If you plan on spending some time at the beach this Thanksgiving week, be careful where you lay out your towel and build your sandcastle!

The boundary line separating sea from shore will be steadily shifting then, and what is high and dry sand at one part of the day could be way under water a little later on. Don’t worry, this major ocean motion is not an early sign of global warming. Instead it’s the purely natural process known as the tides.

Tides are caused by the gravitational force exerted on the Earth’s surface by the Moon and Sun, plus the centrifugal force created by the rotating Earth/Moon system.

The Moon’s pull on our planet is strongest on the side of the Earth which is closest to it. Same story for the Sun.

If there’s a large body of water, like say an ocean, in that part of the world - then the gravitational attraction will cause the water there to bulge outward. We call the bulge high tide. How high the high tide gets depends on the resultant pulling force of the Moon and Sun.

When those two bodies are pulling in the same direction,(new Moon), or in the opposite direction, (full Moon), then their combined force is at its greatest and the bulge is biggest. In addition, the Sun’s pull on our planet is now approaching its yearly peak due to the Earth’s elliptical orbit. We’re closest to the Sun in winter and furthest away in summer.

The combination of a rapidly approaching full Moon and a year ending Winter Solstice will result in a rollercoaster tide ride this week, from ultra high highs to free falling lows. The ocean’s oscillation peaks this weekend when the water rises up to nearly 7.5’ above mean sea level then plunges down to –1.6’ below! That dramatic 9-foot change in the height of the entire ocean along our coast in a period of 7 hours is not exactly in the same league as the Bay of Fundy, but it is about as big as it ever gets here.

Chris Borg

Surfline.com

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