Back in the 1200s, John Abbot published “The Flood at London Bridge,” the first accurate prediction of tidal changes in the ocean. But Mr. Abbott was never able to explain why the tides changed. Many people thought that the Earth itself might be alternately inhaling and exhaling sea water.

Today we know that tides are caused by gravity from the moon (68%) and the sun (32%). Because water is a fluid, it can be moved rather easily by gravity. When the moon is directly overhead, it is powerful enough to lift up the water in the ocean below it by about 6 feet. The flowing water from each adjacent side of Earth leaves behind a low tide, while on the far side of Earth there is a “slightly-lower high tide.”

Because Earth spins a quarter of the way around every six hours, coastal locations experience high and low tide twice each per day, roughly six hours apart. But it’s not exactly six hours, of course, because the moon is moving too. And that is how tides work. Nice job today, everyone. John Abbott would be proud!

January 11 – High and Low Tides (pg406)