Monday, May 17, 2004

Science Friday Monday 

It pays to have friends with varying interests. The other day I was looking out the window of my apartment, which faces South and West over downtown Chicago. I noticed that the sun was setting towards the Northwest, which puzzled me. I always figured that the sun would remain in the southern half of the sky, as we in Chicago are always north of where the sun's direct rays hit. Confused, and without a globe and flashlight to experiment with, I remembered that The Goat is an astrophysics Ph.D. candidate at Harvard. What follows is my email to him and his barnyard-esque reply.

My email:
When looking at the sunset the other day, why did it look like the sun was setting in the Northwest? I would think that it would stay south of, you know, the east-west line out my window, as the sun is hitting the earth far south of Chicago. So why was it setting towards the north?
Goat's response:
To really understand this you can shine a light on a tilted globe from
different angles and then spin the globe. However here's a brief

Recall that the Earth's rotation axis is tilted 23 deg from the axis of
its orbit around the sun, and the Earth spins 360 degrees in a day. So
the Sun takes 12 hours to rotate from due East to due West. On the
equinoxes (Mar and Sep 21) the day is 12 hours long and the Sun rises and
sets exactly east and west.

In the winter, because the northern hemisphere is pointed away from the
Sun, the Earth must rotate farther before the Sun comes into view and the
Sun goes out of view earlier. So, in the winter the day is shorter than
12 hours and the Sun is south of the observer all day long.

However, in the Summer because we're tilted TOWARD the sun, the sun comes into view before it is due East of us, and so it appears toward the north.
The Sun then rotates through due East, takes 12 hours to get from due East
to due West, and then keeps going before it goes out of view. So
yes, during the middle of the day the Sun is south of us, but in early
morning and late evening we are actually looking "over the shoulder" of
the Earth, back toward North, to see the Sun, so it is
north of the East-West line.

You can extrapolate this all the way up to the arctic circle, (above 23
north latitude) where in the middle of summer we can see the Sun go
all the way around to due north without setting.

That make sense?

So there you go.
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