Earth's Rotation and Polaris
May 16, 2013
Photographer: John Chumack; John’s Web site
Summary Author: John Chumack
I captured this image of circumpolar star trails from the Cincinnati Astronomy Club’s Dark Sky location in Adams County, Ohio. Polaris, the North Star, is at center. The circular trails result from the Earth’s natural rotation – stars that never rise or set are said to be circumpolar. So how fast are we spinning? Well that depends on where on Earth you’re standing. I took this photo from 38.8 degrees north latitude, so in this 11 minute exposure, the Earth is spinning at about 800 mph (1,288 k/h). For someone on the equator, the spin rate increases to about 1,037 mph (1,670 k/h). Whether or not a star is actually circumpolar is a function of the observer’s latitude. For instance, any star at my location will be circumpolar if it’s less than about 39 degrees from Polaris. At the left side of the image (top to bottom), notice the blurred remnants of our Milky Way. The faint red dashed line running top to bottom near the center of the image is a jet aircraft. Photo taken on December 13, 2012.
Photo details: Camera Model: Canon EOS 40D; Focal Length: 17mm; Aperture: f/4.0; Exposure Time: 669.000 s; ISO equiv: 640; a single 11 minute exposure. Software: Adobe Photoshop 7.0.