Friday, April 8, 2011

The Sky In Motion

To a first approximation, most of the objects on the sky appear to move as if they were all fixed to the inside of a gigantic sphere with the Earth at its centre (the aptly named "celestial sphere").
That (apparent) movement is the result of the (very real) movement of the Earth in relation to the rest of the universe.

Photography by Oleg Zhukov

As we all know, the Earth rotates around its axis, towards the east, once every approximately 24 hours. The result of this movement is that the objects we see on the sky will seem to rise from the eastern horizon, move across the sky and set on the western horizon some time later. This apparent movement is actually an "orbit" around the celestial poles, parallel to the celestial equator, and this has some interesting effects.
The first is that the actual path of an object across the sky will depend on your local latitude.
The second effect is that, if the declination of a star is larger than 90° minus your local latitude, that star will never set. 
There is, of course, the opposite case: from any location but the equator, some stars will be permanently below the horizon.

Photos - Oleg Zhukov
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