Return of the king – tides, that is!

Going hand-in-hand with this week’s supermoon, king tides are hitting Edmonds beaches, overtopping seawalls, submerging stairs, creating an uphill drive for motorists boarding ferries, and washing all manner of beach debris (and treasures) onto normally dry areas.

Think of king tides as the cousin of supermoons. They’re caused by the same regularly-occurring, twice-daily alignment of the moon, sun and earth as our less extreme tidal cycles, but with a special twist that’s tied up with the irregular shape of the moon’s orbit as it circles the earth.

When you’re talking about the moon’s orbit, circle is a misnomer: the moon’s path around the earth is not a perfect circle, but is more oblong or egg-shaped, with the earth off-center (see diagram below), meaning that at certain times in its travels, the moon is closer to the earth and at other times further away. Astronomers call these two points the perigee (when the moon is closest to the earth) and the apogee (when it is furthest away).

This diagram from NOAA illustrates the egg-shaped orbit of the moon around the earth, and how the combined effect of the moon being at its perigee (closest point to the earth) and at the same time aligning with sun creates the conditions for king tides. Note also that while king tides can occur during both full and new moons, supermoons will only happen when the moon is full (lower diagram).

The closer the moon, the stronger its gravitational pull on the earth.  Land isn’t fluid and can’t flow, but water is and does. So as the moon orbits the earth, tides are created as the waters of the earth are pulled toward the moon. But at those times when the earth, moon and sun align during perigee, the gravitational pull of the moon on the earth — and its waters — is very strong, resulting in higher high tides (king tides, also called perigean or spring tides).

While king tides can occur during both full and new moons, supermoons are only seen when the alignment at perigee happens during a full moon (see diagram). And because at these times the earth is between the sun and the moon, the earth’s shadow will not uncommonly shade the moon, resulting in a lunar eclipse like this week’s “red moon.”

Wednesday’s king tide crested in Edmonds at 7:04 a.m., measuring 12.6 feet.  Another 12.6-foot tide is due Thursday at 7:44 a.m., after which the effect will taper off as the moon’s path takes it further from the earth. But next month look for another supermoon on February 19, with king tides in excess of 12 feet following on the 23rdand 24th.

More information on supermoons and king tides is available here.

— Story and photos by Larry Vogel

 

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