http://antwrp.gsfc.nasa.gov/apod/ap080719.html
Let me be the first to congratulate Johannes Schedler on one of the most strikingly beautiful astroimages I've seen in a long time. The clarity, detail, and rich color in the image is out of this world!
-Noel
M16 / Eagle Nebula (APOD 19 Jul 2008)
(elsewhere) fuelie wrote:Could someone clarify the status of the Southern Cross as a Constellation versus an Asterism?
Serpens (the "snake") was one of Ptolmey's 48 Constellations. It is now 3 of the 88 modern Constellations (Serpens Caput - the head of the snake, Ophiuchus - the snake bearer, Serpens Cauda - the tail of the snake). None of the above qualify as Asterisms in my book, because there isn't a really bright star nor pattern of somewhat bright stars among them. This is the putty between the recognizable Constellations.APOD wrote:M16 and the Eagle Nebula lie ... toward the split constellation Serpens Cauda (the tail of the snake)
To me, an "asterism" is simply a star pattern that is easily recognized by observers (most used describing the Big Dipper as opposed to the entire Constellation "Ursa Major"). To me, a star pattern can be a Constellation, an Asterism, or both. Some Asterisms comprise parts of two or more Constellations. The Summer Triangle is an Asterism (with its bright stars Altair, Deneb, and Vega) that includes parts of Aquila (eagle), Lyra (lyre/harp), Cygnus (swan), and arguably Delphinus (dolphin). The Constellation Cygnus itself contains the Asterism "Northern Cross".
For comparison, in the vicinity of Crux (the Southern Cross), we have the old Constellation named Argo (or Argo Navis, the "ship") listed as one of Ptolmey's 48. Now among the modern 88 Constellations, it has been divided into Carina (keel), Puppis (rear), and Vela (sails). In some cultures, Crux is part of this picture (the "anchor"). Somewhere along the line, I think the ship's "compass" entered into the picture, too.
Deneb is Arabic for "tail". Like the Italian "Coda" in music, "Cauda" is Latin for "tail". In many places among the stars, there are pictures of creatures with tails. Quite a few Constellations, locations within Constellations, and individual Stars have "Deneb" or "Cauda" in their names.
I am intrigued by APOD's usual usage of "toward" in describing the position of a nebula with respect to a Constellation. To me, the sky is divided into 88 regions called Constellations, and everthing in the sky is "in" rather than "toward" one of them. The use of "toward" implies a view that the Constellations are painted on the firmament (celestial sphere) and that nebulae are floating in space between us and the Constellations in the distance.
If a star or nebula is a part of the group of celestial bodies that makes up the recognizable constellation, then I would say it is in the constellation. If it is just in the area of sky designated by IAU as being the constellation, but not actually a part of the picture, then I would say it is toward the constellation. IMHOapodman wrote:I am intrigued by APOD's usual usage of "toward" in describing the position of a nebula with respect to a Constellation. To me, the sky is divided into 88 regions called Constellations, and everthing in the sky is "in" rather than "toward" one of them. The use of "toward" implies a view that the Constellations are painted on the firmament (celestial sphere) and that nebulae are floating in space between us and the Constellations in the distance.