June 07, 2004
How can it be that the Earth turns, when after all, the ground seems so solid beneath us? Mountains, such as the square Castle Dome Peak on the right side of this photo), plants, such as the tall saguaro cactus in the center and animals, such as the photographer behind the camera, all seem to stand firmly on an unmoving surface. The power of our senses is so overwhelming that for thousands of years the prevailing belief was that the entire cosmos was revolving around, and centered upon, the motionless Earth.
It took scientific observation and mathematics to overcome these beliefs and to prove our planet rotates on its axis. This rotation, once every 24 hours, literally "makes our day." We're all familiar with the daily apparent movement of the Sun across the sky. But long-exposure photographs (such as this 8-hour exposure taken in the Arizona desert) reveal that the stars move across the night sky in the same way. Close to the axis of rotation, the stars seem to trace circles on the sky. The northern axis, at least in recent history, happens to point very close to a medium-bright star, Polaris (the short, bright arc in this photo). This coincidence has been a great aid to navigation. However, the Earth also wobbles as it spins, and this motion changes the direction of the axis over a 26,000-year cycle. During the next few thousand years, the northern axis will point farther and farther away from Polaris, and its use as a navigational aid will be lost.