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View Full Version : How far does the moon drag ocean water? East-west and north-south.


rodney101
Mar 16, 2016, 08:08 AM
Hello, I'm an author and really need some tide related info for authenticity of script. I can't find this info anywhere, and hope you might advise.

When the moon sweeps 180 degrees across the sky, over an ocean. I realise the water is lifted by up to a meter, but how far and fast does the bulk of the seawater get dragged east to west by the moon's gravity during this 12 hour period?

Also, is there similar drag of seawater from say -50 degree Latitude up toward the Equator.

It's impossible to find out about this online, but logic suggests the water is dragged a lot , but need a rough idea how far.

Any help or advice will be appreciated, Thanks Rodney

ma0641
Mar 16, 2016, 12:18 PM
Good old Gravity! A lot of what you are asking depends on the earth/moon/sun positioning and where you are located, and to the details of the shape of the beach, coastline, coastline depth and prevailing ocean currents. Tides occur every 6hr.13 minutes. With a new moon or a full moon, tides can be quite substantial. In the middle of the lunar cycle, In the middle of the ocean, particularly at the equator, the tide raises the water level only a small amount. In the Bay of Fundy, it can be 50 ft. Consequently, in the Bay of Fundy, vast amounts of shoreline are exposed and then covered. If you pick any point on the globe, Google "Tidal Chart in ......", you can find the timing and elevation change from low to high but not how much surface is exposed and recovered.

rodney101
Mar 17, 2016, 03:25 AM
Thanks ma0641, I need much a more specific answer. Basically the moon swings across an area of ocean once a day. On average, what distance does the moon drag the bulk of seawater over the time it swings overhead, in the open ocean areas.

ebaines
Mar 17, 2016, 05:44 AM
I had provided the following response when you asked this same question on another forum:

"I found a reference in this article: https://en.wikipedia.org/wiki/Tide which suggests the acceleration of ocean water due to the lunar tide influence is maximum of 1.1 x 10^-7 g, which is approximately 1.1 x 10 ^-6 m/s. If we assume a sinusoidal function with 24 hour period, this works out to a max velocity reached of about 0.01 m/s, and amplitude of displacement of about 200 meters. This would be the average deep ocean movement. Obviously tides near shore typically move much quicker than that - they are greatly influenced by the topography of the shore."

I would add to that original response the following: there are factors other than tidal forces that have much more significant influences on ocean currents - consider for example that currents such as the Gulf Stream are driven by heat from the sun combined with the coriolis force from the Earth's rotation.