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Volume xviii, Issue 11 ~ March 18 - March 24, 2010

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Sky Watch

by J. Alex Knoll

Atop the Celestial Equator

From here, the sun marches north

Winter comes to an end, Saturday, March 20, with the vernal equinox, the day when the sun rises due east, reaches its zenith directly above the celestial equator and sets due west. Around the world the day is divided equally between light and dark. For us in the Northern Hemisphere, this marks the beginning of spring, while for those south of the equator, it is the start of autumn.

Like the ecliptic, the celestial equator is an imaginary line circling the earth. But while the ecliptic is based on the apparent path of the sun, moon and planets around earth, the celestial equator is a ring projected into space from earth’s own equator, intersecting our horizon at due east and at due west. Not only does the line demarcate the planet’s northern and southern hemispheres, it also divides the celestial sphere into north and south.

Imagination aside, the effects of the sun crossing the celestial equator are very real. For the next three months the sun will climb higher into the norther sky, bathing us in more light each day and, as a result, more warmth.

While we can now describe the science behind the sun’s movement, our ancestors no doubt understood its importance and recognized that this day marked the midpoint between both the seasons as well as the sun’s travels from its southern extreme in winter to its northern-most point in summer.

The Mayans aligned their pyramids with the cardinal points of the compass. And at the famous pyramid of Chichén Itza in the Yucatan, the rising and setting sun on equinox illuminates the northern stairway, making the stair’s edge appear as a snake sliding up and down the pyramid.

The Anasazi Indians of Chaco Canyon in New Mexico made a hole between boulders that the sun shines through on equinox, illuminating a spiral painted on an opposite cliff face.

Illustration: © Copyright 1925 M.C. Escher/Cordon Art-Baarn-Holland; Graphics: © Copyright 2010 Pacific Publishers. Reprinted by permission from the Tidelog graphic almanac. Bound copies of the annual Tidelog for Chesapeake Bay are $14.95 ppd. from Pacific Publishers, Box 480, Bolinas, CA 94924. Phone 415-868-2909. Weather affects tides. This information is believed to be reliable but no guarantee of accuracy is made by Bay Weekly or Pacific Publishers. The actual layout of Tidelog differs from that used in Bay Weekly. Tidelog graphics are repositioned to reflect Bay Weekly’s distribution cycle.Tides are based on National Oceanic and Atmospheric Administration and are positioned to coincide with high and low tides of Tidelog.

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