and Sunset Times for
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. Sunrise and sunset are calculated for a flat horizon. Actual times may vary depending upon the terrain.
September begins with a thin crescent moon setting in the western sky about 45 minutes after sunset. If you have a high vantage point during the first three nights of September, the waxing crescent moon accents the planetary display of Mars, Venus, and Mercury low on the WSW horizon about 30 minutes after sunset. Virgo’s bright star Spica may also be visible. On September 9 look for Jupiter near the waxing gibbous moon. A full moon rises on September 15 at 7:26pm. In the early morning hours of September 19 look for the waning gibbous moon west of the bright stars Aldebaran in Taurus and Capella in Auriga. On September 23 the moon rises in the early morning sky with Castor and Pollux, the twin stars of Gemini. A new moon occurs on September 29. As with sunrise, actual times may vary depending upon the
SPORADIC METEOR EVENTS
September has no major meteor showers, but it is a good month for viewing sporadic meteor events. Sporadic meteors originate from the debris left over from interplanetary collisions within our solar system. (Most meteor showers, like the Perseids, Leonids, etc., are associated with particle trails left in the wakes of comets.) Early morning, before twilight, provides the best viewing for meteors because the direction of the earth’s rotation converges with the direction of the earth’s orbit at that time. Meteors are then approaching us as the earth moves into the residual dust left by earlier events. Expect to see 10-20 meteors an hour from random directions during September’s sporadic displays. That’s double the number of sporadic events produced in the spring and early summer. Most meteors burn up in the atmospheric layer known as the mesosphere. The mesosphere measures about 20 miles thick and sits 30 miles up atop the ozone layer at the top of the stratosphere.
If you see a meteor brighter than -4 magnitude (as bright as Venus), it is known as a fireball. The American Meteor Society keeps track of these rare sightings. 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 web site provides a reporting form.
Fall arrives in Moab on September 22 at 9:44am. At that moment the sun will be directly over the equator. This occurs when the ecliptic (plane of earth’s orbit) intersects the celestial equator (extension of earth’s equator into space). The earth’s equator is tilted at an angle of 23.5 degrees away from the ecliptic so these two lines only intersect twice a year. When they do, at the vernal and autumnal equinoxes, the sun’s rays shine perpendicular to the earth’s surface. In spring and fall the earth’s tilt does not influence the seasonal climate as it does at the solstices when the northern hemisphere is tilted either towards or away from the sun. On the equinox the sun rises due east and sets due west, and the period from sunrise to sunset is 12 hours. However, we can see the sun below each horizon due to refraction. Light waves bend around the curvature of the earth, allowing us to see the sun a few minutes before and after it reaches the horizon. As a result, the period of daylight on September 22 is actually 12 hours and 8 minutes.
Note: Hold your hand at arm’s length to measure apparent distances in the sky. Adjust for the size of your hand. 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 hand stretched from thumb to little finger equals 20 degrees. The diameter of both the full moon and the sun spans only 0.5 degree.
Jupiter - Find it high in the southern sky each night, shining at magnitude -2.7 in Sagittarius. Jupiter ends its retrograde motion on September 7 which means its westward drift relative to background stars stops and its position drifts eastward each night.
Mars - Look low on the western horizon for its reddish light which continues to shine at magnitude +1.7. Mars sets about one hour after sunset. On September 11 Venus and Mars move to within one degree, with Mercury about three and one-half degrees below. View it from a high vantage point at the end of civil twilight.
Mercury - Use binoculars to find it hovering below Venus in the orange glow of the setting sun about 30 minutes after sunset during the first half of September. Mercury reaches its greatest elongation (distance) from the sun on September 10. Look below Venus for this elusive planet. Mercury, at 0 magnitude, outshines nearby Mars.
Saturn - Wait until after September 18 to view Saturn. It disappeared into the glare of the setting sun in late August and reappears in the early morning twilight about one hour before sunrise, shining at +0.8 magnitude.
Uranus - Use binoculars to look for a small blue-green dot just east of a nearly full moon on September 12 when it reaches opposition (on the side of the earth away from the sun) Shining at +5.7 magnitude in Aquarius, Uranus is barely visible with the unaided eye in a dark sky.
Venus - Watch it rise a little higher in the WSW sky each night about 30 to 40 minutes after sunset. Venus is the brightest object (excluding the moon) in the night sky, shining at -3.9 magnitude. Use it as a beacon to spot Mars and Mercury as they maneuver around one another throughout the first half of September. The closest approaches occur on the 1st, 10th, 11th, and 16th but don’t miss any opportunity to view this display. A high viewpoint is essential.
Note: Apparent magnitude values range from -4 to +5 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,
CONSTELLATIONS this MONTH
As twilight fades, Scorpius rests on the southwestern
horizon with the Summer Triangle overhead
and the Great Square of Pegasus to its
east. Perseus, Andromeda, and Pisces
line the eastern horizon.
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
night sky from astronomical twilight to midnight. As the night
and the month progress, the constellations will shift toward