Prince of Tides

When we spend time at the seaside, we can notice that the sea water is sometimes close to the shoreline, and sometimes moves away from it. This is the phenomenon of tides and low tides. Two bodies cause the sea to advance to the shore at high tide and retreat at low tide; These are the two most prominent bodies in the Earth's sky: the Sun and the Moon

Author: Zvi Atzmon, young Galileo
In these summer days, many of us spend time at the beach. Spread a blanket (or towel) close to the water line and enjoy. We know that when we spread the blanket close to the waterline, many times after a while the water gets close to it and starts to wet it, and it has to be moved away towards the shore. And sometimes on the contrary - we spread the blanket close to the water, but the water recedes and moves away from us.

The moon is more influential

This "treacherous" behavior of the sea originates from the phenomenon of tides and low tides. At high tide the sea advances and covers an additional area of ​​the beach; And at low tide the sea recedes and reveals a coastal area, and God forbid it returns. Two bodies cause the sea to advance to the shore at high tide and retreat at low tide; These are the two most prominent bodies in the sky: the sun and the moon. The moon has a great influence on the phenomenon of tides, more than double that of the sun. The phenomenon is caused by tidal forces that these two heavenly bodies exert on the earth. The forces of the tides themselves are related to the force of gravity (gravitation, the force of attraction) that they exert on the earth and its oceans and seas. However, tidal force is not gravitational force, but is the difference between the gravitational force exerted by a celestial body on the two sides of the earth - the side closest to the celestial body and the side far from it.
Suppose that somewhere in space, far away from any other celestial body, there is a large body of water - a huge drop of water. The giant drop of water will take the shape of a ball, mainly due to the gravitational force that the water molecules in the giant drop exert on each other. Now let's imagine that some body, for example the moon, approaches the giant drop. The shape of the drop will change - from a ball to an elongated drop.
Why does the shape of the giant drop change? The reason is the difference between the magnitude of the gravitational force acting on the part facing the moon and the gravitational force acting on the part far from it. On the part facing the moon, the force of gravity is greater because the force of gravity is greatly affected by the distance between the two bodies. The opposite side is further away, so the force of gravity acting on it is smaller. Because of this, the side facing the moon is pulled strongly from the far side, and therefore the near side comes a little closer to the moon, and an elongated shape is obtained.
The drop extends not only in the direction of the moon, but also in the opposite direction. why? Suppose we inserted into the giant drop of water a large solid body. A sort of planet is obtained with a solid part (like the Earth's soil) and a watery part (like the oceans). We saw that the water facing the moon is drawn and "stretched" a little in its direction due to gravity. The water on the far side has a weaker gravitational force.
And what about the solid mass inside the giant drop of water? The force of gravity acting on it is like an average between the gravity on the part facing the moon and the gravity acting on the water on the opposite, distant side. Therefore the solid part is drawn towards the moon more than the water on the far side, and therefore the solid part seems to escape a little from the water on the far side, leaving it behind. It turns out that in relation to the solid part, the water "stretches" in both directions: in the direction of the moon and in the opposite direction. Because of the elongated shape of the water on the side facing the moon, the height of the water above the solid part will be large, and so will the opposite side. In these two places there will be high tide. In the vertical direction the height of the water will be small - there will be a low tide: from these areas the water moves in the direction of the tide, and thus the height of the water in the low tide areas is small.

The far side of the moon

The earth rotates on its axis once a day. If we stand on the side facing the moon, there will be a tide. At the end of a complete tidal cycle (which is a little longer than a day; we will immediately explain why) we will return to exactly the same situation. However, in the complete cycle we will pass in another tide - the tide on the far side of the moon. In the cycle we will also pass two low points, from which the water "ran away" towards the high tide points.
In practice, the phenomenon of tides on Earth is more complex. First, the moon orbits the earth once every 27.3 days. Therefore, during the day, when the Earth completes a rotation on its axis, the Moon changes its position slightly in its orbit around the Earth. Therefore, it takes more time until we return to standing exactly in front of the moon. Because of this, the time required for a complete cycle of tides longer than a day - is close to 24 hours and fifty minutes. In 24 hours and fifty minutes, each point will have two high tides and two low tides. This happens in the middle of a Hebrew month, when the moon is full: the sun and the moon are then on opposite sides of the earth - when the sun sets in the west, the moon rises in the east. This also happens at the beginning and end of a Hebrew month when the moon appears as a thin sickle, and then the sun and the moon are on the same side of the earth - both rise at the same time in the east and set in the west.

Four gays and four lows

Except for the moon, the sun also creates tides. The gravitational force of the sun is greater than that of the moon, but we have seen that the force of the tide is not the force of gravity itself, but the difference between the force of gravity acting on the two sides of the earth. Since the Moon is much closer to us than the Sun, the difference between the gravity on the side facing the Moon and the gravity on the side far from the Moon is greater than the difference in the Sun's gravity. Therefore the tidal force that the moon creates is more than twice as great as that created by the sun.
When the moon is only half illuminated, the sun and moon are not in line with the earth, but perpendicular, at an angle of ninety degrees. This is what happens in the first quarter of a Hebrew month and in the third quarter, so the moon causes two high tides and two low tides in each tidal cycle, and the sun also causes two (smaller) high tides and two low tides. In this part of the month, at each coastal point there will be four high tides and four low tides in the tidal cycle, but in this situation they are small because the ocean water is divided between four tides, not between two.

Not only on days

The solid part of the earth (the earth's crust, the continents) is not a completely solid and rigid body. It has some flexibility. That is why the phenomenon of tides exists not only in oceans and seas, but also on continents. But this phenomenon is much smaller and can reach no more than thirty centimeters, and therefore we do not feel it. However, when great precision is required, the continental tides have an effect and must be taken into account: for example in precise GPS measurements and in particle accelerators where precision is critical. Some scientists think that tidal forces can have some effect on the chance of earthquakes.
Even the atmosphere - in its upper parts, at an altitude of tens of kilometers - has a tidal phenomenon, and the atmosphere "stretches" considerably towards the moon.
The phenomenon of tides is known not only from the earth. The tidal forces that the giant planet Jupiter exerts on its closest moon - Io - cause a lot of movement and friction inside the moon Io, and this heats the interior of this moon. This is manifested in vigorous volcanic activity on the moon Io. Jupiter's tidal forces can tear apart a meteoroid approaching Jupiter, and therefore on its way to Jupiter it breaks and falls on it not as one block, but as a series of fragments.
In the Mediterranean Sea, on the shores of which many of us spend our vacations, the tide is relatively small in any case - the sea "rises" by several tens of centimeters. But there are places on the coasts of the oceans where the difference between high tide and low tide can be a difference of meters in height, and then a very long stretch of beach is alternately exposed and covered. On such ocean beaches, the low tide is a good time to look for all kinds of things on the beach that the sea left when it receded at low tide. It is very important to return to Mevamim Beach in time, before the tide comes in...
* Note: the tide phenomenon also has other explanations. The explanation presented here is the appropriate one in our opinion.

The article appeared in the August issue of Young Galileo – Monthly for curious children

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