and Sunset Times for
The long periods of summer daylight shorten by 71 minutes in September. Notice that the change is not evenly distributed. Earlier sunsets advance more rapidly than later sunrises. Twilight, too, fades faster because the earth’s tilt no longer faces directly towards the sun. Civil twilight extends usable daylight by 30 minutes after sunset. Nautical twilight continues another half hour as shapes and color disappear from the landscape. In a final half hour of astronomical twilight, the last rays of sunlight fade from the sky. The reverse progression occurs at dawn. Actual time of sunrise and sunset may vary depending upon the surrounding landscape.
September begins with a waxing gibbous moon high in the southern sky at twilight. On September 2 it rises three minutes ahead of Jupiter in the southeastern sky. The full moon rises in the eastern sky on September 4 at 7:33pm. On September 9 the waning gibbous moon rises with the Pleiades about one hour after twilight ends. The last quarter moon occurs September 11. The moon appears in the northeastern sky in the early morning hours of September 14 in Gemini. Mars rises below the moon about 45 minutes later. On September 16 the moon rises below Gemini and just eight minutes ahead of Venus in the northeastern sky. The new moon occurs September 18. A waxing crescent moon reappears in the western sky just below Antares (Scorpius). The first quarter moon occurs September 25. On September 29 a waxing gibbous moon again pairs with Jupiter high in the southern sky by twilight.
Autumn arrives in Moab on Tuesday, September 22, at 3:18pm. Equinoxes occur when the ecliptic (sun’s apparent path across the sky) intersects the celestial equator (projection of earth’s equator into space). Here on earth that means that at 3:18pm the sun will shine directly over the earth’s equator. On that day there is exactly 12 hours between the time the sun crosses the eastern horizon and the time it dips below the western horizon. Due to physics, however, that is not the way we see it. The sun is visible when it is below the horizon because of refraction. That means that light waves bend when they pass through the atmosphere. It’s similar to the way the portion of your leg under water appears to be offset. The times in a sunrise/sunset table indicate when the sun can be seen on a flat horizon rather than when it crosses the horizon. Should you notice, September 25 is the closest day we’ll have to visually experience 12 hours with the sun and 12 hours without.
No major meteor showers occur in September, but there is still significant sporadic meteor activity this month. 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.
STARGAZE AT OLD CITY PARK
Join Red Rock Astronomers at Old City Park on Saturday, September 19, at 8:00pm for a tour of the night sky. Learn to recognize autumn’s constellations as they overtake the eastern sky. Telescope viewing provided by Alex Ludwig. Meet in the grassy field at the lower southwestern corner of the park. Call 259-4743 (Faylene Roth) or 259-3313 (WabiSabi) for more information or if you have a telescope to bring to the gathering. Sponsored by WabiSabi. All ages are welcome and the event is free. In case of rain or overcast skies, join us the following night, Sunday, September 20.
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 - This month Jupiter rises before sunset and sets in the early morning twilight. Because of its brightness it can’t be missed in the southern sky. It rises in the upper left region of Capricornus and will be easy to view high in the sky by midnight. (Magnitude -2.7)
Mars - The small red disk of Mars does not shine bright enough this month to stand out against the background stars. It rises a few hours after midnight and moves through Gemini in a northeasterly direction throughout the month. (Magnitude +1)
Venus - Look for Venus in the eastern sky after 4:00am. It rises a little later each morning so it will not be as high in the morning sky as it was in August. Venus’s orbit is receding from Earth which explains its diminishing brilliance. By September 30 it barely clears the horizon by dawn. (Magnitude -3.8)
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.
Primary Sources: USGS, U.S. Naval Observatory,
View the Andromeda Galaxy under dark skies with the unaided eye. First find the Great Square of Pegasus. Andromeda is identified by two lines that trail off the northeast corner of the square. Count in two stars from the corner on the top line. Look for a fuzzy blob about four degrees up a perpendicular line from that star.
CONSTELLATIONS this MONTH
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