Full Moon and Tides?

As a result of the whole Sandy thing, Discovery Channel Canada aired a special on the storm during their usual daily science program called Daily Planet. In general, Daily Planet is quite good but they made a major gaffe during part of their narration. When listing the factors that made Sandy such a dangerous storm, they included a line about the tides being higher due to the full moon. My first reaction to that was how could the portion of the lit lunar surface visible to the earth have anything to do with a gravitational phenomenon like tides?

It turns out that while there is something to the tides being higher during a full moon, it really isn’t anything to do with the fullness of the moon but rather the relative positioning of the sun, moon, and earth. In other words, the full moon is not the cause but rather an effect of the same alignment that causes higher than average high tides. The same effect also occurs during the new moon for the same reason.

This is, of course, not the only impact on the tides. Lunar perigee causes higher tides do to increased gravitational influence from the moon – lunar perigee being the closest approach of the moon to the earth during its orbit. If this happens to coincide with the timing of a full or new moon (when the earth, moon, and sun are all in a single line relative to each other), the two effects are cumulative leading to even higher tides.

None of this is terribly surprising and it is no doubt patently obvious to mariners and other folks who deal with tides on a daily basis. But to someone like me who has lived his entire life in the interior of a continent, it was not obvious and it took me some thinking to realize what the mechanism for the effect could be. Of course, now that I have worked out what that mechanism is, the effect is obvious. I haven’t worked out what the magnitude of the effect would be but now it is clear to me that there would be one.

It occurred to me to wonder why they didn’t include an aside comment about the tide effect being due to the alignment of the earth, moon, and sun. About four seconds of thought clued me in. This was not about the tides, but about the storm itself. Had they been discussing tides, no doubt it would have been accompanied by cool graphics depicting alignments and so on. Instead, the simplest reference for the alignment was to use the full moon since during the full moon, the alignment is necessarily there since the full moon is an effect of said alignment.

Apparently, it’s never too late to learn something new. And that is a wonderful thing.


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