November 09, 2007


I've added sidebar links to my constellation posts since I'm starting a collection. I suppose I ought to start doing some more book reviews again, just because I haven't added to that list in a while. I just haven't been reading much lately. My main reading time had been on the exercise bike at the Y, but now I'm lifting weights and walking on the treadmill while listening to podcasts or music, and I can't read while I'm doing either of those.

Today, I'll move a little bit south of Pegasus in the sky and talk about the constellation Pisces. This is a V-shaped constellation that envelopes Algenib, the Southeastern corner of the Great Square of Pegasus. It is very faint, and only one or two of the brightest stars can be seen from within a city. Each end of the V represents one of the two fish, and they are tied together by the (almost) brightest star in the constellation, Al Rischa (or Alrescha in the latinized version), a name that comes from the Arabic Risha for "cord". This is also sometimes referred to as the knot of Pisces, tying the two cords together.

Al Rischa is a very nice binary system and a target for small telescopes. The two stars are separated by only about 2 arcseconds, and both of the stars are themselves spectroscopic binaries (which means we can only detect their companions through features in the spectrum of the blob of light). If you connect Al Rischa in Pisces with Algenib in Pegasus, about halfway along that line is the binary star Zeta Piscium, with a separation of about 23 arcseconds, a very widely separated pair (we know they are together because they move together across the sky with the same proper motion).

Further along that right-hand cord which stretches westward toward Algenib and Markab in Pegasus, you will find the body of one of the fish, also known as the Circlet of Pisces, which looks more like a little pentagon. Just below and to the left of this circle is the current location of the Vernal Equinox, the point where the Sun appears to cross over the Celestial Equator into the Northern Celestial Hemisphere every Spring, the so-called Greenwich of the Sky which marks the point of zero degrees Celestial Longitude (Right Ascension).

Due to Earth's precession, the vernal equinox migrated into Pisces from Aries right around the year 7 B. C., which some scholars say is the year Christ was born. During that winter, Jupiter and Saturn were very close in the sky to the Vernal Equinox point, which some say is the biblical phenomenon of the Star of Bethlehem, about which there is a rich body of literature and speculation. The transition from Aries into the "watery" constellations of Pisces, Aquarius and Capricornus is referred to in Astrology as the dawning of the Age of Aquarius, about which wayyyyy too much has been written.

A very difficult object to locate here is also along the western cord of Pisces, a little further along than Zeta, just below Delta Piscium. It is a lone white dwarf called van Maanen's star about 14 light years away, one of the few (aside from Sirius B and Procyon B) that is easily visible through small telescopes. This is a stellar remnant (also known as a white dwarf), roughly the size of Earth, with a surface temperature of about 50,000 degrees Kelvin and a density about a million times that of water. Burnham has a very nice finder chart for this object.

The only good small telescope deep-sky object here is M74, a faint face-on spiral galaxy only easily visible in the darkest skies, about halfway up the Eastern cord. The features of this nicely symmetrical galaxy are easily seen in this spectacular photo from the Gemini Telescope atop Mauna Kea in Hawaii. It closely resembles the famous Whirlpool Galaxy, which is found near the end of the handle of the Big Dipper, not a good target for this time of year. M74 is right next to Eta Piscium, also known as Alpherg, which is currently the brightest star in Pisces, just a hair brighter than Al Rischa.

The mythology of this constellation is varied. The most common interpretation is that the fishes represent Aphrodite and Eros, who were trying to escape by swimming out of the Eurphrates river away from the underworld and the monster Typhon. They are tied together by a cord so as not to be separated. There may be some tie here, too, to Christianity, since the constellation is associated with the birth of Christ and represents fishes, but that mythology branches off in many different directions with multiple interpretations.

Posted by Observer at November 9, 2007 04:44 PM

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