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Natural Science Forum / Earth Science / Oceanography / October 2003Question about tides
Maybe someone who is knowledgeable about tides can put me out of my misery on a point regarding the simultaneity of neap and spring tides all over the world. In a book about tides, I read : "It is crucial ro realize that spring and neap tides must each occur at about the same time all over the world because the Earth rotates within the tidal bulges. So far, so good, but a few pages after that there's a passage about tidal lags : "... there is a tidal lag, such that high tide commonly arrives some hours after the passage of the Moon overhead. Because the linear velocity of the surface of the Earth with respect to the Moon decreases polewards, the tidal lag is greatest at low latitudes ... but the precise lag is always constant for a particular location". So after all, spring and neap tides can only occur at the same time of day along the same line of latitude, where the tidal lag is equal all around the Earth. At different latitudes, the tidal lag will make them occur at different times. Am I right here? James > So after all, spring and neap tides can only occur at the same time of > day along the same line of latitude, where the tidal lag is equal all > around the Earth. At different latitudes, the tidal lag will make them > occur at different times. Am I right here? No you are wrong! Spring and neap tides occur at the same time of MONTH throughout the world. They would occur at the same time of day along lines of LONGITUDE because of lag if the the oceans covered the entire globe, but this is not true. The tidal waves do not flow around the world. The continent of America ensures that! They actually circulate in tidal basins such as the North Sea, or the South Pacific. See http://www.vims.edu/physical/research/TCTutorial/dynamic.htm or use google to find out about "amphidromic points". HTH, Cheers, Alastair. > In a book about tides, I read : "It is crucial ro realize that spring > and neap tides must each occur at about the same time all over the > world because the Earth rotates within the tidal bulges. I'm afraid that there is in fact no such thing. The Victorian or pre-Victorian concept is still in use as a very badly oversimplified explanation of what may not actually be the truth of the matter. Whereas it is true that tides gain or lose amplitude with each successive "tidal day" it is by no means clear what exactly is happening. > So far, so good, but a few pages after that there's a passage about > tidal lags : "... there is a tidal lag, such that high tide commonly > arrives some hours after the passage of the Moon overhead. Because the > linear velocity of the surface of the Earth with respect to the Moon > decreases polewards, the tidal lag is greatest at low latitudes ... > but the precise lag is always constant for a particular location". It depends on where you are. If you lived in certain parts of Fiji I believe you would find them following the sun. The type of tides seem to vary with latitude. There are a variety of types of tide. But talking about this "Lag": The tides around Britain are high at pretty nearly the same time in Liverpool and Dover but at the same time in Bristol and Hull they are low. 6 hours and 20 minutes or so later, the reverse is true. It is as though a three phase pulse is travelling around Britain and Ireland. It is a mistake (in my opinion) to directly relate the tidal pulse with the position of the moon. After all, if it was directly following the moon it would be racing around the globe at nearly 1000mph. > So after all, spring and neap tides can only occur at the same time of > day along the same line of latitude, where the tidal lag is equal all > around the Earth. At different latitudes, the tidal lag will make them > occur at different times. Am I right here? Spring tides occur at New and Full moon. They reach high or low at different times as stated above. Neap tides occur at the quarter moons with the same effect. Another pointer to the fact that the oceans are not following the moon is that flotsam and boats etc are not set in motion by the moon. If that were so, there would be a jam of detritus swilling around the globe like ice floes. For reasons I don't wish to go into there was an argument going nowhere in the sci.geo.earthquakes newsgroup about this sort of thing. All the posters with differing viewpoints were unwilling to admit that I was right. (Fools!) |
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