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

Constellation of the Month: Aquila, The Eagle

Aquila, the Eagle, was Zeus' thunderbolt bearer. During the war with the Titans, Aquila was always at his side providing him with thunderbolts. Aquila would then retrieve them so Zeus could throw the thunderbolts again. When the gods needed a new cupbearer, Zeus sent Aquila down to Earth, where he found Ganymede and flew him up to Mount Olympus. Aquila also acted as a torturer: When Prometheus stole fire from the gods and gave it to man, Zeus chained Prometheus to a rock and had Aquila eat out his liver. Since he was an immortal, Prometheus' liver grew back overnight and Aquila came back the next day to eat it again. Hercules finally freed Prometheus and killed Aquila. For being a loyal companion, Zeus placed Aquila up into the sky.

Click the map to enlarge it. Looking toward the southeastern horizon around 8 p.m. in September, you can find Aquila about 30 degrees up. It contains Altair, the southernmost star of the Summer Triangle. The other two are directly north (Deneb in Cygnus) and west (Vega in Lyra).

Aquila's brightest star, the first magnitude Altair, is the southernmost star in the Summer Triangle, that obvious trio of first magnitude stars in our summer sky. The triangle is made up of Altair from Aquila, Vega from Lyra and Deneb from Cygnus. The Summer Triangle is an asterism, not a constellation--an unofficial grouping of stars. The triangle's rising in the morning sky heralds the beginning of summer, and its setting in the evening the beginning of winter.

Altair is a white star glowing at magnitude 0.8 that is 1.6 times the size of our Sun. Surprisingly, Altair spins on its axis every 6.5 hours, almost a hundred times faster than our Sun. Centrifugal force from the rapid spin causes Altair's equator to bulge out, making this star look like a squashed beach ball, twice as wide as it is high. Of course, you cannot see this, even in a telescope, since all the stars are too far away to see their shape.

Another star in Aquila is the 3.7-magnitude Alshain. This star is relatively nearby, only 255 trillion miles (44 light years) away. It is a yellow star, much like our Sun, but a little larger and five times more luminous. Unlike our Sun, Alshain does not travel alone through the galaxy. It has a companion, a small red star, known as Alshain B. This companion star is only two-fifths the diameter of our Sun and only 1/500th of its luminosity.

As we view this pair from Earth, they are 13 seconds of arc apart. This would put Alshain B 17 billion miles from Alshain A, which is 180 times the distance from the Earth to the Sun. If you were able to drive from A to B at 80 miles per hour, it would take you 24,000 years. (And you thought Albuquerque was far away!) Imagine driving day after day as you slowly watch Alshain A shrink away behind you and Alshain B gradually get larger over the 24,000=year road trip, and remember that these two are close together in astronomical terms!

The Planets for September 2005

Venus and Jupiter continue their dance from last month. Venus is heading east among the stars and passes Jupiter on the Sept. 2. The next evening, Spica, Venus and Jupiter will be in a horizontal line in that order from left to right. Venus will continue moving eastward, passing closest to Spica on the evening of Sept. 4. Venus will continue moving eastward the rest of the month. Even though it continues to move away from the Sun, it is moving south along the ecliptic and so will not get much higher in the sky. Venus is magnitude -4.0, and through a telescope it is less "full" than last month and 16.1 seconds of arc across.

Jupiter is a poor telescopic target, being low in the west after sunset. Still in Virgo, Jupiter is 31.2 seconds of arc across and dims to magnitude -1.7.

Mars is getting brighter and larger as it comes closer to the Earth. It is an increasingly tempting telescopic target now at magnitude -1.3 and 15.9 seconds of arc across. Mars rises in the east around 10 p.m. There has been a rumor that Mars was going to come close the Earth in late August and be as big as the full moon, but that is just not true: Mars will not reach its closest point to the Earth until late October and will still be a telescopic object at that time.

Saturn reappears in the morning sky in September. Still in Cancer, Saturn is a slowly improving telescopic target as it rises higher in the sky each day. It will be 30 degrees above the horizon on the Sept. 15 as the sky starts to lighten. Saturn's ball is 16.9 seconds of arc across, while the rings are 38.4 seconds of arc across, with the southern face tipped toward us at an angle of 19.3 degrees. The Galileo spacecraft, now orbiting Saturn, was out of communication with us from July 22-27 as Saturn appeared to skim the surface of the Sun as viewed from Earth.

The Earth goes through the Autumnal Equinox on Sept. 22. At that instant, the centers of the Sun and Earth will be in a straight line, with the noontime equator of the Earth directly between them. After that date, the noontime equator will be above the Sun-Earth line, making the Sun appear south of the celestial equator. This marks the beginning of the astronomical season of autumn. As the weather cools, you can spend more time outside in the evening so you can "keep watching the sky"!

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

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