By Jim Kaler

In a video made at a Harvard graduation, graduates, professors, and visitors were asked about the origin of the seasons. Most answered confidently. And most were wrong. The popular view is that the seasons are caused by the changing distance to the Sun. And indeed the distance (which averages 93 million miles) does vary, though by just 3.4 percent. More telling, we are closest around January 4, in the dead of northern winter, and farthest around July 4 (Happy Independence Day). And if distance were the culprit, the seasons would be the same everywhere on Earth. But when it's winter in New York, it's summer in Buenos Aries.

Earth's seasons are instead caused by the 23.4 degree tilt of our planet's rotation axis (about which we go every 24 hours) relative to the perpendicular to the orbit that takes us annually around the Sun. There is no particular reason for the tilt. It's an accident of Earth's formation. All planets have their own peculiar rotational tips, from near zero for Venus (actually 180 degrees, since the planet rotates backwards) to a weird 98 degrees for Uranus, making the distant planet spin on its side. If the Earth's rotation axis were straight up and down, the Sun would hang perpetually over the equator. But it isn't. The 23.4 degree tilt makes the Sun appear to travel on a path, the ecliptic," that is inclined by the same 23.4 degrees to the equator. From our perspective in temperate northern latitudes, the tilt causes the Sun to appear to shift back and forth from highest in the sky at noon on June 21 to lowest on December 21. When the Sun is high, a sunbeam hits Earth more vertically and has a higher heating efficiency than when the Sun is low and the beam is more spread out across the ground. Hence our glorious seasons, without which we might not know Vivaldi.

We are too far north for the Sun to pass overhead. It can, however, cross the zenith between latitudes 23.4 degrees north and 23.4 degrees south, which define the tropics, Cancer in the north, Capricorn in the south. The ecliptic also defines the familiar constellations of the Zodiac. The Sun appears against the constellation Gemini on June 21 (at the Summer Solstice) and against Sagittarius on December 21 (at the Winter Solstice). Because of a 26,000-year wobble in the rotation axis, in classical times the solstices were in Cancer (one constellation east of Gemini) and Capricornus, and thus the names of both the terrestrial circles and the novels.

But as they say on TV: "Wait, there's more." Subtracting 23.4 degrees from 90 gives 66.6, which are the northern and southern latitudes that define the Arctic Circle and Antarctic Circles, the limits to the midnight Sun. At the Earth's rotation poles the Sun is up for six months straight. But it never gets more than 23.4 degrees above the horizon and, as anticipated above, it's cold, even in summer.

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Copyright © James B. Kaler, all rights reserved. These contents are the property of the author and may not be reproduced in whole or in part without the author's consent except in fair use for educational purposes. First published in the July-September 2015 Clark-Lindsey Village Voice.