees above Spica, the brightest star in Virgo.  Mars is about 10 degrees (the size of your fist when your arm is extended) to the left of Saturn.  Each evening Saturn is more difficult to see as Spica and Saturn disappear into the sun’s glare.

Mars has a fast orbital motion to the east.  On the evenings of September 14 and September 15, Mars passes below Zubenelgenubi by about 1 degree.  On September 19, the faster moving moon overtakes and appears near Mars and the star just after sunset.  In the classic bookThe Stars in our Heaven:  Myths and Fables, Peter Lum notes that on early star maps, Zubenelgenubi and Zubeneschamali were part of Scorpius, representing the southern claw and northern claw of the scorpion.  “It was the Romans who first used this particular group of stars as [part of the] modern zodiac, and they originally placed there the figure of Julius Caesar holding a pair of scales in his hand as a token of his infinite wisdom and justice.  The figure was soon eliminated” (p. 177).

The month ends with Mars low in the southwest, nearly midway between Zubenelgenubi and Antares, also known as the Rival of Mars, low in the southwestern sky during early evening.

The Solar System

The chart above (click it to see it larger) shows the visible planets’ positions on September 15, 2012.  Venus and Jupiter are on the morning side of Earth.  Mercury is behind the sun.  Mars and Saturn are on the evening side of our planet.

Step outside on the next clear morning to look at the spectacle of Morning Stars and find a clear horizon in the evening to see Saturn (early in the month) and Mars in the southwestern sky.

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