Circumpolar stars are those stars that are nearest the north star Polaris. These stars are in a special category because they are always visible every night and rather than rise in the east and set in the west like most of the stars in the sky (and the moon and planets), instead they circumscribe Polaris every 24 hours, hence the designation circumpolar.
From our latitude in San Francisco, 38 degrees north of the equator, we see Polaris 38 degrees up above the horizon due north. Although it is a celebrated star because of its unique location, Polaris is not the brightest star in the sky - that honor goes to Sirius in Canis Major. However, being at the point in the sky where Earth's north pole projects into space means that Polaris does not move over the course of a day or even over the course of a year, a truly unique star!
The "circumpolar region" of the sky is a circle that stretches from Polaris 38 degrees down to the horizon and 38 degrees in every other direction around it. Everything that you can see in this zone remain above the horizon every night. All the stars and constellations move in a counter-clockwise direction around Polaris, much as you might imagine a pinwheel that is anchored to a center point but the body of the wheel can spin in a circle. The same is true of the constellations that are within that 38 degree circle.
Today I was at the San Francisco Waldorf School talking with the sixth grade class about Circumpolar Stars and many other topics of interest for the young astronomer. The students in this class had been creating a number of illustrations of star patterns including the zodiac and the circumpolar stars. The illustration is taken from the workbook of one of the students (Sophia) and is a fine illustration of the circumpolar stars as seen this time of year shortly after sunset. Polaris is the star at the end of the handle of the Little Dipper and the other constellations show are all within the 38 degree circle around Polaris. As mentioned, over the course of 24 hours these stars will all move in a very large circle in a counter-clockwise direction. Thus in the early evening Cassiopeia is near the top of its path looking like an "M" but over the course of the night it will swing around to the left of Polaris eventually moving down low on the horizon by morning. In contrast to this, the Big Dipper is low on the horizon at nightfall but by early morning will have swept around to the right of Polaris and will be high in the sky at sunrise.
For those wishing for a more advanced lesson on circumpolar stars, you'll be interested to know that as you move north from San Francisco, Polaris appears higher in the sky and the circumpolar region becomes larger. What happens if you move south toward the equator? It's a fun thing to ponder.
The Sky This Month – April 2015
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