Circumpolar Constellations: Cassiopeia

By Peter Forrester | February 23, 2019

Today I begin a series on the constellations that are close to the North Star. Constellations that never set from a person’s perspective are called “circumpolar”. They rotate once around Polaris every 24 hours but never go below the horizon. Which constellations are circumpolar depend on your latitude, or distance from the equator, but from northern latitudes between 40 and 50 degrees north, there are five constellations that are definitely circumpolar, with four of them having at least some fairly bright stars.

Milton’s latitude is 43º 24′ (43 degrees 24 minutes) North. If you imagine a circle sitting on the horizon, with the North Star, Polaris, at its center, then everything inside that circle is circumpolar. So I may extend this series out over more than just the first five parts.

I begin with the one called Cassiopeia (one of Ptolemy’s original 48 constellations in the 2nd Century AD), because it is currently high and easily visible in the early evening. The five brightest stars in this constellation form an “asterism” or group of stars with a very recognizable shape, which depending on its direction can be described as a funny “E”, “W”, “M”, or “3”.

If you imagine another circle around Polaris, this one with the Big Dipper on the edge, then Cassiopeia is also on this circle, across from the Big Dipper. In other words, it’s about the same distance away from Polaris as the dipper, but on the other side. So you will always find her in a northerly direction.

Cassiopeia was a vain queen in Greek mythology, who bragged that she and her daughter Andromeda were more beautiful than the Nereids, the daughters of the sea god Nereus. In anger, the god Poseidon punished her kingdom, and she and her husband were persuaded to sacrifice Andromeda to a sea creature Cetus, but she was fortunately rescued by the hero Perseus. Some depictions of Cassiopeia show her chained to a chair of a type once used in torture, which is said to be the gods’ final punishment.

The brightest star in Cassiopeia, Schedar, or α (Alpha) Cassiopeiae, has an apparent magnitude of 2.2. The γ (Gamma)  star sometimes brightens up to 1.6 but is normally around 2.3 in magnitude. Two of the three other stars in the “W” asterism are between 2.3 and 2.7 in magnitude. The dimmest of the five stars in the asterism has a magnitude of 3.3.

The Milky Way (what appears to be a long, thin cloud but is actually the edge of the disk of the galaxy) runs through Cassiopeia. Thus there are many “deep sky” objects such as star clusters and nebulae, and two supernova remnants (including a famous supernova from the year 1572). There are also several galaxies within Cassiopeia. Some of these deep-sky objects can be seen with the naked eye, but generally it takes a telescope to get a good view.

Cassiopeia was one of the first constellations I learned to identify as a teenager because it is so bright and always so close to the North Star. I hope you will take the time to look up and see if you can find it, and happy stargazing!


References:

The Urban Astronomer. (2008, December 17). Circumpolar Stars. Retrieved from http://urbanastronomer.blogspot.com/2008/12/circumpolar-stars.html.

Wikipedia. (2019, February 12). Cassiopeia (constellation). Retrieved from https://en.wikipedia.org/wiki/Cassiopeia_(constellation) on February 19, 2019.

Wikipedia. (2019, January 22). Cassiopeia of Ethiopia. Retrieved from https://en.wikipedia.org/wiki/Cassiopeia_of_Ethiopia on February 19, 2019.

Wikipedia. (2018, January 28). Circumpolar constellation. Retrieved from https://en.wikipedia.org/wiki/Circumpolar_constellation on February 19, 2019.

Wikipedia. (2019, February 11). Latitude. Retrieved from https://en.wikipedia.org/wiki/Latitude on February 19, 2019

Author: Peter Forrester

I have been interested in astronomy and stargazing for many years, and now delight to offer some of my learning to others through my weekly blog posts.

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