If those myths are sprouting up again, it must be spring

Today at 5:31 a.m. is the official beginning of spring, at least astronomically speaking.

This is the vernal, or spring, equinox, when the noonday sun is directly over the equator and will continue to rise higher in our sky through the summer solstice in June. It's also typically a time of new beginnings and an appropriate point to dispel some common equinox myths.

Myth No. 1: You can balance an egg on its end only on the equinox.

Not true! This is usually the only day when people (usually television news anchors) try it. However, there is no scientific basis for this myth, and it has been disproved numerous times.

All you need to balance an egg is a raw egg, a hard, flat surface and a steady hand. And it will work any day of the year. A good explanation is on the Bad Astronomy Page, www.badastronomy.com/bad/misc/egg_spin.html

Myth No. 2: The lengths of day and night are each 12 hours on the equinox.

This is not quite true. A look at sunrise and sunset tables will show that the actual date when sunrise and sunset are 12 hours apart is several days earlier for the spring and several days later for the autumnal equinox.

This is because sunrise is calculated from the time the upper edge of the sun becomes visible and the time of sunset is when the upper edge disappears. Each of these skews the length of day by a couple of minutes. For a more eloquent explanation, see the U.S. Naval Observatory's site, http://aa.usno.navy.mil/faq/docs/equinoxes.html

Myth No. 3: This is the end of the rainy season.

Nope. The spring equinox is an astronomical date and has nothing to do with the rainy season. Here in the Bay Area, winter storm systems generally become more scarce as the storm track gradually shifts north during March and April, but there are typically a couple of more days of rain in March and an average of five or six days of rain in April. In other parts of the country where spring and summer storms bring considerable rain, this is only the beginning.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to weathercorner@ggweather.com. Please indicate in your e-mail where you live.