I wanted a little exercise to help me study for my comprehensive exams in my PhD program in Marine Science. So i asked you all to write to me with your burning ocean science question, and i will answer right away, in practice for the exam. Here are some of the q's and a's.
Q: When low tides happen, is the "middle" of the ocean higher? How can it not 'fall' back toward the shores?
Q: When low tides happen, is the "middle" of the ocean higher? How can it not 'fall' back toward the shores?
A: I love this question – I had the same question in my mind
when trying to grapple with the idea of tides!
Tides are the product of gravity, and the force of gravity exerted by
the moon, and the sun. And for each of
these heavenly bodies, there are two forces to consider: gravitational force (gravity) and centrifugal
force. For gravitational force, the
water on earth is pulled toward the sun, toward the moon. For centrifugal force, the water on earth is
pulled away from the sun, away from the moon.
So now envision the Earth, orbiting around the sun, and picture a lens
around the sphere of the earth. Imagine
the earth at 3 o’clock, with the sun in the center, and at 9 o’clock – the
gravitational force pulls the water to opposite sides of the earth at each of
those times. Now the gravitational force
is stronger with the sun (the moon is closer to earth than the sun, but the sun
is MUCH bigger), but both forces play a role in each other’s gravitational
forces and the tides. SO. You have the
earth orbiting the sun. And you have the
moon orbiting the earth. And you have
the earth spinning on its axis each day, as well as the annual orbit of the
sun. So there’s a LOT of motion going
on, and this is why the tides are so complicated at each given place!
To answer your question – yes! When its low tide on Muir Beach, the “middle”
of the ocean is higher – out in the deep Pacific. However, thanks to our gravity field on
earth, and the fact that the ocean basins are so enormous, the basins
essentially absorb that bulge. But this
also creates high tide somewhere else – with the Muir Beach example, lets say
it’ll be high tide at Fukushima, maybe, at the same time. Essentially, the water moves like this
because of the amazing gravitational fields of the earth, sun and moon, and
because of centrifugal forces.
3 comments:
Addendum from your resident physical oceanographer: those sun and moon forces are the drivers of the tide, but the response is set by the resonant frequencies of each ocean basin--just as when you swirl water in a glass, there are some speeds at which you get a big smooth wave going around and others at which you just get a mess. It's better to think of the tide in a particular basin as a pinwheel going around, highs and lows around the edges sweeping counterclockwise (in the N Hemisphere) along the coastline. Here's a movie!
https://www.youtube.com/watch?v=5zi7N06JXD4
THANKS Neil!!! I wish YOU had taught my phys-o class. :( this is awesome, this movie is awesome! thank you.
ps - after watching that movie, i'm wondering: i heard that the tides are highest/lowest at the mid-latitudes because there are both diurnal and semidurnal tides. but the movie seems to suggest that the effect of how large the tide is could be the resonant frequency, as well/instead?
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