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Volume 15, Issue 1 ~ January 4 - January 10, 2007

Sky Watch

by J. Alex Knoll

Oddities of Sun and Earth

Fifty-cent words explain the strange interactions of the earth orbiting the sun

Some may blame this spat of unseasonably mild weather on global warming, but perhaps they’ve got it all wrong, as January 3 marked earth’s perihelion, its point in orbit nearest the sun, a mere 91.4 million miles distant. That’s more than three percent closer to the sun than at aphelion, which falls in early July, our farthest point from the sun, roughly 94.5 million miles.

Alas, our proximity to the sun might explain January’s heat in the Southern Hemisphere, which is now in the midst of summer. But for those of us north of the equator it’s a red herring, as it is earth’s tilted axis that causes our changing seasons.

Our elliptical orbit, however, combined with earth’s 231⁄2-degree tilt is responsible for another phenomenon, called the the analemma, an imaginary figure-eight shape based on the sun’s noontime position throughout the year. The peak of the upper loop represents the sun’s noontime position at summer solstice; the bottom of the lower loop marks the sun’s noontime position at winter solstice. The overlap between the two loops is the sun’s position in the sky at noon during both spring and autumnal equinox.

Like a figure eight, the lower loop of the analemma is larger than the upper loop. Near perihelion, earth literally moves faster in orbit due to the sun’s added gravitational pull.

Now imagine a vertical line splitting the figure eight of the analemma equally in two. This is the meridian, a line extending from true north and from true south into infinity. If our orbit around the sun were a true 360 degrees, the noontime sun would never stray from the meridian, nullifying any west- or eastward shift in the sun’s place over the year.

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