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Treebeard's Stumper
6 December 2002

Time and Tide

It was easy to miss the new moon on Wednesday, but we all noticed the beautiful full moon two weeks ago during the Leonid meteor shower. Surfers know that there were extreme tides both times, like the -1.5 ft low tide for Hike Club at Surf Beach yesterday. We all know the moon causes tides, but why do full and new moons both cause extreme tides? Why don't they work against each other? This is related to another classic stumper. The earth turns once every day on its axis under the moon, so why are there two high and low tides every day instead of just one?

It was like being at the end of the world skimboarding on the tidal flats in the fog at Surf Beach last week under the new moon. There's a 8.3 foot difference between that -1.5 ft low tide and the 6.8 ft high tide just a few hours earlier. It's impressive to look up at your extended arm and think: sea level was up there!

The photo on the right was an even lower -1.7 ft tide at Devereux Point on the night of the winter solstice on December 22, 1999. See my photo of the rising full moon that evening at my More Lunacy (21 Jan 2000) stumper.

The moon is on opposite sides of the earth relative to the sun during new and full moons, so how can they both cause extreme tides?

Answer


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Copyright © 2002 by Marc Kummel / mkummel@rain.org