SKY HAPPENINGS

The Sky for September 2015
By Faylene Roth

The Summer Triangle—composed of bright stars in Lyra, Deneb, and Aquila—dominates the overhead sky. The Great Square in Pegasus fills the eastern sky while the long arcs of Andromeda and Perseus stretch into the northeastern sky. Scorpius lies along the southwestern horizon, and Fomalhaut—loneliest star in our skies—lights the void above the southern 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.

DAYLENGTH
A rapid decrease in daylength occurs over the next two months—74 minutes each month. Rapid because Earth’s orbital speed increases when the sun’s gravitational grip tightens as the Earth approaches the halfway point in its orbit toward perihelion (nearest point to the sun). Decreasing because the tilt of the Earth’s axis is moving away from a direct face-off with the sun causing the sun to rise and set over the steep bulge of the equator.

MOON HAPPENINGS
September 5 – Dark evening skies. A waning last quarter moon rises after midnight.
September 13 – Dark sky period for several days before and after the new moon (12:41am).
September 21 – Waxing first quarter moon brightens evening skies until after midnight.
September 27 – Largest supermoon of the year rises at 7:03pm, becomes full at 8:50pm, reaches perigee one hour later. Supermoon has come to mean any full moon within 90 ̊ of perigee (closest distance between moon and earth each year). This year we have three supermoons: in August, in September, in October. The September full moon is the true supermoon of the year, and it promises a total lunar eclipse as well. Since the moon clears a presumed flat horizon at 7:03pm, the eclipse will be in progress by the time we see it.
Partial phase begins – 7:07pm
Total phase begins – 8:11pm
Greatest eclipse occurs – 8:47pm
Totality ends – 9:23pm
Partial phase ends – 10:27pm
(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.)

AUTUMNAL EQUINOX
On September 23 at 2:21am MDT the sun’s apparent pathway across the sky (the ecliptic) crosses the celestial equator. Anyone standing on the Greenwich Mean Line (aka 0 ̊ longitude) will see the sun in the eastern sky at 8:21am GMT. If the sun were eclipsed at that moment, s/he would see the constellation Virgo behind it. The September star chart shows where the ecliptic crosses the equator in Pisces. This marks the sun’s position at the Spring equinox. The zodiacal light is visible in the morning pre-dawn sky for about one month before and after the equinox. About one-half hour before astronomical twilight begins, look for a white cone of light due east above the horizon. The zodiacal light is created when sunbeams directed straight up from the horizon illuminate dust motes circulating within the inner solar system. The cone of light extends as high as 65 ̊ above a level horizon.

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.

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.