Observers of marine life in the intertidal zone can take advantage of this week's extreme low tides.

The highest and lowest tides of the year 2000 occur today, Thursday, January 20. Why today? There are three primary reasons. First, the moon has reached full phase. At this position (and at new moon, too) the sun and moon pull in the same straight line, stretching the ocean water in opposite directions over two opposing hemispheres. (Tonight, coincidentally, the full moon we are having undergoes a total eclipse -- best viewed around 8 or 9 p.m.)

Second, today's moon is only one day away from perigee, the time during the moon's monthly journey when it is closest to Earth (the moon is close now, but not nearly as close as it was during the well-publicized "bright full moon" episode last month). The closer the moon is, the more it flexes its gravitational muscle on Earth. Indeed, the difference in the moon's pulling force on opposite Earth hemispheres -- a difference that tides directly respond to -- grows in accordance with the inverse cube of the separation between Earth and the moon. Politely translated, this means that the closer moon has more "leverage" than you might think.

Third, we are now only a month past winter solstice, when the sun lies almost below us in the middle of the night and the full moon passes almost directly overhead at the same time. This alignment tries to put one big hemispherical bulge of water almost straight over our coastline for the occasion of the high tide (but with a substantial delay in time). One-fourth of a day later Earth has turned about a quarter turn and San Diego lies between the bulges; we get a low.

The highest high tide this year (+7.2 feet above mean lower low tide) is due at 8:08 a.m. this morning, and the year's lowest low tide (-1.9 feet) is due at 3:10 p.m. this afternoon. Tomorrow's tides are similar: a high of +7.1 feet at 8:54 a.m. and a low of -1.8 feet at 3:53 p.m. Saturday's not-quite-so-extreme high (+6.8 feet) and low (-1.6 feet) occur at 9:39 a.m. and 4:34 p.m., respectively.

What's in it for you? A lot -- at least for the low tides -- if you like to venture on foot into certain areas of coastline normally covered by the surf, and if you enjoy exploring marine life at the lowest level of the intertidal zone. San Diego County's 60-mile-long coastline, highlighted by several rocky stretches of oceanfront, offers plenty of opportunities. Better spots for tidepooling include Swami's in Encinitas; South Cardiff, just north of Solana Beach; La Jolla Shores (both north and south of the wide, sandy beach); the rocky headland north of Pacific Beach; Sunset Cliffs Park in Point Loma; and the tidepools at Cabrillo National Monument.

Tidepool life is protected, so don't think you can harvest a bit of dinner there or lay your hands on an ornament. Tread lightly on the rocks, taking care to maintain your balance, and strive to keep as little flora and fauna underfoot as possible. Above all, enjoy yourself. If thick clouds aren't rolling in, stick around for the crimson sunset, currently playing at around 5 p.m.