Day and Night Length
Name: Stella G.
I was born in Russia. Throughout many of my childhood
years I watched white nights. Can you please explain me why and how they
occur? I am currently taking an astronomy course at the University of
Toronto and my astronomy book gave a brief explanation about length of
day and night in different parts of the world, but I did not really
understand it. Thank you for your help in advance.
The best way to understand this is to look at a globe of the earth. Most of
these are mounted so that it spins about an axis that is slightly off the
vertical. This is what you want.
These globes are made to demonstrate the orientation of the earth's
rotational axis with respect to the sun. The sun, you see, can be thought
of as being level with the globe. Whether the globe's north pole, south
pole, or neither is closest to the sun depends on the time of the year.
In the summertime, the earth's north pole is angled toward the sun. So look
at the globe, and imagine the sun being in that direction. (If the earth
had a curved bar holding the axle like the globe does, the sun would cast a
shadow onto the earth right underneath this bar.) Spin the globe about its
axis, and see what happens to the area of the globe near the North pole.
The arctic circle never goes into the shadow!
Your home of St. Petersburg is just a little bit south of the arctic circle.
So, in the summertime when the arctic circle is in full view of the sun, St.
Petersburg at its darkest is just ever so slightly in the shadow. (On your
globe model, this is when St. Petersburg is as high as it can rotate.) The
earth's atmosphere extends high enough that the sky above you can catch and
scatter some sunlight, making the sky bright.
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
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Update: June 2012