D  e  s  e  r  t     E  x  p  o  s  u  r  e     August 2005



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Constellation of the Month: Sagitta, the Arrow

Archery has been with us for thousands of years, and the "bullet" it uses is our Constellation of the Month, Sagitta, the Arrow. Sitting high in our northeastern sky, Sagitta was seen as an arrow by the Persians, Hebrews and Greeks, but it took the Romans to actually name it. Sagitta's stars are on the faint side, but can be seen in dark skies or with binoculars.

(Click the image for a larger map.)

Facing south and looking almost overhead, Sagitta is to the left (east) of overhead, in the Summer Triangle. This view is at 9 p.m. on Aug. 15. The west end of the Summer Triangle is the brilliant Vega in the constellation Lyra. To the east, Deneb in Cygnus and Altair in Aquila bracket our Constellation of the Month, Sagitta, the Arrow. Hercules is almost directly overhead.

While the Arrow was widely recognized, the archer and its target are less certain. For some, it is Hercules' arrow to kill the Stymphalian Birds—represented by surrounding constellations Aquila, Cygnus and Lyra
—as one of his Twelve Labors. Hercules is on the other side of Lyra, however, so it looks like the arrow missed.

Another of Hercules' labors was to free the Titan Prometheus, who stole fire from the gods and for mankind. Zeus chained Prometheus to a rock and sent the eagle Aquila to eat his liver in unending torture. Hercules killed the eagle with his arrow as his 11th labor and freed Prometheus.

Sagitta is the third-smallest constellation, at only 80 square degrees. Its only interesting deep-sky object is the loose globular cluster M71, at magnitude 8.2 and 7.2 minutes of arc across. M71 was discovered by Philippe Loys de Chéseaux in 1745 and is now believed to be about 13,000 light years away.

A Month of Meteors

August is the month of the Perseid meteor shower, which often produces good displays and can reach 90 meteors an hour. This shower will peak on Friday morning, Aug. 12. The Perseid meteors appear to come from the constellation Perseus, which is in the northeastern sky every morning this month. The tiny dust particles that burn up as they enter our atmosphere are from Comet P/Swift-Tuttle.

The orbits of certain comets, like Swift-Tuttle, come very near the Earth. When the Earth passes through the comet's orbit, the particles from its tail and coma hit our atmosphere at 36 miles per second; their energy of speed is converted into heat and light and we see a brief streak in the sky. All the comet particles are traveling together, so they streak into the atmosphere in the same direction, like railroad tracks. And just like railroad tracks, they are parallel, but appear to come together at a far distance. This perspective effect makes all the meteor tracks appear to come from one place in the sky—in this case, Perseus.

The best way to view the meteor shower is to put a chaise lounge out at a dark site facing east. Remember, the meteors can be anywhere in the sky and you'll be seeing only about one a minute, so be patient.


The Planets for August 2005

Venus and Jupiter are the only two planets left in our evening sky. Saturn is too near the Sun for viewing and Mercury has moved into the morning sky. But Venus and Jupiter are the two brightest planets in our sky, and Venus will be creeping toward Jupiter all month. They will rendezvous early next month. Venus continues to move away from the Sun, but at the same time it is moving further south in the sky, nearer the west-southwestern horizon, making it harder for us in the northern hemisphere to see it. Even though Venus is moving closer to us, it will remain magnitude -3.8; through a telescope it is getting larger and becoming less of a "full Venus." At mid-month it will be about 80 percent illuminated and 13.4 seconds of arc across.

Jupiter is heading for the Sun and becomes increasingly harder to see in the western sky. Each day as Jupiter sinks in the sky, we view it through more of Earth's atmosphere, which absorbs some of the light and makes Jupiter harder to see. Because the atmosphere is always in motion, too, the more turbulence you look through, the more the light from celestial objects is distorted. So Jupiter will seem to dance around in a telescope, blurring its details. Jupiter remains in Virgo and fades to magnitude -1.8 on Aug. 15, when it will be 32.7 seconds of arc across.

Earth is continuing to catch up with Mars, making it appear brighter and larger than last month. At mid-month Mars will be magnitude -0.7 and 12.6 seconds-of-arc, 24 percent larger than this time last month. Mars is now only two months from its closest approach to the Earth this apparition.

Little Mercury makes a brief, low appearance in the morning sky. The Messenger of the Gods will appear on the east-northeastern horizon at 5:45 a.m. on Aug. 13. Watching it each morning at the same time, it will continue to get higher each day until it reaches 6 degrees up on Aug. 19. It will then turn around and head back toward the Sun, becoming lost in the morning twilight by month's end. Enjoy Mercury's little performance and "keep watching the sky"!

 


Astronomical Events (all times MDT)

Aug. 4, 9:05 pm—New Moon
Aug. 7, evening—Venus near the Moon
Aug. 9, evening—Jupiter near the Moon
Aug. 12, morning—Perseid Meteor Shower
Aug. 12, 8:38 p.m.—First Quarter Moon
Aug. 14, evening—Antares near the Moon
Aug. 19, 11:53 a.m.—Full Moon
Aug. 23, 5 p.m.—Mercury farthest west of the Sun
Aug. 25, morning—Mars near the Moon
Aug. 26, 9:18 a.m.—Last Quarter Moon

 

An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.

 

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