Knowledge: Tides

January 26, 2010

As an at least nominally educated human, I know this much about ocean tides: They happen about twice a day, get real exciting in places like Nova Scotia and Mt. St. Michel, France, and that they have to do with the moon. You want a low one when going razor clamming. You want to get out of the way for high ones.

In pursuit of my cyclical madness, I decided to learn a bit more about tides.

Proxigean Spring Tides

In the simplest explanation, earth’s ocean tides happen because of the moon. Our moon and our earth are attracted to each other in that way that all bodies are because of gravity. While the moon is in no way near to plummeting down to earth or vice versa, there are effects from its slight gravitational tugging on us. The moon, in effect, tugs up a bulge of water: more water is piled up on the side of the earth closer to the moon, and a matching bulge pushes off of the opposite side of the earth as well—that’s high tide.

And that is a gross oversimplification. Already you might wonder, well, all right, if the moon pulls on the earth and the water and stuff, how come there are two high tides (one on the side toward the moon, one on the opposite)? Why not just one big tide? Sigh. It’s because it’s not the moon yanking on the water that pulls it up, not directly. The moon exerts gravitational oomph on the earth and the earth distorts slightly, horizontally. It’s actually some machinations involving the tidal forces in this ellipsoid squashing that causes the tides, but at this point it devolves into physical complexity beyond what I’m willing to deal with (hey, I’m not getting paid).

So that’s the moon and the earth and the diurnal tides that we know intuitively. Just like the moonrise, they come 50 minutes later each day, in lockstep with that satellite of ours. But there are some other factors that can make things interesting: how close we are to the moon, and where the sun is at any given time.

Higher tides—spring tides—take place during full and new moons, when the sun and moon are aligned with respect to the earth. The combined force of the two bodies make for higher tides. Like this:

The moon is closer to earth during different parts of its 27-day orbit and different times of the year, varying by about 50,000km during an average month. Because tidal forces are non-linear, this change in proximity makes a significant impact. Sometimes, not more than once every 1.5 years, things line up perfectly. The moon is in perigee, or, even better, in proxigee, the closest it will be all year. There is a new moon, meaning that the moon is right between the sun and the earth, exaggerating the tidal pull. When this heavenly syzygy occurs, a walloping high water called the Proxigean Spring Tide can occur. The “best” Proxigean tides occur during the new moon because this puts both the sun and the moon on the same side of the earth, and their pulls are combined.

Funny I should choose to learn about this now. Next Saturday, January 30th, will be, if calculations are correct, the biggest, most Proxigean tide since December 2008. It will be of the full moon variant, however. The next new moon Proxigean tide isn’t until 2017. In the graph below, you can see the predicted tidal level amplitude in San Diego get bigger as the perigee/full moon approaches.

San Diego Tides

Predicted San Diego tides for Jan. 25 - Feb. 5, 2010 from

Tidal Bores

The Aegir, River Trent, Morton, Gainsborough, Lincolnshire on Flickr

This shot, just outside of Gainsborough, shows the last Aegir of 2006 (these striking tidal bores don’t happen all of the time!), having traveled about 50 miles to this spot. (Photo by Lincolnian)

Unlike tsunamis, which are often labeled as such incorrectly, a tidal bore is actually a tidal wave. This intriguing phenomenon doesn’t happen in too many places on earth, only in places where high tidal ranges and the right conditions combine with funnel-like local topography that channels the incoming tide into narrow streams or rivers. This causes a marked wave-like effect. The so-called “Aegir” of Trent (oh, so quaint and Anglo-Saxon, that moniker!) in England is a more well-known example of a tidal bore. Just to make it a bit more intriguing and spooky, tidal bores are often accompanied by low-frequency sounds caused by trapped air bubbles. Technically, yes, you can surf on some of these waves.

A slightly scarier-looking tidal bore, in Alaska's Turnagain Inlet.

Though significant tidal bores do not occur anywhere in the continental United States, the highest tides on earth are boasted by Nova Scotia and Anchorage (both experience tidal bores). Nova Scotia is famous for the Bay of Fundy, where resonance between the tidal period (about 12 hours 25 minutes) and the amount of time it takes a wave to traverse the continental shelf and the bay cause enormous tidal fluctuations. So much water pours into the bay during high tide that the entire province of Nova Scotia tilts a bit under its weight. That’s a lot of water and a lot of energy, but damming an arm of the bay might actually increase the resonance and lead to untold flooding nightmare scenarios—okay, it’s probably not that bad, but no one knows quite what would happen.

A similar phenomenon to local tidal bores happens in larger scales, too. In the photo here, taken by astronauts aboard the Intenational Space Station (neat!), you can see a massive-scale tidal bore as the the tide rushes through the channel created by the Strait of Gibraltar.

“In this image, the tidal bore creates internal waves…that propagate eastward and expand outward into the Mediterranean in a big arc (near bottom). Other features can be traced in the sun’s reflections. Linear and V-shaped patterns…are wakes of ships, providing evidence for the heavy ship traffic through the narrow waters between Spain and Morocco.”
Visible Earth Web Site (NASA)

Earth Tides

Water moves around a lot and is where we notice the tides. But tides are all a byproduct of gravitational acceleration, and our planet itself is not immune. The Earth bulges up to about 30cm as a result of the sun and moon’s pulls, and, while this is infinitesimal on our human scale it is enough to affect construction and calibration of the Large Hadron Collider (LHC) near Geneva. It is intriguing to think of the solid planet flexing like that.

How this works and the relative effects of sun and moon and other things involve some Rather Incredibly Complex Math.


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