Thursday, November 5, 2009

Circling the Circlet

Like Aquarius, Pisces the Fishes is another faint constellation that’s a little difficult to spot. But if you can find the Great Square of Pegasus, you can locate an asterism (recognizable star pattern) in Pisces and thereby orient yourself to the fishy constellation.

On classical star atlas maps, Pisces is typically represented as two fishes, each tied with a string at the tail, and the two strings joined with a knot. One fish heads west, and the other fish heads north.

Pisces is a constellation of the zodiac. The zodiac is a band of twelve constellations that straddles 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.

Pisces in Johann Bode’s 1782 star atlas
Courtesy of
Linda Hall Library of Science, Engineering and Technology

Let’s circle our quarry.

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 the Circlet
Star maps created with
Your Sky

2) First, locate the Great Square asterism in Pegasus, high in the southeastern sky.

3) Now look under the Great Square for a circular arrangement of five naked-eye stars. If you don’t see it, you’ll need to try again at a darker site, away from urban and suburban light pollution. This is the Circlet asterism, the most recognizable star pattern in Pisces. It lies a little east of the Water Jar asterism in Aquarius. The Circlet marks the body of what is known as the Western Fish, the fish that points westward.

The Circlet of Pisces

4) The stars in the Circlet do not have traditional names, but just west of the Circlet is Fum al Samakah, which is Arabic for fish’s mouth, and which marks the gaping mouth of the Western Fish. Fum al Samakah is a blue-white dwarf star, nearly 500 light years away (one light year is nearly six trillion miles).

Next time, we’ll locate the Northern Fish. Then you’re all invited to the fish fry.

Astronomy Essential: There are planets around other stars.

Thus far, astronomers have discovered over 350 planets orbiting stars other than our Sun. These are known as exoplanets. It seems the familiar eight planets of our solar system are not unique in the universe.

Astronomers use a number of methods to detect the presence of planets around distant stars. Orbiting planets can make stars wobble a bit, and that wobble can be measured to confirm the existence of a planet that can’t be directly observed. The periodic dimming of a star when an orbiting planet passes in front of it--that is, transits it from our perspective on Earth--can be measured. And in rare cases, exoplanets can be directly imaged.

The exoplanets found thus far are primarily gas giants and ice giants. The holy grail of current exoplanet detection research is to find terrestrial planets--rocky planets like Earth--orbiting in the habitable zone of their suns where, potentially, liquid water and life might exist.