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Dr. Bozidar Mitrovic: To make a diagram similar or analogous to
the one that I made for the summer solstice, here is the Earth, sunlight
shining on from this side, the North Pole, the South Pole.
This direction is vertical to the ecliptic and this angle between the rotational axis
and that vertical is the largest on the day of winter solstice.
It's 23.5 degrees.
The equatorial plane bisects the North-South axis, and we know that
this angle must be 66.5 because the two complement each other to 90
degrees.
Here is our Arctic Circle. Again, latitude 66.5 degrees North and
here's the Antarctic Circle, latitude 66.5 degrees South. Now, this
vertical...Actually, the sunset line will be along this. This side
of the Earth is in shade and this one is lit up.
You can see that on the day of winter solstice, there's no daylight
above the Arctic Circle. Just the situation that was occurring
above the Antarctic Circle on the day of summer solstice. But now,
on December 21st, on the first day of winter, we have that the
latitudes greater than 66.5 degrees South, one has 24 hours of
daylight. Just what we had on the day of summer solstice above the
Arctic Circle, so the things get reversed.