Fall, that colorful blend of summer and winter without either's extremes, arrives Saturday morning promptly at 7:49 PST. That's the autumnal equinox, when the sun's zenith is over the Earth's equator. This happens again around March 20; the rest of the year the tilt of the Earth's axis inclines it either slightly toward, or slightly away from, the sun.

"Equinox," or "aequinoctium" derives from the Latin "aequus" (equal) and "nox" (night). However, Sept. 22 is not divided exactly into two, 12-hour halves. Think of it as an elusive moment in time, gone before you can say, "sunrise."

The duration of the sun passing the horizon accounts for the small variation. On the year's two equinoxes in March and September, the sun's center sets 12 hours after it rises. Before and after that, the days get longer or shorter until the next equinox. A "day" begins when the upper edge of the sun reaches the horizon, but does not end until the sun has fully set. Keep in mind that we see sunlight a little longer after it sets; the Earth's atmosphere refracts the sun's light, as if bending it over the visible horizon.

It hasn't always been on this date. Julius Caesar, when he developed the first 365-day calendar around 45 B.C., set the spring equinox at March 25 and assumed time between vernal (spring) equinoxes was 365.25 days. He was off; it's actually about 11 minutes shorter now.

The discrepancy accumulated. Over the next 16 centuries, the equinox moved back until it reached March 11. Motivated by desires to cement celestial events with religious ones, Pope Gregory XIII established the Gregorian calendar, focused on Easter but also annulling the equinox's wide shift. Because the spring equinox was tied to Easter celebrations, the Roman Catholic Church really didn't like the moving target. The Gregorian calendar also deleted 10 days from the old Julian calendar, and returned the off-sync lunar calendar to the actual phases of the moon.

Equinox celebrations are many. Also known as Alban Elfed, Cornucopia, Feast of Avilon, Festival of Dionysus, Harvest Home, Harvest Tide, Mabon, Night of the Hunter, Second Harvest Festival, Wine Harvest, and Witch's Thanksgiving, autumn's emergence was called Efnniht in Old English. Stonehenge and other megaliths were aligned to capture it. Pagan celebrations were grand, typically marking the end of harvest and thanking nature, as do some Native American cultures. The church replaced them with the tamer Michaelmas feast, still observed in some countries. The Mayans erected beautiful pyramids displaying triangles of maximized sunlight.

These examples are only a few. Saturday's equinox also marks the astrological sign of Libra, symbolized by evenly balanced scales. Nature's time of balance is brief - all the more reason to celebrate it.

Sholeh Patrick is a Libran columnist for the Hagadone News Network. Contact her at sholehjo@hotmail.com.