The movements of the stars have taxed the human mind throughout the ages — from ancient Babylonians seeking to predict sky events, to Greek philosophers wrestling with the structure of the universe, to beginning skywatchers today trying to figure out where's the Andromeda Galaxy.

The turning of the celestial sphere perplexes everyone who takes up skywatching, but sooner or later the picture snaps into place and the whole setup becomes obvious. However, those who think the sky's motion is inherently simple should try explaining to a beginner why every star follows a different curved path across the sky at a different speed. And why do some stars move from west to east while most move east to west? Can you explain why some constellations turn somersaults during the night while others just tilt from side to side?

To bring the sky's motion down to Earth, astronomers for millennia have built little mechanisms that duplicate it. A working model not only illustrates how the sky turns but can help locate objects at any given time. The simplest sky model is a planisphere.

Untold numbers of these star finders have been designed and published in the last century. Even the most experienced observers rely on them, especially at unfamiliar hours of the night. The word "planisphere" simply means flat sphere. It has a map of the sky that pivots at the celestial pole. As the map revolves around the pivot, it slides under a mask that represents your horizon. Turning the map mimics the apparent daily motion of the sky, complete with risings and settings at the horizon edges.