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About the cover

  D e s e r t   E x p o s u r e    August 2008


Constellation of the Month: Ara, the Altar


Another constellation that just peeks over our southern horizon is this month's constellation of the month, Ara, the Altar. This constellation appears even lower in the sky from Greece. So how did Ptolemy know about it? The Greeks borrowed many of their constellations from the seafaring Phoenicians, who sailed the Mediterranean and had colonies in northern Africa, which Ara was easily seen. But the Phoenicians got the constellation myths from the Assyrians, who got them from their forebears, the Sumerians. As different people translated these myths, we ended up with different versions of the same myth.

(click the image for a larger map)


Just peeking over our southern horizon as it gets dark, Ara, the Altar, is far south on the celestial sphere. The Milky Way passes through this constellation, making it home to a number of open and globular star clusters, including NGC 6397. The brightest star in Ara is Beta Ara at magnitude 2.85.

Ara was originally named Ara Centauri, the Altar of the Centaur. The centaur referred to in this myth was the wisest creature on Earth, Chiron — the immortal tutor to Greek heroes like Jason, Hercules, Achilles, Asclepius and Actaeon. Chiron sacrificed Lupus, the Wolf, to the gods on the altar of Ara. Zeus put the centaur, wolf and altar into the sky. A slightly different version (from last month's myth of Lupus) has Lycaon sacrificing a child to Zeus on Mount Lycaeus; the child was turned into a wolf and placed in the sky, as was the altar (as Ara). Another myth says the altar was used by Zeus in preparing for his war with the Titans.

In size, Ara is 67th of the 88 constellations. It lies a little south of Sagittarius, where the center of our Milky Way galaxy is located. When we look toward Ara, we are looking near our galactic bulge. Orbiting many galaxy's bulges, including ours, are the globular clusters. One of these in Ara is NGC 6397, which is about 26 minutes of arc across, glowing at magnitude 5.9.

NGC 6397 is one of the two closest globular clusters to us, only 7,200 light-years away. Compare this to the distance to the center of our galaxy, some 28,000 light-years.

The stars in a globular cluster all formed around the same time, allowing astronomers to study a virtual zoo of similarly aged stars all at the same distance. NGC 6397's closeness to us makes it easy to see the individual stars and study how their different sizes shape how they are formed and age. In the creation of a cluster, only a few large, massive stars form, while the smaller, lighter stars form more often. So, by counting the total number of stars, we can determine how light a star can actually form. If very light stars form, the total number of stars should be very high in the cluster, most of them low mass. Hubble Space Telescope images of the fringe of NGC 6397 show only half as many stars as would be there if very low mass stars existed. This implies that the universe does not seem to form stars that are less than 20 percent of the mass of our Sun.

The Planets for August 2008


Now that we are well away from the June solstice, the Sun is again heading south, making our days shorter and our nights longer. Our August evening sky has all five naked-eye planets visible at the same time, four low in the west as it gets dark and Jupiter way over in the east.

Watch the Skies

(all times MDT)


August 1, 3:12 a.m. —
New Moon,Total Solar Eclipse

August 6, evening —
Venus 1.1 degree north of Regulus

August 8, 1:20 p.m. —
First Quarter Moon

August 12, morning —
Perseid Meteor Shower peaks

August 13, noon —
Venus 0.2 degrees south of Saturn

August 15, 5 p.m. —
Mercury 0.7 degrees south of Saturn

August 16, 2:16 p.m. —
Full Moon-Partial Lunar Eclipse

August 22, 10 p.m. —
Mercury 1.3 degrees south of Venus

August 23, 4:49 p.m. —
Last Quarter Moon

August 30, 12:58 p.m. —
New Moon

Mercury makes an appearance in the evening sky this month. Appearing low in the west just after sunset, Mercury will be near Regulus on August 9, near Saturn on August 15, and with Venus the rest of the month. Mercury will be moving eastward from Cancer, through Leo and into Virgo, coming around from the far side of the Sun. When Mercury first becomes visible, it will be small and full. As the month progresses, Mercury will start to swing around the Sun, becoming larger and less full. When it passes Saturn, the Messenger of the Gods is 86.7 percent illuminated, with a disc 5.3 seconds-of-arc across, shining at magnitude -0.5 and setting around 8:45 p.m.

Venus is also moving out from the Sun, becoming larger and less full every day. On August 5, Venus will be near Regulus as the planet continues to move eastward through Leo and into Virgo by month's end. At mid-month, the Goddess of Love is magnitude -3.9, and its 10.4 second-of-arc disc is 94.6 percent illuminated. Venus also sets around 8:45 p.m.

Staying in Leo this month, Saturn is still moving slowly eastward away from Regulus, setting around the same time as Venus. As Venus passes Saturn on August 13, the rings will be 36.5 seconds-of-arc across while the disc is 16.1 seconds-of-arc across. The southern face of the rings is still visible, tilted up 6.3 degrees.

Mars also starts the month in Leo and moves into Virgo. Mars sets around 9:15 p.m. At midmonth, Mars is magnitude 1.7, and the disc is a tiny 4.0 seconds-of-arc across.

The last planet in our evening sky is the King of the Planets. Jupiter will already be above the eastern horizon as the Sun sets. It will be visible most of the night, setting around 3:15 a.m. Jupiter is just above the handle of the Teapot in Sagittarius, moving slowly westward. At mid-month, Jupiter will be magnitude -2.6, its disc slowly shrinking at 45.3 seconds-of-arc across.

There will be a total solar eclipse on August 1, running around the North Pole, from northeastern Canada, northern Greenland and the Arctic Ocean into Russia, Mongolia and northern China. A partial lunar eclipse will be visible in Europe and Africa on August 16. The Perseid meteor shower peaks on the morning of August 12, but it will be almost full moon, so it will not be a good display. Meanwhile, "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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