Hi Jeff, >about the *difference* between the amount of sunshine, >twilight, and darkness at the poles and arctic circles >- do you suppose this has to do with the Earth's >aphelion and perihelion differences? Due to the different speed in its elliptical orbit around the sun (faster at perihelion and slower at aphelion) the northern hemisphere summer is a few days longer than the southern hemisphere summer. That explains the difference in sunshine hours etc. between the South Pole and North Pole (and the general unsymmetry between northern and southern hemisphere). But the differences between the equator, arctic circle and poles need some further explanation. If I take the sun as a point in the sky and leave out atmospheric refraction in the sunshine calculations (in other words, if I calculate the number of hours that the geometric center of the sun is higher than 0 degrees) the differences become much smaller. I don't know whether the flattening of the earth is also involved. Maybe somebody else has further ideas? When I did these calculations a few months ago I thought I understood everything. But now I find my head spinning again. I'll need a few more cloudy and sleepness nights to think it over! Gerhard HOLTKAMP Darmstadt, Germany ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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