Constellations and Asterisms
Published on Dec 24, 2005 at 7:45 pm.
3 Comments.
Filed under constellations.
Well, here it is Christmas eve, and family have arrived. It is sad that I have such low expectation, but high hopes, and it is always the expectations that come about. I know that I’d be happier if I had someone special to spend the holidays with, but sometimes I think that with the family dramas and slights and so forth that goes on around here that in lieu of what I want, then spending the day completely alone might be better. Well, next week I am going to be out of town, and perhaps things will be much smoother. The holidays seem to be almost set up to make me feel worse.
Anyway, I figured that I’d say a few words about constellations and asterisms. I can’t remember if I did this already or not. In the distant past, people imagined patterns in the sky, and these led to the first constellations. Many of our constellations that we use today go back to the Romans. They had 66 constellations. Parts of the sky had no particularly bright stars useful for making patterns, so these were considered regions of unformed stars. Many of the constellations were figures in stories of classical mythology. So, we have Hercules, Pegasus, Perseus, Andromeda, etc. Of particular interest are twelve constellations along the ecliptic. If you imagine dimming the sun to the point that you could see the stars behind it, then it would appear to be in different constellations on different days as the Earth orbits. The apparent path of the Sun through the sky is called the ecliptic. The planets and the Moon also move through the sky within a few degrees of the ecliptic, so these are also the constellations that they move through. (Actually, there are 13 constellations along the ecliptic now.) These constellations along the ecliptic are known as the zodiac. From Rome, not all of the sky could be seen. So, when explorers ventured far enough south, they saw stars and patterns not seen from Rome. Different people made different patterns. Later astronomers made more pattern from the dim stars ignored by the Romans, filling all parts of the sky with constellations. This was in part due to a desire to designate in what constellations new discoveries, such as nebulae, were found. Also, naming stars required that the stars be in a constellation for both Bayer designations and Flamsteed numbers (see my post on Star Names). Well, there was little agreement between astronomers, particularly astronomers from different nations, as to what constellations existed, and what they were called. So, in 1933, the International Astronomical Union declared 88 official constellations covering the entire sky. Under this definition, the constellations are no longer patterns in the sky, but rather they are regions of the sky. Many of the old patterns are contained within the new regions, but those patterns are no longer of anything but historical interest to astronomers. Now, you can look on a map and see the borders of the constellations drawn. Embarrassingly, some stars with Bayer designations or Flamsteed numbers associated with one particular constellation are now technically located in another constellation. Oh, well. For historical reasons, we won’t rename them. Now, the patterns are not important to professional astronomers, but they are to the amateur or the layman. People like to see the patterns, and of course if you have a small telescope that you point yourself, you need to know which stars are which, and how to find things in the sky. To do this, you rather need to recognize some patterns in order to know what star chart to use. Many really easy patterns, though, do not belong to a constellation, or perhaps they are composed of only part of the stars in a constellation. The most famous example are the seven stars that in North America are called the Big Dipper. Really, these are stars that make up part of the constellation Ursa Major, the big bear. Another famous pattern is the Summer Triangle, composed of the stars Altair, Vega, and Deneb, each of which is in a different constellation! These unofficial, but useful, patterns are called asterisms. So, there is your Christmas astronomy lesson. -Astroprof
Richard Schwartz on March 18, 2007 at 10:59 am: 1
What do they call it when people see patterns in stars, clouds, sidewalk cracks, pizza crust, and such patterns do not really exist? There is a name for this tendency of our brains to organize information into patterns… what is the word?
Astroprof on March 18, 2007 at 1:09 pm: 2
I know that the brain is wired to see patterns, and often sees them where there are none. I don’t know what the name of the phenominon.
Brad Hoehne on October 2, 2008 at 10:35 am: 3
The word for that phenomenon is “Pareidolia”.