Why is it that the tides two miles from here are an hour different than the tides here? If the tidal bulge follows the moon at 1,000 miles per hour, how can the difference be so great?
When the water tries to follow the moon, it runs up against a lot of obstacles, including its own inertia, the shape of the coastline, and the resonances that are set up by the continual tidal motion. In some cases the tides are fighting a permanent current, e.g., going up a river, and this slows down the tidal crest. The result is that the tides at any one place at any given time don’t have a whole lot to do with the moon any more. Q: Why are there two high tides per day, anyway? How is this possible? A: The standard simple answer to this question is that the water on the side of the earth opposite the moon bulges out due to decreased lunar gravity in the same way that the water on the side of the earth nearest the moon bulges out due to increased lunar gravity. This is counter-intuitive in that one might expect all of the water to just rush over to the side where the moon is. To explain this, I quote from “Our Restless Tides,” a NOAA tutorial at http://tidesandcurrents.noaa.gov/restl
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