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Seasons explained: It's all about tilt

Posted: Sunday, January 05, 2003

One could just say it's warm and thus summer when we're close to our sun and cold and thus winter when we're far away. However, that cannot be correct, because we had our closest approach to the sun at 91.4 million miles Friday and are farthest at 94.5 million miles on July 3 or 4 -- the average distance being 93.1 million miles.

Also, the northern and southern hemispheres have opposite seasons. For example, Australia has summer right now.

Instead, seasons must be explained differently: It's the tilt of Earth's axis that produces them; not because we like that theory, but because it delivers the correct explanation. And, taking it to the extreme in Alaska, we can really see and feel what that tilt does to the sun's appearance in the sky -- low arc in the winter and short days, high arc and long days in the summer.

To understand seasons best, model the seasons by using a globe and a bright lamp. Position the globe about three feet from the lamp, so that the northern part of the axis is tilted toward the lamp and also points at a corner in your room. The corner substitutes for the north star. Notice that our sun shines more directly onto the northern hemisphere and that we get long days, that means it's summer.

Now make the Earth orbit the sun with the axis pointing at the corner all the while keeping a distance of about three feet and turning the lamp so it keeps shining on Earth. On the opposite side, the Earth's axis is now pointing away from the sun. We get sunlight under a much more shallow angle, which produces a lower arc, and days are short, that means it's winter.

Notice that the southern hemisphere gets more direct sun and longer days. Thus, it's summer for them. Fall and spring are in between these extremes.

Check out my seasons lecture at http://www.wncc.net/courses/aveh/lecture/lecmove.htm#Seasons.

The following description and diagram are applicable all winter in the evening and throughout the night, from December on and even into April.

Find Jupiter in Cancer and Saturn between Gemini and Taurus.

While stars are fixed relative to each, you'll notice that planets move. The dominating constellation is Orion, the hunter. It is many people's favorite because of its brilliance. Four stars arranged in a rectangle (with red Betelgeuse, blue Rigel, Bellatrix and Saiph), which outline the body, while three stars make up the belt with the Orion nebula as the sword beneath it.

Above Orion you find Taurus, part of it in the shape of an arrowhead, including red Aldebra. And above that are the Pleiades or Seven Sisters -- a must for binoculars. Left of Taurus is the pentagon Auriga with yellow Capella. Beneath Capella appear Castor and Pollux, the twins in Gemini.

Then we close this circle toward the horizon, with Procyon and Sirus -- the brightest star in the entire sky. Since Jupiter resides further east, also look for Cancer and Leo. (The Kenai Peninsula College book store sells star finders for $5.)

As for the morning sky, you can't miss bright Venus above the southeastern horizon in the predawn sky and reddish, but much fainter Mars, close by. While Venus will vanish into the dawn sky by February, Mars will get ever better this year, appearing in the morning during winter and spring, and in the evening during fall and next winter. Notice that bright Jupiter appears in the morning above the western horizon.



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