How do tides work?

Question

How do tides work?

Answer 1

Tides are caused by the gravitational attraction of the earth to the moon and to the sun.  Although the moon is a lot smaller than the sun, it is a lot closer to the earth; so the moon's gravitational effect on the tides is about three times that of the sun.

The highest tides ocur when the sun, the earth and the moon are all in line, and the gravitational attraction of the sun and the moon is aligned. These are called 'spring tides', and they happen during new moon (when the sun and the moon are on the same side of the Earth), and full moon (when the sun and the moon are opposite each other).  When the sun and the moon are at right angles, relative to the earth (during half moon), the tides are at their weakest, called 'neap tides'.  Also, during its orbit around the sun, the earth is closer to the sun at times; and during these times, the spring tides are at their highest; called 'king tides', ot technically 'perogee syzygy' tides - meaning 'closest in line'.

It is easy to understand why there are spring tides when the sun and the moom are on the same side and 'pull together'; but spring tide when the sun and the moon are pulling 'against each other' during full moon seems counter-intuitive. Why this happens is as follows:

Ignoring the effect of the sun for the time being and just considering the effect of the moon, we discover that high tides ocurr on opposite sides of the earth at the same time.  This is also difficult to understand; surely if the tide is caused by the gravitational attraction of the moon, it is obvoius that the seas on the side closer to the moon would bulge out towards the moon. Why then, would there be a high tide on the opposite side? - You would think that on the side furthest away from the moon, the effect would be that the tide there should be lower, because of the bulge on the side nearest the moon.

The reason has to do with the fact that gravitational attraction  towards the moon is weaker on the side of the earth facing away from the moon.  In effect,  the seas closer to the moon are attracted towards it, but at the same time, the gravitational centre of the earth is attracted towards the moon MORE than the seas on the furthest side; and so the earth is 'pulled' away from those seas.

The net effect is that the seas closest to the moon are subject to a net pull TOWARDS the moon (relative to the earth), and the seas on the opposite side are subject to a net pull AWAY from the moon (relative to the earth).

Now bringing the sun into the equation, we can see why tides are highest at full moon:  Then, the seas facing away from the moon are pulled AWAY from the moon relaive the the earth; but at the same time are pulled TOWARDS the sun, relative to the earth.  The effect is reversed on the side facing the moon: net attraction TOWARDS the moon and AWAY from the sun.

Now it can be seen why tides are highest at full moon.

The effect is called tidal stretching, and is caused by the fact that gravitational attraction between two bodies decreases with the square of the distance between them. This has startling effects near very strong gravitational objects (the most powerful being a black hole) An object falling into a black hole would be infinitely stretched, because of the tidal effect.

Answer 2

Tides are generated by gravitation from big celestial bodies near Earth. The two that have the greatest influence are Moon and Sun, since Moon is very close and Sun is not that close, but is very big. The effect by the Moon is stronger.

Gravitation from the Moon is mostly canceled as it is in free-fall status respect to Earth, but because the Moon is a solid body, it stops gravitation from contracting it to a single point (gravitation also attracts a body to itself), leaving that part of the force non-free. The difference in position of the sides of the Moon respect to the Earth (left and right, closer and further) cause it to produce a non-uniform pull on Earth. As Earth is mostly solid too, only the sea is actually displaced in a substantial level, being high tides the one that point toward the Moon and on the opposite side of the Earth. As the Moon moves around Earth, the tides also change, in the process eroding the surface of the Earth and heating it.