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Chapter 6: Seasons of the Year

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The first section (See the chapter “Stargazers and Sunwatchers" in the From Stargazers to Starships FlexBook on www.ck12.org) described the observed motion of the Sun across the sky, in different seasons of the year. This section tries to explain what is seen.

If the Earth's axis were perpendicular to the ecliptic, as in the drawing below, the Sun's position in the sky would be halfway between the celestial poles, and its daily path, seen from any point on Earth, would stay exactly the same, day after day.

Seasons 1

An Earth rotating on an axis perpendicular to the ecliptic.

Each point on Earth would be carried around the axis AB once a day. On the equator (point C) the sun would always rise until it was overhead, then again descend to the horizon. At the poles (A and B) it would always graze the horizon and never get away from it. Except at the pole, every point would be in the shadow half the time, when on the right of the line AB, and would experience night; the other half it would be in the sunlight, experiencing day. Because the motion is symmetric with respect to the line AB, day and night anywhere on Earth are always equal.

Actually, the axis of rotation makes an angle of about 23. 5 degrees with the direction perpendicular to the ecliptic. That makes life a lot more interesting.

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Feb 27, 2012

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Apr 29, 2014
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