D e s e r t E x p o s u r e
September 2009
Constellation of the Month: Capricornus, the Sea-Goat
Our constellation of the month has some of the oldest mythology of any constellation.
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Capricorn, the second faintest zodiacal constellation, is about 25 degrees up in our southern sky. Jupiter is in Capricorn this month as is M30, a globular cluster that is the last object in a March Messier Marathon. Amateur astronomers often have trouble locating this object low in the east during the Marathon since by the time it is high enough to be found in March, the morning twilight is getting too bright to allow M30 to be found. |
Capricornus, the sea-goat, is a goat from the waist up and a fish from the waist down. This is one of the more unlikely representations in our sky. As far back as the Middle Bronze Age (roughly 2000-1550 BCE), this constellation was represented by a sea-goat. The Sumerians associated it with the god Enki who brought humankind civilization from the sea. Capricornus is low in the southern sky during September, but as the second-faintest zodiacal constellation, it is a little hard to find.
In Greek mythology, Capricornus represents the half-man, half-goat Pan, god of shepherds, flocks, mountain wilds, rustic music and even theatrical criticism. Pan liked to spend his time consuming alcohol and chasing the ladies. He attempted to seduce nymph Syrinx, but she was unimpressed and turned herself into reeds. When Pan came near, the wind made such a lovely sound as it blew through the reeds that he picked some and embedded them in a wax base, creating the panpipes. (He could also make a sudden loud noise that would cause panic — hence the origin of that word.)
Pan rescued the Greek gods twice. During their war with the Titans, Pan blew a conch shell and the sound panicked the Titans into flight. And when the monster Typhon was about to attack the gods, Pan saw him coming and advised the rest of the gods to disguise themselves as animals. Pan dived into a river, but could not decide what animal he wanted to be. He finally decided on a goat, but his trunk was underwater and turned into a fish.
Zeus took on Typhon with some of his trademark lightning bolts. The defeated monster was buried under Mt. Etna in Sicily, where his "breath" can still be seen coming out of the volcano. In gratitude, Zeus placed Pan's image into the sky.
While there are a number of deep-sky objects in Capricornus, the one that is the nemesis of amateur astronomers is M30, a globular cluster. M30's stars are bright enough to be observed in telescopes as small as four inches in aperture. It is magnitude 7.50 and 11 minutes-of-arc across, a relatively easy object. But amateur astronomers have a competition in late March when the Moon is not too full, called the Messier Marathon, and M30 can spoil it for them.
Charles Messier (1730-1817) was a French astronomer who was interested in finding comets. As he swept the sky with his telescope looking for these celestial interlopers, he often tripped over the same fixed fuzzy objects. To keep from wasting time on these non-comets, he made up a catalog of 110 Messier Objects, numbered from M1 to M110. Amateur astronomers use this list to find these relatively bright objects, including open clusters, globular clusters, galaxies and nebulae.
There is a zone east of Capricornus where there are no Messier objects. This means that when the Sun is in that area in late March, it is possible to start finding Messier objects elsewhere as it gets dark. Continuing down the list, the last object in the morning just as it starts to get light is M30. It is often difficult to find low in the east in late March, so an amateur will get 109 objects, but be foiled in the attempt to complete the list. The sky is often just too bright to find M30 in eastern Capricorn by the time it is high enough to be observed.
The Planets for September 2009
Jupiter is now past opposition and appears in our east-southeastern evening sky as it gets dark. This is still a good time to observe Jupiter, with its midmonth disc being 47.2 seconds-of-arc across. Visually, it will be magnitude -2.8, moving slowly westward among the stars of Capricorn. It will pass about 17 minutes-of-arc from the 4.8 magnitude star Iota Capricorni on Sept. 21. The King of the Gods will be highest in the sky around 10 p.m., and sets around 3:45 a.m.
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Mars rises around 1:15 a.m. at midmonth, and is visible the rest of the night. Shining its reddish light at magnitude 0.9, Mars is moving toward opposition at the end of January 2010. At midmonth, Mars is 6.2 seconds-of-arc across. The God of War spends the month in Capricorn, moving from the western edge almost to its eastern boundary by month's end.
The last planet up in the September morning sky is Venus. Starting the month in Cancer, Venus moves rapidly eastward into Leo, almost making it to the eastern edge by month's end. This will be the last month this year Venus will be in the morning sky. Rising around 4:30 a.m. over the eastern horizon, Venus' midmonth disc is 87% illuminated, and becomes increasingly full all month. Venus is down to magnitude -3.9, and its disc is 11.8 seconds-of-arc across. By the end of the month it will be only 11.3 seconds-of-arc across as the Goddess of Love starts to slip into the morning twilight.
For us, Mercury and Saturn are too close to the Sun for observing this month, but they will both be back in October. The Sun rolls through the September Equinox, starting the season of autumn in the northern hemisphere. The equinox this year is on Tuesday, Sept. 22, at 3:19 p.m. MDT.
With the end of summer we will be getting cooler and more comfortable observing weather, so plan to "keep watching the sky"!