Sky Lights

Equinoxes mark changes of seasons throughout the solar system.

By Bob Berman|Thursday, March 2, 2006

The vernal equinox, which occurs this year at 1:26 p.m. EST on March 20 and marks the official beginning of spring, has been a festive occasion from time immemorial. It heralds the seasonal period of renewal when the days become longer than the nights and when warmth and life return to the land. The early Egyptians marked the arrival of spring with a feast and built the Great Sphinx so that it points toward the rising sun on that day. For centuries, Christians have celebrated the resurrection rituals of Easter on the Sunday following the first full moon after the vernal equinox.

The astronomical cause of all this excitement is simple geometry: Earth's axis, which tilts at a 23.5 degree angle, points sideways relative to the sun on the equinox, so that the sun is positioned directly over the equator. With light and shadow cutting neatly across the globe, every populated region enjoys a near-even 12 hours of day and 12 of night. For the next three months the North Pole points increasingly sunward, causing days to grow longer and ushering in summer in the Northern Hemisphere. The equinox orientation of our planet also has a more subtle effect. It angles Earth's magnetic field in a way that most easily catches charged particles flowing from the sun. As a result, March is one of the best months to see an aurora display.

The Sky This Month

March 1 Mercury is the sole bright "star" beneath the crescent moon shining low in the west 40 minutes after sunset.

March 10 Saturn is near the moon. The ringed planet is bright and high in the sky all month, lovely through a small telescope.

March 18 Jupiter is near the gibbous moon; both objects rise around 11 p.m.

March 29 The moon's shadow creates a total solar eclipse through a strip of Africa, across the Mediterranean Sea, and into Turkey and Central Asia.

All month Daylight increases at its fastest pace of the year: it grows longer by three minutes per day in New York City and Denver, seven minutes daily in Fairbanks, Alaska.

All Month Venus is dazzling in the east before sunrise, shining at magnitude –4.5.


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