Objects of a certain size – mass – attract other objects that are also of a certain size. We speak of mutual attraction.
The different celestial bodies also attract each other. There is attraction between the sun and the earth, the moon and the earth, the sun and the moon and so on.
The fact that the objects do not collide with each other because of this attraction is due to the fact that these objects move through space at high speed in elliptical orbits. Forces are needed to change the direction of the celestial bodies over and over again, which causes them to describe this elliptical orbit. These forces are in balance with the pull forces.
Not only the size of the body determines the attraction to another body, but also the distance between them. The mass of the sun is many times greater than the mass of the moon. But because the sun is about 400 times further away from the earth than the moon, its attraction to the earth is still smaller than that of the moon.
- High and low tide
The discussed attraction forces cause the tidal movements.
We assume that the water of the seas and oceans is like a fluid shell around the earth. This attraction is, of course, also exerted on the water. On the side of the earth that is facing the moon (see point 1 in Figure 25) a hump of water is formed. But also on the opposite side of the earth such a hump occurs (point 3). Because that side is further away from the moon, the moon does not pull as hard on that side. The water does not move as fast towards the moon, and ‘hangs’ as it were: there is also a lump of water on that side. At points 2 and 4 the water is low, because the water has flowed to points 1 and 3.
For the sake of clarity, the water in Figure 25 has been drawn excessively thick. You can see that the water is stretched in the direction of the moon and in the opposite direction. The water mountain at point 1 is the most attractive and the mountain water at point 3 is the smallest. This is the explanation that both water mountains at points 1 and 3 are the same height. In the drawing it will be high tide at points 1 and 3 and at points 2 and 4 it will be low tide.
The earth rotates counter-clockwise around its axis (see arrow). In one such circular motion, the earth does 24 hours, one day.
In theory, therefore, in those 24 hours there will be two high tides and two low tides. If it is high tide at 00.00 hours, it is low tide at 06.00 hours, high tide at 12.00 hours and low tide again at 18.00 hours.
In reality, high and low tides do not entirely follow this model. The tidal movement takes a little more than 24 hours: each time high tide falls about 25 minutes later than the last time. Why is that?
The moon rotates counter-clockwise around the earth and takes 29.5 days to do so. For the sake of convenience, we’ll make it 30 days.
You already know that it is high tide at a point on earth that is right under the moon and on the other side of the earth. If point 1 of the previous drawing has returned to its starting position 24 hours later, it should be high tide again, according to the model. That’s not true because the moon hasn’t stopped either and has shifted a bit in the meantime. So, point 1 has to go on for a while to ‘catch up’ with the moon. Look at the next drawing.
A circle always has 360°. If the moon has travelled a complete circle around the earth in 30 days, then it has travelled 360°. In one day, the moon lays 360° : 30 = 12° off. So that 12° still has to travel point 1 after 24 hours to get right under the moon again.
How long does point 1 take? Let’s go figure that out. If point 1 has returned to its starting position after 24 hours, the point has travelled a 360° circle.
So per hour point 1 lays 360° : 24 = 15° off. It only has to ‘catch up’ with 12°, so that takes 12/ 15 hours. 1/15 hours is 60: 15 = 4 minutes, so
12/15 hours = 12 x 4 minutes = 48 minutes.
Twice a day there are high water levels. Each subsequent high or low tide is therefore about 24 minutes later than the previous one (48 minutes divided by 2 equals 24). Check that on a calendar with the water levels on it or in a book of hours. Do you see it’s true?
High and low tides are caused by a combination of attraction forces of sun, earth and moon on each other. In the foregoing we assumed that only the moon is the cause of high and low tide