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Wyoming Skies for September

September 1, 2009

A monthly look at the night skies of Wyoming, written by Ron Canterna, professor in the University of Wyoming Department of Physics and Astronomy.

As the sun sets in September, the summer triangle (Vega, Deneb and Altair) is directly overhead and dominates this month's night sky.

This is again a great opportunity to view the plane of our Milky Way galaxy, which stretches from the southwestern horizon, arching up directly overhead, and plunging eastward on the northern horizon.

Overhead is the Northern Cross or Cygnus "the swan." As the night time hours pass the two most prominent constellations rise from the eastern horizon -- Andromeda, "the princess," and Pegasus, "the winged horse." Pegasus can easily be spotted since its body is dominated by the famous Great Square of four stars.

To the north Cassiopeia (the great "W" in the sky), and Perseus, are seen on the northeast horizon. Jupiter can be seen during most of the night. Look for the bright object in Capricorn in the south. After midnight be on the lookout for Mars, rising around 2 a.m. and then Venus, the bright "morning star."

September 2009 Interest: The Autumnal Equinox

Fall officially begins Sept. 22, 22 hours and 18 minutes Universal Time. The date marks the time when an important celestial event triggers several noticeable events on Earth.

The important celestial event is the sun is passing through the celestial equator (a projection of the Earth's equator onto the sky) going from the north celestial hemisphere to the south. The ancient Babylonians, Egyptians, Mayans and Greeks were able to mark this event.

From our human perspective we notice that the daylight hours not only are getting shorter, but less than 12 hours since the word equinox means equal day and night time hours. We will also notice the sun sets directly in the west and will rise directly in the east. So any windows facing those directions capture these events.

Finally we all know what fall implies, the sun gets lower in the sky and cold weather fast approaches. Can we wait another six months until spring arrives?

For more information, visit the Wyoming Skies home page ( or send an e-mail to


Posted on Tuesday, September 01, 2009

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