# How tides work (Original)

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## 13 thoughts on “How tides work (Original)”

1. Dear Steve,
An Interesting article and it is easy to see that the Earth and the sea would be pulled as one object towards the moon if they were a point mass at the Earth’s centre and because they are not, and the Moon’s gravity varies across the Earth’s diameter, then the near-side sea is pulled slightly more and the far-side sea is pulled slightly less, leading to the two opposite tidal bulges.
The Moon’s near-side and far-side gravity strengths are 3.497 micro-g and 3.272 micro-g, which is a very small difference and I was toying with the difference between the heights of the opposite bulges. The near-side tidal force is 115 nano-g and the far-side is 109 nano-g, which would seem to indicate a 5.5% difference.
To what extent, if any, is the impact of the difference in centripetal force at the Earth’s surface necessary to cause circular motion about the Earth-Moon barycentre important ?
Assuming a radius for the Earth at 6371 km, a barycentre radius of 4671 km, and a rotation of 27.32 days, the far-side centripetal field required is 7.978 micro-g and the near-side field required is 1.228 micro-g. These are very different values and far exceeding the tidal field force.
I presume that it makes little difference, but I see some references elsewhere to inertia in explaining the opposite bulge. How to put this to bed ?

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2. Hi Steve,
I wonder if you can explain tidal friction in a little more detail. In particular your explanation implies that the moon arrives (at a particular point above the surface of the Earth) after the high tide has already occured at that point. I have read elsewhere that it is the other way around which seems more intuitive. I have also read that the delay is only generally a matter of about 12 minutes. Is this correct?
Many thanks!
Ben

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3. An interesting read. I can see why there is a high tide on the seas facing the moon but why is there also a high tide at the point furthest away from the moon?

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1. The bulge D may be understood as the moon’s tidal force pulling the planet (not the ocean) toward it. (source: National Geographic)

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2. Hi Graham, Thanks for your comment,
As stated in the post,
The tidal force due to the Moon at a given point on the Earth is the difference between the Moon’s gravitational field at that point and the Moon’s gravitational filed Earth’s centre of mass (which is the same as the Earth’s centre, because the Earth is spherical.

Tidal Force at location x = gravitational field at x – gravitational field at the Earth’s centre.

At the point on Earth closest to the Moon, the Moon’s gravitational field is stronger than it is at the Earth’s centre, so the tidal force points towards the Moon.
At the point on Earth farthest from the Moon the Moon’s gravitational field is weaker than it is at the Earth’s centre, so the tidal force points away from the Moon.
At the Earth’s centre the tidal force is always zero.
At another places on the Moon tidal force is not zero and depending on the result of the subtraction, it may point towards the Moon away from the Moon, or indeed at any direction to the Moon. Some examples of the tidal force at different locations on the Earth are shown in the diagram near the bottom of the post labelled ‘Direction of the Tidal Force’

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4. Interesting part of nautical science as well. One of my favourite books is A History of Marine Navigation by the astronomer Per Collinder.

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5. Fascinating. I grew up by the sea and thought I understood tides. Now I realise that I only half understood them! Thank you.

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6. Hi Steve,
A very good explanation, & very well-worded so that your intended readership, once they’ve started, are happy to keep on reading. I suspect you’ll need to be prepared for questions about the Sun’s influence, additive and subtractive – and what would have happened in the Moon’s absence.

I’d imagine that nearby aliens – who no doubt have somewhat better instrumentation than ours – could measure changes in Earth’s shape & diameter, and work out how much of the surface is water-covered; probably how deep the oceans are at various points and observe the much smaller “tidal” bulges of solid land-masses. They’d no doubt go on to deduce quite a few facts about Earth’s interior.

You mentioned the emergence of life: If Darwin’s “small warm rock pool” origin scenario is right, then I’m sure* that tides in a previously-sterile ocean would have been instrumental in its origin. If it originated in deep ocean, e.g. hydothermal vents, then it might have already had time to evolve to a high degree, and the contribution of tides would have been to allow it to spread (rather reluctantly!) onto land.

* well, almost sure; disregarding panspermia etc.
Regards, David.

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1. Thank you David for your comment. Yes it is interesting to speculate what Earth would be like without the Moon, but I strongly suspect that we wouldn’t be here !
Steve

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