Skywatching September 2013


The Late Summer Milky Way
by Alan Dyer

(Want to see some exquisite astronomy photos? Read Alan Dyer’s blog  here. )

As summer reaches its last lap, the Milky Way presents itself for magnificent viewing from dark locations during the early evening.  The feeble light arches across the sky from north to south as shown in Alan Dyer’s photo above.  The rim of our galaxy is revealed as a great accumulation of stars, gasses and dust particles.

The Moon’s Phases (CDT)

New Moon — September 5
First Quarter — September 12
Full Moon — September 19
Last Quarter — September 26

The Sun

The sun’s rising and setting points move rapidly south this month.  The farther south the sun sets, the daylight  hours decrease.  We lose an hour of daylight this month.  On September 22, the sun rises exactly east and sets precisely west.  On this day of “equal night” (equinox, 3:44 p.m. CDT), the sun is seen directly overhead from the equator at noon, although it never passes exactly overhead from the mid-latitudes.  From now until next March, sunlight reaches Earth’s surface more directly at southern latitudes with the shift of warm seasons from the northern hemisphere to southern.

The Evening Sky

Venus continues to shine in the western sky after sunset, although it sets about 90 minutes after the sun.  Like the sun, Venus’ setting point moves south.  (Read more about Venus as an Evening Star.)

On the evening of September 5, Venus passes about 1.5 degrees to the upper right of Spica.  (For perspective, the full moon is about 1/2 degree in diameter.  Your index finger at an arm’s length is about 1/2 degree. )  Venus and Saturn appear to pass each other later in the month.  On this evening the planets are about 14 degrees apart.

A few nights later, September 8, the moon enters the evening sky, appearing about 2 degrees from Venus.  The Venus-Saturn distance is under 11 degrees.

On the next evening, September 9, the moon pairs with Saturn.

For the next several evenings watch the planetary pair get closer.  Venus passes about 4 degrees to the lower left of Saturn on September 19.  The pair appears close, but they are about 900 million miles apart! Zubenelgenubi appears nearby.

On September 23, Venus passes about 1.5 degrees below Zubenelgenubi.

Meanwhile, Mercury makes an appearance low in the western sky during bright evening twilight.  On the evening of September 24, this elusive planet passes less than 2 degrees to the upper right of Spica with Venus 22 degrees to the upper left.  Find a clear western horizon and use binoculars to find Spica and Mercury.  Once you locate them with optical assistance, can you see them without binoculars?

Morning Sky

Jupiter and Mars appear against a bright starry background in the morning sky.

Early in the month, the moon appears near both planets.  The chart above shows the planets with the moon on the mornings of September 1 and 2.  On the morning of September 2, the planets are about 22 degrees apart and separating quickly.

Mars continues its quick orbital motion toward the east.  On the mornings of September 8 and 9, Mars passes the Beehive star cluster, a mile post of sorts along the ecliptic (plane of the solar system).  Away from city lights, a smudge of light appears near Mars.  This is the star cluster.

 Catalogued as M44 and NGC 2632, the Beehive star cluster can be seen as individual stars through binoculars.  The optics will also reveal Mars’ color.  The cluster is about 500 light years away and it has at least 300 stars.
In Robert Burnham’s Celestial Handbook he writes that the star cluster was used in ancient times as a weather indicator.  Aratus and Pliny have both stated that the invisibility of the [star cluster] in an otherwise clear sky was considered to forecast the approach of a violent storm (p 345).
Mars was last in the region in October 2011.  It appears here again on August 20, 2015.
By month’s end Jupiter and Mars are over 30 degrees apart.  The moon returns to the region passing both planets.  The chart above shows the moon near Jupiter on the mornings of September 27 and 28 and near Mars on September 30 and October 1.

The chart above shows the visible planets on September 15, 2013.  (Click the image to see it larger. ) A line drawn from the Earth to the sun and extended into space shows the noon line and the midnight line.  Notice that no planets are behind the sun and no planets are visible at midnight.  Jupiter and Mars are on the morning side of Earth and Mercury, Venus and Saturn are on the evening side of Earth.  So all the visible planets are visible just before sunrise or just after sunset and none around midnight.

Be watchful of the changing season that September brings.  The planets are convenient for easy viewing this season.

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