Constellation of the Month: Piscis Austrinus, the Southern Fish
There are two constellations in the sky that represent fish: Pisces, the Fishes, and Piscis Austrinus, the Southern Fish.
Unlike Pisces, which is a Zodiacal constellation, Piscis Austrinus is not near the ecliptic, but can be found low in our southern sky during October. This constellation is part of Ptolemy's original 48 constellations, rather than being a product of the Renaissance. The Southern Fish is composed mostly of fainter stars, except for the first-magnitude star that marks the fish's "mouth," Fomalhaut.
Eratosthenes, the Greek astronomer, recorded a story that Piscis Austrinus is the parent of Pisces, but the mythology of this constellation is not well known. It is, however, an ancient one, and originally included what is now the constellation Grus, the Crane (the bird, not the lifting device). Many of Grus' stars still have names that refer to their original constellation.
The mythology of Piscis Austrinus comes from the ancient Babylonians, who called this constellation Al Hut al Janubiyy, the Large Southern Fish. The story starts with Atargatis (the Greek translation is Derceto), the Syrian fertility goddess. Her usual consort was Hadad, but on one occasion, she fell in love with Caystrus, a young Syrian. From this love affair Atargatis bore a daughter, Semiramis. In her shame, Atargatis killed her lover and abandoned her daughter, who was brought up by doves and eventually became queen of Babylon.
Atargatis was still so distressed that she tried to commit suicide by diving into a lake at Ascalon in Palestine. There she was turned into a mermaid, half woman and half fish; in reverence to her, the Syrians do not eat fish. Another version has Atargatis falling into a lake at Hierapolis Bambyce near the Euphrates in Syria, where a large fish saved her; the heroic act is commemorated by Piscis Austrinus.
The star Fomalhaut derives its name from the Arabic, Fam al-Hut, which means "mouth of the fish." It is the only first-magnitude star in our evening October sky. Fomalhaut is a fairly young star, only some 100 to 300 million years old. A spectral class A3 star, it is somewhat bluer, 17 times brighter, and twice as large as our Sun.
When astronomers compared A3 stars in the early 1980s using NASA's Infrared Astronomy Satellite (IRAS), they discovered that Fomalhaut gives off more light in the infrared part of the spectrum. This was confirmed in 1998 with very short wavelength radio images, demonstrating that Fomalhaut is surrounded by a donut-shaped disc of dust.
In 2004, when astronomers got a look at Fomalhaut with the Hubble Space Telescope, they saw that the inside edge of the 21.5-billion-mile-diameter disc was much sharper than expected. The ring was also off-center from the star, indicating something else inside the ring was distorting the shape. This was clear if indirect evidence of a planet orbiting inside the disc that had swept up the dust. Among the bright specks in the image, astronomers found a planet orbiting 10.7 billion miles from Fomalhaut, about 10 times the distance Saturn is from our Sun. This was the first time a planet outside our Solar System had been imaged in visible light.
When Fomalhaut was imaged again in 2006, astronomers could see that the planet had moved, indicating an 872-year-long orbit. The planet appears to be between one and three times the mass of our Jupiter, but is brighter than would be expected, leading to the possibility that the planet is surrounded by huge rings that may someday coalesce to form moons like Jupiter's.
The Planets for October 2009
Look to our southeast as it gets dark. The bright star shining there is Jupiter, now the only planet in our evening sky. Jupiter starts the month moving westward in Capricornus, but it becomes stationary on Oct. 13 and then turns around and heads back east. The King of the Gods' disc is 43.5 seconds-of-arc across, at magnitude -2.6. It is highest in the sky around 9 p.m. and sets by 2 a.m.
Watch the Skies
(all times MDT)
The Red Planet starts the month in Gemini and moves into Cancer by month end. At midmonth Mars's disc is 7.1 seconds-of-arc across, growing slowly larger as we move toward opposition. It glows with a ruddy light at magnitude 0.7. Mars rises around 12:45 a.m. and is visible the rest of the night.
Venus is finishing up its appearance in our morning sky, now heading rapidly eastward toward the Sun. Beginning in Leo, Venus travels down that long constellation and into Virgo, another long constellation, where it ends the month not far from Spica. At midmonth Venus rises around 5:30 a.m., shining at magnitude -4.0. Its disc is 93% illuminated and 10.8 seconds-of-arc across.
Saturn has finally gotten high enough in our morning sky to be seen. While it was invisible near the Sun in September, the Earth passed through the Ring plane, so when you see the Rings again, you will be seeing their north face. Saturn is magnitude -1.1 at midmonth and moves eastward among the stars of western Virgo. Saturn's disc is 16.0 seconds-of-arc across while the Rings are 36.3 seconds-of-arc from end-to-end, tipped down 2.3 degrees. On the morning of Oct. 13, Venus will pass half a degree south of Saturn. Through a telescope, you can compare the brightness of Venus' surface, brilliantly lit by the nearby Sun, with the darker appearance of Saturn, 173 times dimmer because of its remoteness from the Sun.
Mercury makes a morning appearance early in the month. At the beginning of October, Mercury is 16 degrees above the Sun, rising at 5:45 a.m. Mercury's disc is a 30% crescent, 8.3 seconds-of-arc across. By mid-month, the Messenger of the Gods is 85% illuminated and 5.6 seconds-of-arc across at magnitude -1.0. A week later it is even fuller as it becomes lost in the morning twilight, sinking down toward the Sun. Mercury passes Saturn on Oct. 8, a scant 22 minutes-of-arc south.
It is getting dark much earlier than it was in summer, so go outside early to enjoy the cooler temperatures and darkening skies, allowing you to "keep watching the sky"!
An amateur astronomer for more than 35 years, Bert Stevens is co-director of Desert Moon Observatory in Las Cruces.