Q:

Why does the sun move across the sky?

A:

The sun appears to move across the Earth's sky as a consequence of the Earth's rotation. As Earth turns on its axis, an observer on the surface sees the sun appear to come up somewhere over the eastern horizon, rise to a peak around midday and then begin its descent toward the west. This cycle varies slightly from one latitude to another, as well as from one day to another.

Earth spins once on its axis every 24 hours. To a stationary observer at most locations on the surface of the Earth, this motion imparts the appearance of motion to the sun. The exact spot on the local horizon where the sun appears to rise is dependent on where on the Earth the observer is located and what time of year it is.

In addition to spinning on its axis, the Earth inclines at about 23 degrees. This tilt brings first one and then another hemisphere into position to point more directly toward the sun. For observers in tropical latitudes, this motion causes the sun to cross the sky higher and higher throughout the spring until the summer solstice, when it begins moving back down toward the horizon. To an observer near the pole, the sun rises above the horizon only from late spring to early fall. It appears to circle the horizon during this time and disappears entirely through the winter months.


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