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Venice residents watch the Adriatic sea's rise and fall with anxious eyes, keenly aware of how weather can affect its twice-daily ebb and flow. But what is the primary cause of the tides there and elsewhere? Despite what frequent Venice visitor Galileo Galilei thought, tides are indeed related to the motion of the moon (and to a lesser extent, the sun).*
Two oceanic bulges—one on the side of the Earth facing the moon and the other on the side opposite from the moon—move in relation to our satellite as it orbits us, causing in most places two high tides a day as our planet rotates on its axis. The moon's gravity pulls the ocean toward itself to create the near-side bulge, but this doesn't account for the bulge on the far side.
Here, follow a step-through animation that explains why water rises on the side of the Earth away from the moon.
What Causes the Tides? (55K)
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Tidal Curiosities
Why is there sometimes a lag of up to several hours between the arrival of the moon overhead and the arrival of high tide? Why do the biggest tides happen when the moon is full? And why is it that sometimes, in some places, there is only one high tide (and one low tide) in a day instead of two? Find out here.
* According to Galileo, tides were caused by the combined motion of the Earth rotating on its axis and its movement around the sun. To find out more, see His Big Mistake within the "Galileo's Battle for the Heavens" Web site.
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Flash is a plug-in that allows for increased interactivity. If you can see
the animated boxes at left, the plugin is already installed. If you do not see
the boxes, you can install the Flash plugin, or select this feature's
non-Flash version.
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Further Reading
"Our Restless Tides." NOAA/NOS Web site,
http://co-ops.nos.noaa.gov/
restles1.html
"Tides and the Earth-Moon System," by Peter Goldreich. Scientific American, April, 1972.
Bores, Breakers, Waves, and Wakes, by R.A.R. Tricker (New York: American Elsevier, 1965).
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