Sunday 13 July 2003

The best thing about the summer for me is the ocean, or its milder cousin, the bay. Vacationing at the shore means attending to the tide, since what you can do at the water depends on the tides.

We’ve all learned that the moon’s gravitational pull causes high tides, but I’ve never adequately understood why it is that there are two high tides in a day, one nearest to the moon (I get that one), and one farthest from the moon (huh?).

The answers are, of course, to be found on the web, though there’s always the slippery problem of judging the authority of the source. As with many difficult queries, the answer to the double tide question has a number of differing answers out there.

Nova provides the best-illustrated answer: centrifugal force, combined with a non-obvious orbital pattern of the earth-moon pair. This explanation is bolstered by an appealingly terse and fact-filled page from the University of Hawaii (who you would think would know something about oceans and tides), and by the National Ocean Service Tides and Currents page.

The other theories:

  • From the newsletter of the Omaha Astronomical Society, the core of the earth is pulled toward the moon, leaving the far water behind, causing the second high tide.
  • From a page about the Quoddy Loop, water is attracted not just toward the moon, but toward the axis connecting the earth and the moon, so on the far side, it bunches up trying to get to the axis.


Basically, it's 'cos the water nearest the Earth is the most strongly attrracted, the earth itself, being *slightly* further away, is *slightly* less strongly attracted, and the water on the far side of the Earth is less strongly attracted still. This produces a 'stretching' effect - hence the double tide.

There is also a squeezing effect, because the water on the 'sides' of the earth is attracted *directly* towards the Moon's center. Picture this in your head - most of the force is sideways, but an element of the force is pushing the wather inward, towards the center of the Earth.

Bugger - this would be easier with pictures! Does this make *any* sense?

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