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What a Difference a Place Makes
By Jeff Kanipe
posted: 05:39 am ET
15 May 2000
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Brought to you by Starry Night Brought to you by Starry Night Monday, May 15 If you live at high northerly latitudes, you have no doubt noticed that it's taking longer and longer for night to fall. In some extreme cases, like Alaska, it never gets completely dark. This time of year, daylight prevails because the Northern Hemisphere is inclined toward the sun, thanks to Earth's faithful 23.3-degree axial tilt. But the amount of daylight is not the only thing that varies with latitude. So does the position of the constellations with respect to the horizon. In the Northern Hemisphere, this latitudinal effect is particularly conspicuous around Polaris, the North Star. In the region immediately surrounding Polaris, the stars never rise or set but simply pivot concentrically about the celestial pole. This part of the perennial sky is referred to as "circumpolar." Every place on Earth has a circumpolar sky. Let's take three examples: Dallas, latitude 32 degrees; Minneapolis, latitude 45 degrees and Anchorage, latitude 65 degrees. No matter where you are in the Northern Hemisphere, the elevation of Polaris equals your latitude. So from Dallas, Polaris will be found 32 degrees above the north horizon point; Minneapolis, 45 degrees and Anchorage, 65 degrees. Consequently, the altitude of Polaris also determines the amount of circumpolar sky visible at that location. Hence, the circumpolar sky in Dallas lies within a 32-degree radius in any direction from Polaris. In Minneapolis, the circumpolar sky has a radius of 45 degrees; in Anchorage 65 degrees. (Check out today's star maps for a visual comparison.) So, if you're standing at the North Pole, how much of your sky is circumpolar? The answer: All of it! Tonight's moon phase. 
** Put the sky in the palm of your hand. Download SPACE.com's Skywatch, along with the latest space news, into your Palm Pilot or other handheld device. **Jeff Kanipe is the author of A Skywatcher's Year, an astronomy guide just published by Cambridge University Press. He is a former editor at Astronomy and StarDate magazines and a writer for the Earth & Sky radio series. The images in Skywatch are produced by Starry Night software.
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