Thursday, March 03, 2005

Vernal Equinox on the Horizon

Vernal Equinox on the Horizon
by Dave Adalian

Despite winter’s best efforts, spring is finally showing. Buds are turning to blossoms and temperatures are rising. The Sun is making the occasional appearance between rainstorms and soon it will let us know the season has changed.

Imagine the Sun, stars and planets as points of light on the inside of an enormous sphere surrounding Earth. Next, project a circle from Earth’s equator onto the sphere, forming a celestial equator dividing the sky into northern and southern halves. Then imagine another circle on the sphere, this one marking the Sun’s apparent path during the course of a year.

The two circles don’t match, and it’s that misalignment that causes Earth’s seasons. During winter months, the Sun sits south of the celestial equator so the northern hemisphere gets less light and temperatures drop. In the summer, the Sun appears to be north of that line and daylight hours are longer and hotter.

Twice a year, once in March and again six months later in September, the line marking the Sun’s progress and the celestial equator intersect. These events are known as equinoxes, Latin for equal night, and they mark the changes of summer to autumn and winter to spring.

This year’s vernal--or spring--equinox will happen at 4:34 a.m. PST on Sunday, March 20, when the Sun crosses the celestial equator and moves into the northern half of the sky. At that moment spring begins, and for a few days on either side of the event nighttime and daytime will be the same length.

Not coincidentally, the Sun rises exactly east on a few days surrounding the equinox and sets directly west. During the course of summer, the point on the horizon where the Sun rises and sets will appear to move north until summer solstice in June, when it again decsends toward fall.

The vernal equinox has been an important event for humans since the dawn of civilization. Ancient Egyptians paid homage to it by building the Sphinx to face sunrise that day, and closer to home the Mexican Pyramid of Kulkulkan, honoring the serpent god Quetzalcoatl, was designed so the shadow of the god appeared to slither down the steps on the equinox.

But, forget the egg balancing. Raw eggs do not balance on their ends more easily on the equinox. A Chinese New Year’s ritual performed about six weeks before the vernal equinox is the source of the myth.


This column appeared originally in the Visalia (Calif.) Times-Delta on March 3, 2005.