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Full moons pull.


paul schofield
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I have just seen a report on Newsround regarding the storm hitting the east coast of America later this evening. The report claimed that the ocean waves will be made higher by the full moon ?

Can this be so ? or am I missing something ?

I thought the moon had the same gravitational pull on the oceans regardless of what phase it was in. Surely they must mean the Moons position. Or am I being a bit thick ?

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Well, the Moon's phase *is* its position :)

There are a couple of effects at work. The first is that when the Earth, Moon and Sun line up (called "syzygy"), the gravitational effects causing tides reinforce each other and they cause higher high tides. These are called "spring tides". Water being a fluid, the highest tides don't necessarily absolutely coincide with the alignment, but may occur slightly later.

Obviously if the moon is at perigee there's a stronger gravitational effect than when it's at apogee and that causes higher tides too, but the effect is apparently smaller.

James

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I thought that the higher tides were when the moon was between the earth and the sun so the gravitational forces add to each other and that would mean a new moon, not a full moon.

I wonder if this is a bit of journalistic licence, otherwise called hype.

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I thought that the higher tides were when the moon was between the earth and the sun so the gravitational forces add to each other and that would mean a new moon, not a full moon.

I wonder if this is a bit of journalistic licence, otherwise called hype.

You wonder incorrectly in this instance. Spring (and neap) tides are well understood and documented.

James

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I assume that the cause of this high tide is owing to the 2 high tides on opposite sides of the earth from here the moon is, which I know that 2 exist and have seen an explanation for but never quite intuitively understood.

I promise never to skipper a boat, or to change to being a ships pilot, otherwise lots of shipwrecks.

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Tides are only a part of it. The main threat apart from the wind itself is the storm surge.

Agreed, certainly. But what the OP was referring to was the news reports saying that the effect would be even worse because of the spring tide associated with the full moon. I can see that possibility exists, though the sea level rises at high tide are actually far more complex than that.

James

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You're right, they're fiendishly complicated and influenced by the coastal and local sea bed topography, wind, local air pressure, the forces generated by the planet's rotation and then there's the Sun as well. When the Sun and Moon line up we get the spring neaps and ebbs, when they're at 90 degrees to each other we get the smaller tidal ranges.

Tides and weather are fairly important where I live ;)

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The phase of the Moon has no bearing on the earths tides. The distance of the Moon from Earth does. The closer it is in its monthly cycle...........the stronger the pull.

Well, in the sense that the phase of the Moon and the spring and neap tides have the same cause (the relative positions of the Sun, Earth and Moon) then they are related to the Earth's tides. The point of the Moon relative to perigee or apogee makes very little difference. The height of the tides due to the progression of the Moon through its orbital period can vary the tide height by metres, whereas the difference due to perigee/apogee is, apparently, a matter of a few centimetres.

James

Edited by JamesF
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Well full moon or not Sandy has just been devastating to the New Jersey shoreline. Most of the barrier islands average about 5 feet above normal high tide. The storm surge at this time is estimated between 6 to 11 feet and a possibility of up to 15 feet. Some of the islands have flooded completely across from ocean to bay at the last low tide. Tonights high tide is around 8pm with the storm surge impacting between 5-7pm. It looks like Sandy is going to be one for the record books.

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Not sure why the amplitude of the waves would be any higher... the sea level at high tide will be higher, however, meaning the waves don't have as far to reach to crash inland. Likewise, the sea level will be lower than usual during low tide.

So, it does seems to me the news story is a little misleading.

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I think somebody somewhere along the communication lines has been playing chinese whispers with this one. It sounds like a bit of a muddled report to me. Waves are caused by the friction of air movement against the surface of a body of water. The stronger the winds, the greater the amplitude of the waves. You can have high tides with big waves or small waves ;)

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I don't think anyone was suggesting that the amplitude of the waves would be greater. Just that the absolute wave height might be higher than would otherwise be the case because the tide would be close to its maximum high, giving a higher baseline water level.

James

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