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Understanding Tides, the basic physics to prediction

For many, tides are an everyday mystery of nature. Something to admire and a window to the mud flats for romping or digging shellfish before the waters return. But talk to most folks along the coast about how tides work and the discussion becomes murky. The following rudimentary overview will help readers better predict tidal heights. Knowing this will help you time your favorite ocean activity, whether surfing, fishing, sailing up a tidally restricted harbor entry, or digging some steamers for the summer cookout.

The Moon, the driving force

Think about the moon and it's interaction to Earth. How often is the moon full? Funny, but the longer we spend perusing social media in air-conditioned comfort, the further we stray from basic geoscience that shape our world. We experience a full moon roughly once a month, thus the basis for the months of the Julian calendar. About 2 weeks later we view the dark side of the moon, the new moon. Think of Moon's orbit like a Tim Wakefield knuckle ball slowly orbiting Earth. For this discussion, Moon orbits roughly around Earth equator (in truth it crosses over both Northern and Southern Hemispheres). Like that elusive knuckleball, it orbits SLOWLY. Takes one whole month orbit once, and like that knuckleball we see the same side because the spin rotation is equally slow. Great, so one month roughly from full moon to the next full moon. We've already spun through 30 days in that time. Got it. Next part: if you were to track the moonrise daily from the same location, you'd find it rises 50 minutes later every day. Thus, while our Solar Day is 24 hours long, the lunar day we experience as Earth dwelling terrestrials (time it takes for moon to return to same spot) is 24 hours plus the added 50 minutes. Cool huh. But I thought we were talking tides here. Stay with me and you'll see where this is flowing.

moon tide phases graphic

Tides at sea are caused by the gravitational pull of the moon. Think of Earth as that solid baseball. Now cover it with 70% water. Water flows towards the strongest gravitational pull (moon), creating two bulges on the sides facing and opposite the moon. This graphic below helps illustrate the concept.

lunar gravity differential field creating tidal bulge

So two bulges of water on either side of Earth are aligned with Moon's pull, like lenses of water. All the while, inside that lens of water, Earth is spinning. On East coast U.S., we will pass through these bulges twice in 24 hours. Thus, we have two high tides in a 24 hours, each high 12 hours apart. Low tide is half way between, 6 hours after the peak of high tide. As the earth completes it's rotation (solar day) the moon has slowly continued on it's track around the earth. It will take an additional 50 minutes (lunar day is 24 hrs 50 min) to see the moon in the same spot.

So to recap, in this area of the world we have two high tides and two low tides in 24 hrs. The proper name for this is "semi-diurnal tides." Other areas of the world can have mixed diurnal or diurnal tides, so if you plan on cruising the world, dig a little deeper than this discussion. From this info, we know that if we have a noon high tide here, the next high will be roughly midnight. Furthermore, because moon is doing it's knuckleball orbit around the earth, the following day high tide will be roughly 12:50 (50 minutes later). Pretty simple right? Pretty cool too. Now we can figure it out without having a tide chart in hand.

diurnal tide graphs

Remember, this discussion is simply the vertical rise and fall of the tides (high/low/stand), not the horizontal flow of currents caused by tides (ebb/flood/slack). Currents vary greatly and are much more complex to predict. Use a reference such as Eldridge or Reeds to figure these out as current often varies greatly from rise and fall of tide.

Sun's magnetism

So if you've read this far, I assume we're all still riding on the same bus and you're ready for the next progression. If you're utterly flustered, stop now and visit this again later. Tides will vary in strength depending on the alignment of Moon, Earth, Sun. The moon dictates the tides. But the sun also has an auxiliary gravitational pull. When they all line up, we see either a full or new moon. During these times, the high tide will be highest and the low tide will be lowest. It's called Spring Tide. When Moon, Earth, Sun alignment is a right angle, we see the crescent moon, and the sun lessens the pull of moon. Tides are less, meaning a high tide that doesn't rise as far up a piling, and the low doesn't expose as much beach. Called Neap tide. Makes sense right. So looking up in the sky on a cloudless night gives an indication as to strength of tide. How about them apples!

spring and neap tide graph

Well, that's enough for now. Time to put it to use. Now go on, impress your friends like your Crocodile Dundee with your innate tidal wisdom as you strap the surfboard to the roof or toss the clamming rakes in the car.

- Michael Reardon

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