NIGHT SKY HAPPENINGS

The Sky for September 2016
By Faylene Roth

The Summer Triangle—formed by Vega (Lyra), Deneb (Cygnus), and Altair (Aquila)—dominates the overhead sky with the Milky Way running through it from Cassiopeia high in the northeastern sky to Sagittarius low in the southern sky. Look for Cepheus in the middle of the triangle formed by Cassiopeia, Ursa Minor, and Cygnus. Cepheus looks like a simple drawing of a house with a steeply pitched roof (but upside down). It actually represents the crown of Cepheus, an Ethiopian king. Libra, Scorpius, Sagittarius, Capricorn, Aquarius, Pisces, and Aries represent the constellations of the Zodiac from west to east across the southern sky. The Great Square of Pegasus rising in the eastern sky marks the prelude to summer’s end and autumn’s beginning as Boötes—with its signature red-orange star Arcturus, once the harbinger of spring in March skies—retreats toward the western horizon.

Twilight extends the period of daylight in three stages at each end of the day. Morning twilight begins with astronomical twilight—about 1-1/2 hours (nearly 2 during summer months) before sunrise—as the eastern horizon brightens. Nautical twilight continues—as the overhead sky turns blue and color returns to the surrounding landscape—for another 30-40 minutes. The final stage—civil twilight—provides adequate light for most outdoor activities for the half hour before the sun crests the horizon. The opposite progression occurs after sunset.

VISIBLE PLANETS
Evening (Before Midnight)

Mars – On September 2 Mars and Saturn both set just minutes before midnight. Depending upon your viewpoint, it could be earlier. Distinguish Mars by its reddened disk. Mars moves rapidly eastward compared to Saturn—from Scorpius to Ophiucus on September 2 and on to Sagittarius on September 21. It sets before midnight. (Magnitude -0.2)

Saturn – Watch Saturn set twice on September 1—at 12:04am and again at 24:00 midnight. Unless you find a high vantage point, the planet will drop below the surrounding western rim before midnight both times. This golden yellow planet remains in Ophiucus throughout August and can be distinguished by its brilliant yellow color. By month’s end it sets around 10:00pm. (Magnitude +1.2)

Venus – As bright as it is, Venus will be difficult to catch because it sets within an hour of sunset. Look for it low on the western horizon during nautical twilight as overhead skies darken. (Magnitude -3.8)
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.

MOON HAPPENINGS
September 1 – New moon (3:03am) yields dark skies for several nights.
September 9 – Waxing first quarter lights the evening sky then sets soon after midnight.
September 16 – Full moon (1:05pm) rises at 7:36pm.
September 23 – Dark evening skies return with the waning last quarter moon rising after midnight.
(The moon rises later each day—as little as 30 minutes to as much as one hour. Time of moonrise and moonset may also be delayed in mountainous terrain.)

Twilight is often the best time to look for Venus and Mercury because they frequently rise or set within one-half to one hour of sunrise or sunset. Twilight transitions between night and day in three stages at each end of the day. Morning twilight begins with astronomical twilight as the eastern horizon brightens —about 1-1/2 hours (nearly 2 during summer months) before sunrise when the sun is 18 ̊ below the horizon. Nautical twilight takes over for another 30-40 minutes—as the sun passes 12 ̊ below the horizon and the overhead sky turns blue and color returns to the surrounding landscape. The final stage—civil twilight—begins when the sun ascends to 6 ̊ below the horizon and provides adequate light for most outdoor activities for the half hour before the sun crests the horizon. The opposite progression occurs after sunset. Civil twilight covers the period after sunset during which daytime light quality persists for about one-half hour. Color then fades from the landscape during the 30-40 minute period of nautical twilight during which the overhead sky darkens while the western sky retains color. Astronomical twilight then transitions to night skies that are now darkened along the horizon.

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.

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

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