Explanation:
As the Earth spins on its axis, the sky seems to rotate around us.
This motion, called
diurnal motion,
produces the beautiful concentric
trails
traced by stars during time exposures.
Partial-circle star trails are
pictured above
over Grants Pass,
Oregon,
USA last month.
Near the middle of the circles is the North Celestial Pole (NCP), easily identified as the point in the sky at the center of all the star trail arcs.
The star Polaris,
commonly known as the
North Star,
made the very short bright circle near the NCP.
About 12,000 years ago, the bright star
Vega was the
, and in about 14,000 years, as the
Earth's spin axis slowly continues to
precess, Vega will become the North Star again.
digg_url = 'http://apod.nasa.gov/apod/ap090909.html'; digg_skin = 'compact';
