The tide –
a grand uncanny:
water pulling back
and pushing forward,
water in transit as we transit,
the moon transits and
still, is it?
Getting a little more concrete about the “Grand Uncanny”
Several times each day water is pulled back and forth by the mingling of lunar and solar gravitational forces with the earth’s rotation. Wind, weather and even the shape of the land can play a part in these complex liquid movements that we call tides.
The most common type of tidal cycles are semi-diurnal tides. These consist of two high tides of about the same height and two low tides, also about the same height, each day. Semi-diurnal tides occur on Europe’s Atlantic coast and on America’s Atlantic coast, where I first experienced the ocean as a young girl. Our family vacationed at my maternal grandparents’ home on a coastal barrier island every spring. There, I watched migrating birds, ghost crabs and coquina clams on wide, sandy beaches with the Atlantic as a backdrop. I took the regularity of the tides for granted. We planned activities around them, like walking way out to a spit of land only accessible at low tide, or going to the dock to catch Blue crabs with baited traps at high tide. If I was at the ocean it was the Atlantic, and understanding the tides was straightforward. I just needed to visualize the smooth oscillations of high and low tides on a tide chart and remember that the peaks and troughs would hit around 45 minutes later each day.
Then I moved to the West coast. Actually, I was far from the actual coast, which was a place to visit from time to time for a change of scenery. The pounding surf, beautiful blue-green water and mammoth logs littering the shores of Washington, Oregon and California took my breath away. Amid all that drama I paid no attention to the tides. Then we moved again, this time to a small island far from the Pacific ocean but surrounded by salt water thanks to its location near the end of a long strait that is so big it’s called the Salish Sea. Living here has prompted me to get to know the tides again, but I didn’t know how complex tidal cycles can be.
The tidal cycles here are called mixed semi-diurnal tides: there are two unequal low tides and two unequal high tides each day. There are higher high tides and lower high tides, and lower low tides and higher low tides. Did you get that? Apparently mixed tides are a West coast thing, occurring from Mexico to Alaska, along the Chilean coast and in some other locations. My (east coast native) partner likes to theorize about the congruence between left coast attitudes and left coast tides. I thought all tides were as regular as the semi-diurnal ones back on the east coast, but when I look at a local tide table I see irregular waves, with peaks and troughs that vary from deep to average to almost non-existent. Here’s an example: the tide chart for December 25th, 2019.
In addition to daily tidal cycles there are spring and neap tides, which occur everywhere but which, to my mind, might make predicting tides here even more challenging. Spring and neap tides are tidal changes (also called differentials) that are bigger or smaller, depending on the moon phase. At the new and full moon the earth, moon and sun line up and their gravitational pull increases, making high tides higher and low tides lower. At the quarter moons the gravitational pull is lessened, resulting in lower high tides and higher low tides. The upshot is that tide charts are essential around here, whether you’re going fishing, want an easier time getting your kayak in the water, or are looking for Geoducks.
If you read this far you know that much more can be said about that Grand Uncanny we call the tides. Maybe I’ll write again as I learn more. For now just remember: ebb and flow, ebb and flow, ebb and flow….