Tides - 7

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Let us do a simple "Thought Experiment"...

I take a big wooden sphere, dip it in oil, and take it out so that the wooden sphere is covered with a thin uniform film of oil.

And I place this sphere at the center of an elevator falling freely under Earth's gravity, its cable having been snapped.

What do I see?

If the gravity of Earth were uniform, the sphere with its oil film would fall freely down.

The film of oil would cover the sphere uniformly, gravity having disappeared (due to the weightlessness in the elevator). 

But the gravity of the Earth is NOT uniform. 

It obeys the inverse square law.

So the downward gravity at the bottom of the sphere (being closer to the Earth) would be slightly greater than the downward gravity at the center of the sphere.

And the downward gravity at the top of the sphere would be slightly lesser than the downward gravity at the center of the sphere.

RELATIVE to the center of the sphere (i.e. as felt by the sphere),  there is a slight resultant gravitational force downwards at the bottom of the sphere; and a slight resultant gravitational force upwards at the top of the sphere. 

It is simply a matter of subtraction of two collinear vectors.

In other words the film of oil at the bottom of the sphere is pulled down at the bottom and pulled up at the top.

The oil at the bottom leads and the oil at the top lags compared to the center of the sphere. 

So there is a piling up (bulge) of oil at both the bottom and the top of the sphere.

So there is a high tide at the bottom and a high tide at the top.

What about the oil film at the level of the center of the sphere?

The oil having flowed away both towards the bottom and towards the top, there is a thinning down at that site...in a first approximation.

So there is a low tide of oil there.

Hence Proved! QED!

In other words, it is not gravity but the gradient of gravity that causes tides.

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Let us now revert to our Earth, and the seas that cover it like a thin film.

For the moment let us ignore the Moon (an unpardonable sin).

And consider the Earth and the Sun. 

Earth is freely falling towards the Sun (like that sphere in an elevator with a broken cable).

So at the areas facing towards the Sun, Earth sees a high tide of seas.

Also a high tide at the areas facing away from the Sun. 

And low tides at the areas in between (equatorial).

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So also the tides due to the Moon.

Seen from the Moon, Earth is rotating around it (at the period of nearly 28 days).

So Earth is freely falling towards the Moon as well.

So at the areas facing towards the Moon, Earth sees a high tide of seas.

Also a high tide at the areas facing away from the Moon. 

And low tides at the areas in between (equatorial).

So the high tides are truly high on a New Moon Day as well as a Full Moon day.

Because, the high tides due to the Sun and the Moon assist each other, Moon and the Sun being in a near straight line.

And very low tides in between.

These are called the Spring Tides (nothing to do with the spring season...more like the spring in a shock absorber of a Chetak scooter).

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In reality, however,  the tides due to the Moon are larger than the tides due to the Sun.

Although the Moon is so much smaller than the Sun, it is so much closer to the Earth than the Sun is.

If the Sun were imagined to come in to reside at the site of our Moon, the Moon would be like our Aamne-Saamne neighbor (on a scale).

Merey saamne waali khidki mien ek chand ka tukda rehta hai

So the gradient of Moon's gravity is larger than the gradient of Sun's gravity.

Take now the Ashtami Day (shukla or bahula).

The Sun and the Moon are perpendicular to each other as seen by the Earth.

So the high tides due to them must cancel each other.

They do, to some extent.

But the tides due to the Moon, being larger than the tides due to the Sun, they overcome them.

So there are high tides on Ashtamis as well.

But these are not as great as those at the Poornima and Amaavaasya.

These are called Neap Tides ('neap' meaning powerless).

Sitting on that blessed rock near the Scandal Point on the Vizagh beach, I would therefore see 4 tides every lunar month (two spring tides and two neap tides) on Amaavaasya, Shukla Ashtami, Poornima, and Bahula Ashtami.

And since my Earth and I are spinning around themselves once in 24 hours, I would see these high and low tides repeating every 6 hours.

Phew!

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To be Continued

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