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Name: Don David S.
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: Thursday, August 22, 2002


Question:
Why does the changing day length speed up and slow down as we approach the solstices?


Replies:
First, let us get some motions "out of the way":

The earth undergoes a number of motions. The motion around the polar axis accounts for days and nights. The motion around the sun accounts for the length of the year.

Now the polar axis of the earth is tipped about 23.5 degrees with respect to the plane of motion of the earth around the sun, and this angle stays approximately constant (not exactly because the earth is affected by planetary motions, the fact that the center of gravity of the earth is not at the "center" of the earth, the moon, and other things). But the earth's orbit about the sun is elliptical, not quite circular. Kepler/Newton's principle states that the earth (and other planets) equal areas per unit of time. So, when the distance of the earth from the sun is larger, the rate of motion is different than when the distance from the earth from the sun is smaller. You can visualize this on a very good animated web site:

http://www.opencourse.info/astronomy/introduction/03.motion_earth/

Vince Calder


Don David,

The sun's latitude changes. The sun lights about half of the planet at any moment. As the Earth turns, we enter the lit area, experience a day, and then leave the lit area. When the sun is toward the south, our path through the lit zone has a shorter length. When the sun is toward the north, the lit path has a longer length.

Extreme effects occur at the north and south poles. Consider the north pole. When the sun is toward the north, the north pole is lit. The north pole doesn't move much, so it stays lit all summer. When the sun moves toward the south, the north pole is not lit. It stays dark all winter.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College



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