SKY HAPPENINGS

The Sky for September 2011
By Faylene Roth

DAYLENGTH
The evening sky darkens rapidly this month as summer twilight fades nearly two minutes earlier each day. By month’s end the period of daylight from sunrise to sunset will be less than 12 hours. Civil twilight extends about one-half hour past sunset. Nautical twilight continues for another 30 minutes with colors and shapes less apparent. Astronomical twilight begins when color and detail disappear from view. Darkness overtakes the sky about 30 minutes later. The reverse progression applies to dawn.

MOON HAPPENINGS
Sept. 4 – First Quarter Moon sets soon after midnight.
Sept. 12 – Full Moon occurs at 3:27am and rises at 7:23pm.
Sept. 20 – Last Quarter Moon rises shortly after midnight.
Sept. 27 – New Moon occurs at 5:09am.

METEOR EVENTS
No major meteor showers occur in September, but there is still significant sporadic meteor activity. Most meteor showers originate in regions dense with particles left in the wake of comets. Sporadic meteor events are the result of random distribution of debris from interplanetary collisions within the solar system. These meteors may appear in any region of the sky and vary in size, color, and speed. If you see a fireball or a bolide, look up The American Meteor Society web page. It tracks these rare sightings. Fireballs are brilliant meteors of -4 magnitude, as bright as Venus. Bolides are meteors that end in an explosion. Make note of the time, location, brightness, color, duration, distance traveled, compass direction, angular elevation, and its track across the sky relative to background stars and constellations. The AMS website provides a reporting form.

AUTUMNAL EQUINOX
An imaginary line called the ecliptic traces the path of the sun across the sky relative to the background stars. Earth’s orbit around the sun creates the seasons because the equator is tilted from the plane of the ecliptic at an angle of 23.5 degrees. An extension of Earth’s equator into the celestial sphere creates the celestial equator. The fall equinox occurs at the point in the earth’s orbit where the plane of the ecliptic intersects the celestial equator.

Neither the northern hemisphere nor the southern hemisphere tilts towards the sun on the equinox. As a result, the direct rays of the sun fall perpendicular to the equator. Sunrise will be due east and sunset will be due west. The autumnal equinox occurs September 23 at 3:05am MDT. At that point the sun will appear directly overhead at 180 degrees E/W longitude—a line running through the eastern tip of Siberia south through the western Pacific Ocean. The length of day and night should be equal, but according to the sunrise/sunset table, day length is seven minutes longer on September 23. This occurs because the atmosphere refracts sunlight around the curvature of the Earth. At sunrise we see the sun before it reaches a horizontal plane with where we are standing. At sunset we continue to see the sun after it dips below the horizon. On September 26 the sunrise/sunset table does show equal periods of day and night. By this time, the days are actually shorter than the nights.


Note: Hold your hand at arm’s length to measure apparent distances in the sky. The width of the little finger approximates 1.5 degrees. Middle, ring, and little finger touching represent about 5 degrees. The width of a fist is about 10 degrees. The fist with the thumb extended at a right angle equals 15 degrees. The hand stretched from thumb to little finger approximates 20-25 degrees. The diameter of both the full moon and the sun spans only 0.5 degree. Adjust for the size of your hand.

Primary Sources: USGS; U.S. Naval Observatory;
Your Sky at http://www.fourmilab.ch/yoursky/

To find out when the space shuttle and International Space Station are visible
from your location, go to: http://spaceflight.nasa.gov/realdata/sightings/index.html and click on Sighting Opportunities.

VISIBLE PLANETS
Jupiter - Look for Jupiter in the eastern sky a few hours after sunset. It remains in the sky throughout the night and will be seen in the western sky at morning twilight. Jupiter can be found east of the Great Square of Pegasus. It moves from Aries into Cetus this month. (Magnitude -2.7)

Mars - Look for a small, but bright, red disk south of the twin stars of Gemini. Mars does not rise until after 2:00am. It is high in the eastern sky by morning twilight. (Magnitude +1.2)

Saturn – Unlikely to be seen without a wide view of the western horizon from a high vantage point. Look for it about 30 minutes after sunset. Follow the arc of the handle of the Big Dipper to bright Arcturus in the western sky. Continue a steep arc down to Spica—the next bright star—in the constellation Virgo. Saturn appears to the right of Spica and is brighter. It will definitely be out of view by month’s end. (Magnitude -0.8)


Note: Apparent magnitude values range from -4 to +6 for most planets and visible stars. The lower the value the brighter the object. A decrease of 1.0 magnitude is 2.5 times brighter.

Follow the Milky Way from Cassiopeia in the northern sky through Cygnus and the Summer Triangle and on to Sagittarius in the southern sky. Gaze through the stars at the western edge of Sagittarius and into the center of our galaxy
—over 26,000 light years away..

Hold the star chart high above your head and match the compass directions to the direction you are facing. Adjust the star chart by orienting Ursa Major (Big Dipper) to match its position in the sky.

The star chart approximates the sky from astronomical twilight to midnight. As the night and month progresses, the constellations shift toward the northwest. The celestial equator is measured in hours (h).
The ecliptic is measured in degrees.