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Volume 14, Issue 38 ~ September 21 - September 27, 2006

Sky Watch

by J. Alex Knoll


Better thank your lucky stars for earth’s 231⁄2-degree axis of rotation

Three minutes past the stroke of midnight Saturday morning marks the autumnal equinox, when the sun transits above the celestial equator, an imaginary line extending from earth’s own equator outward into space. During this 24-hour rotation of the planet, the sun remains perched above the equator, shedding light equally on all parts of the earth and equally dividing day and night.

For the three months hereafter, the sun will dip farther and farther south, shedding less and less light on those of us in the Northern Hemisphere and leading us closer and closer to winter. The sun reaches its farthest point south on December 22, winter solstice, when it shines above the Tropic of Capricorn, 23 degrees and 27 minutes below the equator.

After that, our shortest day of the year when the sun rises and sets its farthest south, the entire process reverses, leading to the vernal equinox and then the summer solstice, when the sun hovers above the Tropic of Cancer, now 23 degrees and 27 minutes north of the equator. All this shift is a result of earth’s 231⁄2-degree orbital tilt.

Were earth without this axis, it would spin fully upright like a well-balanced top. Worse still, the sun would remain forever perched above the equator, robbing everyone of our seasons. It might be pretty nice for those of us around 40 degrees latitude, especially if you love temperate spring and fall weather; but the heartland of America’s agricultural belt would take a punch in the gut. And for those closest to the equator, it would be forever summer — and a much hotter one at that. The farther from the equator one lived, north and south equally, the colder and more perpetually so the weather.

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