By Jim Kaler

While not the brightest star in the sky, Polaris, the North Star, is among those with the broadest of stellar roles. Ranking 50th, it's the luminary of Ursa Minor, the Smaller Bear, and sits at the end of a beloved pattern, the Little Dipper. The star is easy to find, as the two front bowl stars of the Big Dipper point toward it. It's close to the North Celestial Pole, the point about which the stars seem to rotate over the course of the night. Face Polaris and you look north. As you walk toward the star, it and the celestial pole rise higher above the horizon. The first rule of celestial navigation is that the elevation of the pole above the horizon in degrees is your latitude. We are at a latitude of 40 degrees north. Look 40 degrees up from the horizon and there it is. 'Twas not always so. The Earth's spin axis wobbles about the orbital axis (to which it is inclined by 23.4 degrees, the cause of the seasons) over a period of 26,000 years. The "North Star" thus keeps changing. The current alignment of Polaris near the pole (actually 3/4 of a degree away) is thus an accident. In Homeric times, the Little Dipper's brighter front bowl star (Kochab) was near the pole, while the ancient Egyptians followed Thuban in Draco (the celestial dragon). In 12,000 years we get bright Vega in Lyra, and in 26,000 we're right back to Polaris. About 150 BC, the Greek astronomer Hipparchus divided stars into six groups, from first "magnitude," the brightest (22 of them), to sixth, the faintest he could see. We use his scheme yet today. Polaris, which is visible any time from almost all the northern hemisphere, initially set the modern system's mathematical scale at magnitude 2.0. But Polaris's brightness is slightly variable, which makes it a poor standard. It's an ageing type of variable called a "Cepheid" (after the constellation Cepheus, in which the first one was found) with a mass some six times that of the Sun. Cepheids were the initial candlesticks that allowed Edwin Hubble to discover the expansion of the Universe in the 1920s. Polaris is unique in that over the twentieth century its variability nearly ceased, allowing us an unusual opportunity to watch stellar evolution in action. For previous "Astronomy Corners," go to

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Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the Winter-Spring 2016 Clark-Lindsey Village Voice.