Thursday, November 19, 2009

Tropical Fish

There’s a third fish in the autumn sky, less heralded than the Pisces duo and--in the Northern Hemisphere--best seen from warmer latitudes such as the southern U.S. and Mexico. Considered a Southern Hemisphere constellation, this tropical creature is Piscis Austrinus the Southern Fish.

One would have to travel well south of the equator to, say, Santiago, Chile or Sydney, Australia in order to see Piscis Austrinus at the zenith, directly overhead. However, many northerners with a clear view of the southern horizon should be able to locate it easily because its brightest star is among the top twenty brightest stars in the night sky. Folks in far northern latitudes, such as Fargo, North Dakota, will have a bit more of a challenge because the star sits only about a fist-width above the southern horizon. A fist-width is the width of your fist, held at arm’s length against the sky, and measured across the knuckles.

Piscis Austrinus (PIE-siss aw-STRY-nuss) swims solo, south of Aquarius. In fact, the stream of water from the Water Bearer’s Water Jar has traditionally been depicted as pouring into the open mouth of Piscis Austrinus.

Piscis Austrinus (under Aquarius) in John Flamsteed’s 1729 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

I’m ready for a dip in warm, tropical waters. How about you?

1) About an hour after your local sunset time, face south. If you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

Looking south to Fomalhaut in Piscis Austrinus
Star maps created with
Your Sky

2) Look south to find the Southern Fish, floating just east of the meridian. You can use the Bandanna of Capricornus to locate it, especially this year while the Bandanna is occupied by brilliant Jupiter. Jupiter is the brightest “star” you’ll see in the southern sky and the first celestial landmark to become visible after sunset.

3) The luminary you’re looking for is the only bright star in the Northern Fish and the only one with a traditional name: Fomalhaut. Fomalhaut (FOAM-uh-lott) is from the Arabic for mouth of the fish, and it marks the gaping maw of the oddly thirsty fish.

Only 25 light years away, this seemingly ordinary white star is surrounded by an emerging solar system four times the diameter of ours. A planet several times larger than Jupiter has been imaged in orbit around Fomalhaut.

It’s difficult to mistake Fomalhaut for any other star, since there are no other bright stars near it. In fact, Fomalhaut has been characterized by a number of writers as “lonely.” Once you spot it and take in the oceanic expanse of dark sky and dim stars around it, you’ll understand why its alternate classical name was Piscis Solitarius--the Solitary Fish.

Astronomy Essential: The zodiac is the band of 12 constellations that lie along the ecliptic.

The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, the Moon, and the planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system. This is why the Sun, Moon, and planets all appear to move along the ecliptic and through the constellations of the zodiac.

Although conventionally we say the Sun, Moon, and planets move “through” the zodiac, we need to remember that the stars of the zodiacal constellations are much farther away than the Sun, Moon, and planets. In essence, they form the backdrop for those solar system bodies.

The constellations of the zodiac are: Aries the Ram, Taurus the Bull, Gemini the Twins, Cancer the Crab, Leo the Lion, Virgo the Maiden, Libra the Scales, Scorpius the Scorpion, Sagittarius the Archer, Capricornus the Sea Goat, Aquarius the Water Bearer, and Pisces the Fishes.