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Topic Last Updated on 11-07-2024
THE MOON IS TO BLAME
If you go to the shore in the early morning and stay there all day, you can observe an interesting phenomenon. Leave your towel in the sand about 30 feet from the water. In a few hours, you’ll find that the surf is splashing nearby or has already swept it off into the deep. If you keep watching, you’ll notice that after a certain period of time, the water begins to recede and the towel falls back farther than it was even at the beginning of your observation. This is due to gradual changes in the water level, called high tide and low tide. The more gradual the shore’s slope, the more visible these changes are.
Many of you have probably heard that the tide is connected to the Moon. It’s true: our planet’s satellite unevenly affects the World Ocean with the strength of its gravitational pull — the closer to the Moon, the stronger the effect. Let’s recall the formula for the force of gravity. It is inversely proportional to the square of the distance between the objects in question. When dealing with the distance to the Moon, such distances as the radius and diameter of the Earth are important. Therefore, due to the differences in force, water is “distorted” into bulges.
Let’s imagine that the Earth rotates inside of these bulges of water. From one point on the planet, an observer will experience the sequential phase of high tide and low tide within one rotation (24 hours). It seems logical enough, right?
The biggest waves | Explanation
In reality, though, we observe that, in a 24-hour period, the water level changes four times. Twice a day, the beach will grow very narrow — corresponding to the high water level, called high tide. Twice a day, it will reach its maximum width — due to the water level reaching its minimum, called low tide.
Our reasoning thus far has explained the occurrence of only one low tide and one high tide. So, where do the other two come from?
IT’S ALL MORE COMPLICATED THAN YOU THINK
The majority of people think that the Moon orbits around the Earth, but that’s not exactly the case. In actuality, the Earth and the Moon revolve around the center of mass in the Earth-Moon system. Because our planet is twice as heavy as the Moon, the center of mass is located inside the Earth, at a point situated at a distance from the Earth’s center that is equal to about 2/3 of the planet’s radius. This is why it seems to us that the Moon revolves around our planet.
As it works out, the Earth also completes a small revolution around this center of mass. This is not observable to us, but the water that covers our planet’s surface, due to inertia, tries to “run away” from the center of this revolution. Centrifugal force is identical at every point on Earth.
At point A, the closest to the Moon, it slightly weakens the force of gravity, while at point C, on the opposite side. The force grows stronger and pulls the World Ocean into a bulge.
FORCES AT PLAY IN HIGH
As a result of the two forces involved, an effect appears, forming the stretched out, oval-like shape on which the observer rotates and, therefore, sees two high tides and low tides in a 24-hour period.
The biggest waves | More facts:
There’s still one other phenomenon — the time between high tides changes slightly. Why is this so? We have to take into account the fact that during the time in which the Earth completes one rotation, the Moon changes place. One full lunar cycle is composed of 28 days, meaning that, in one 24-hour period, the Moon traverses 1/28 of its orbit. The maximum water level, in turn, depends on the Moon, so in order to catch a full high tide, an observer on Earth has to wait another 50 minutes or so (in one day, there are 24×60=1440 minutes, 1/28 of which is approximately equal to 50 minutes)
HIGH TIDE DEPENDS ON THE MOON, WHICH ALSO CHANGES POSITION
That is, one complete lunar cycle of high tides and low tides for the observer finishes in 24 hours plus 50 minutes. In order to determine the time between high tide and low tide, this period should be divided by four. As a result, the time between a full low tide and a full high tide equals 6 hours and 12 minutes
