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The moon鈥檚 gravity causes the tides by pulling up a lump of ocean under the moon, and creating another, because of the centripetal reaction, on the opposite side of the earth. As the planet spins, these lumps of water stay aligned to the moon, creating a local sine wave from which the handy 鈥渞ule of twelfths鈥� approximation is derived (see Albrecht D眉rer鈥檚 1525 drawing).

Because the moon鈥檚 monthly orbit is in the same direction as the earth鈥檚 spin, the harmonic period is a little more than 12 hours, and the two daily high tides get later every day.听 The sun鈥檚 gravity also contributes, so when the sun and moon are approximately aligned, twice a month at new moon and full moon, you get bigger tides called spring tides.

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But tides can get higher than the regular bi-monthly springs. That鈥檚 partly because the distances and alignments of the earth, moon, and sun vary on time scales longer than the lunar month, giving extra-high tides, for example when there鈥檚 a super-moon. We know when to expect these, but we鈥檙e not always so good at predicting the wind, which also has strong effect on the tide. This year鈥檚 Storm Antoni brought breaking waves almost over the narrow ridge of the spit, driving seafoam like drifting snow, and propelling a talented kite-surfer across a temporary lake that had formed over the reeds near the Cley windmill. And on the 30^th of September, the full moon spring tide coincided with winds that pushed more water than expected into Blakeney Harbour, and up to and into the Old Life Boat House. It also brought an , which was caught by students doing the M.Res. in Biodiversity, Evolution & Conservation, and released safely back into the harbour.

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