Twice a day, Humboldt Bay inhales and exhales a vast amount of water through the half-mile-wide harbor entrance between north and south spits. How much water? Picture a cube straddling four city blocks, that is, two blocks long, wide and high. That's the volume, on average, that passes through the "throat" of the bay every six or so hours.

Another way to think of it is in comparison with your own breath. You'll inhale and exhale, in your lifetime, some 15 million cubic feet of air in your allotted 200 million breaths. (I'm allowing you to live to 80 -- I trust you're OK with that.) It would take 200 lifetimes of breathing to match the volume of water that goes through the harbor entrance in just one tide.

When I said "twice a day," I meant, twice every 24 hours and 50 minutes, the interval it takes for the moon to be highest in the sky (seen from one location -- say, Eureka) from one day to the next. The reason it's not exactly 24 hours is that, in the time it takes for the Earth to rotate once relative to the sun, the moon is swinging in its orbit around the Earth (which takes a little less than a month), so that Eureka has to "catch up" to the moon, adding an extra 50 minutes a day.

I love living by, and kayaking on, the bay. My morning routine is to walk down Eureka's boardwalk to my favorite coffee shop, checking the state of the tide en route. The water of Humboldt Bay reminds me that no planet is an island, and that Earth participates in a continuous dance with the moon and the sun, controllers of our tides.

Here in our bay, we'll see just a couple of feet between high and low tides one day, while on another we can have a 10-foot difference. The moon contributes about two-thirds of the net tidal effect, while the sun takes care of the other third. When the sun and moon are acting in concert (that is, close to new moon and full moon), we get high-high and low-low tides. Conversely, when they're working against each other (at quarter and three-quarter moons) we get minimal tidal differences.

To fully appreciate the sun and moon's influence on Humboldt Bay, download JTides (www.arachnoid.com/JTides/). Enter "Eureka" in the site menu and marvel at the graphic sinusoidal curve, visual representation of our daily dance with the sun and moon.

For those who want to check the math, tidal influence is proportional to mass divided by the cube of the distance. The sun's mass is 30 million times that of the moon, but it's also 400 times farther away. The net result is that the sun exerts about half the influence on tides that the moon does.

Barry Evans (barryevans9@yahoo.com) is a recovering civil engineer living in beautiful Old Town Eureka. His book Everyday Wonders: Encounters with the Astonishing World Around Us led to a four-year stint as a science commentator on National Public Radio.