D e s e r t E x p o s u r e
July
2008
Constellation of the Month: Lupus, the Wolf
This month's constellation of the month is just to the east of last month's. Lupus, the Wolf, can be found along our southern horizon in July. This poor wolf is almost on its back from our point of view.
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Click the image for a larger map
Lupus, our constellation of the month, rides low in the southern sky in July. The Milky Way runs through this constellation, making it home to dark nebulae, as well as open and globular clusters. There are even a few galaxies managing to shine through the dust and gas in the plane of our galaxy. |
Lupus started out as a part of Centaurus. Around 200 BC, Hipparchus of Bithynia separated it out and named it Therion (meaning "beast"), after an unidentified animal Centaurus was sacrificing to the gods. When Johann Bayer published his catalogue in the early 17th century, he identified Lupus as a wolf, based on the Latin translation of Ptolemy's work.
The mythology behind this wolf is rather blurry. In one tale, the impious Lycaon and his 50 sons invited Zeus to a banquet. When Zeus arrived and was seated at the table, Lycaon presented him a dish made out of human flesh, possibly Lycaon's own son Nyctimus or Zeus' son Arcas. Outraged, Zeus zapped Lycaon and his sons with his lightning bolts — or turned them into wolves and placed them in the sky as Lupus.
A slightly different version has Lycaon sacrificing a child to Zeus on Mount Lycaeus; immediately after the sacrifice, the child is turned into a wolf and placed in the sky. This gave rise to the legend that a man was turned into a wolf at the annual sacrifice to Zeus Lycaeus. The wolf could regain human form by abstaining from eating human flesh for 10 years. The cult of Zeus Lycaeus, attributed to Lycaon, was eventually replaced by the familiar Hellenic Zeus after the Ionian invasion of Greece. Lycaon was transformed into an evil spirit — apparently another case of the victors rewriting history.
Lupus contains the dark nebula B 228. When we think of objects in the sky, we often think of bright glowing stars, red-colored nebulae and glittering galaxies. But a dark nebula shows itself not with its glow, but with the lack of glow. These objects are not so different from any of the bright nebulae: Instead of the gas and dust being illuminated (reflection nebulae) or energized (emission nebulae), the nebula is simply a dark cloud obscuring our view of the stars behind it.
Many of these dark nebulae are located in the plane of the Milky Way. As we move along the Milky Way, we see the glow of the stars and glowing dust and gas of our galaxy. We then come upon a dark irregular blob. No stars shine in this blob, even though there are many stars all around it. This blob of unilluminated dust and gas absorbs the light from anything behind it, leaving a "hole" in the sky. If a really bright star is behind the dark nebula, it will show through, but the dust and the gas will absorb the blue end of the spectrum, making the star appear very red. This is just like the way the Sun gets red as it heads toward sunset.
The designation B 228 is from Edward Emerson Barnard's (1857-1923) catalog of dark nebulae. The "B," of course, stands for "Barnard" and the number is the sequence in the catalog. Barnard issued his first catalog of dark nebula in 1919 with 182 objects, then extended the list in a posthumously published 1927 paper. In the 1919 article, dark nebulae were referred to as "black holes" — certainly not what we think of when we hear that term today.
The Planets for July 2008
Venus finally moves away from the Sun into our evening sky this month, moving from Gemini through Cancer and ending up in Leo, not far west of Regulus. Look low in the west-northwest during the last week of the month, and you will find Venus below and to the right of the Regulus, Saturn and Mars grouping. On July 31, Venus is magnitude -3.9, and its 10.1 second-of-arc disc is 96.7 percent illuminated as it moves out from behind the Sun.
Mars moves eastward among the stars
Watch the Skies(all times MDT)
July 1, morning — Mercury farthest west of Sun.evening — Mars 0.7 degree north of Regulus
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of Leo this month. Starting out less than a degree from Regulus on July 1, Mars passes about three-quarters of a degree south of Saturn on July 10 and continues eastward for the rest of the month. At midmonth, Mars is magnitude 1.7 and the disc is a bare 4.3 seconds-of-arc across.
Saturn is also moving eastward in Leo, but much more slowly than Mars. The Earth is still moving away from the Ringed Planet, so Saturn continues to shrink. At midmonth, Saturn's disc is 16.5 seconds-of-arc across while the Rings span 37.5 seconds-of-arc. The southern face of the Rings is still visible, tilted up at 7.9 degrees. Saturn sets by 10:30 p.m.
Just east of the Teapot in Sagittarius, Jupiter is moving slowly westward during July. It rises around 7:40 p.m. and sets around 5:45 a.m. In opposition on July 9, Jupiter will be at its maximum size for this year, 47.3 seconds-of-arc across and shining brightly at magnitude -2.7.
The only planet in July's eastern morning sky is Mercury. It is farthest from the Sun (22 degrees) on July 1, and immediately starts moving back toward the Sun. At that time, the Messenger of the Gods is magnitude 0.5; its disc is 8.1 seconds-of-arc across and 36.0 percent illuminated. During July, Mercury will travel from eastern Taurus through northernmost Orion and into Gemini, where it disappears into the solar glare just after midmonth.
July 4th will see man-made fireworks fill our sky. Next month, we will be able to see some of nature's best natural fireworks, the Perseid meteor shower. So get some sleep this month so you can stay up and see nature's fireworks as you "keep watching the sky"! k
An amateur astronomer for more than 35 years, Bert Stevens
is co-director of Desert Moon Observatory in Las Cruces.