Day Length Rate Change
Name: Don David S.
Date: Thursday, August 22, 2002
Why does the changing day length speed up and slow down
as we approach the solstices?
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:
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
Illinois Central College
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Update: June 2012