April 12, 2009 in Keepers Log, Questions for the Keeper

Question for the Keeper: tide

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How does the tide clock work? Does high tide occur at the same time everyday?

At the lighthouse, a tide clock adorns the kitchen wall above a regular analog timepiece. The tide clock has a single hand that sweeps around its face pointing towards high tide or low tide or somewhere in between. The hand is geared slightly faster than the hour hand on a regular clock. A regular clock keeps pace with the sun; a tide clock keeps pace with the moon. Unlike a 24-hour solar day, a “lunar day” lasts 24 hours and 50 minutes, the average between successive moon rises. This happens because the moon revolves around the Earth in the same direction that the Earth rotates on its axis. It takes an extra 50 minutes to for the Earth to “catch up” to the moon. As the Earth rotates, two high tides and two low tides occur each lunar day. Thus, high tides occur approximately 12 hours and 25 minutes apart. For figuring out the timing of high tide, it’s easier to consult tide tables than doing the calculations yourself.

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