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Tides and the Moon

We were given a fascinating talk at one of our meetings on sea fishing for bass some people seemed to be confused over the link between the Moon and the tides. I will try to explain. (This is the simplified version and some of you may be able to spot the simplification! If you want the full works then ask me).


All the diagrams above are viewed from above the Earth’s North Pole


Figure 1 shows the moon in orbit around the Earth with light from the Sun falling on both Earth and moon. When viewed from the Earth the Moon will appear “new” at d, full at b, and half Moon at a and c.

Since the Moon orbits the Earth once every 28 days then 1week passes between each of the positions a,b,c and d.

The next 4 diagrams are most definitely not to scale! The gravity of the Earth is sufficient to keep the Moon in orbit, but the effect that the gravity of the Moon has upon the Earth is much less. All it can manage is to pull the water of the Earth towards the side of the Earth that is facing the Moon. This gives a bulge in the water on the side facing the Moon. Inertia causes a similar bulge on the opposite side. As the Earth rotates, the “bulge” is held in place by the Moon’s gravity so a coastal area experiences two high tides every 24hrs (almost).

Unfortunately, things are never so simple! The Sun, although much larger and further away, also has a gravitational effect on the oceans.

Figure 2 shows the Sun, Moon and Earth all in a straight line so that both the Sun and Moon pull the water “bulge” in the same line. The same thing happens in figure 4. The water bulge is at its greatest and we experience a spring tide. From figure 1 you can see that this occurs at new and full Moons.

Figures 3 and 5 show the Moon and Sun pulling at right angles. The water bulge is now at a minimum. The tidal range is also at a minimum and we experience neap tides. Looking at figure 1 shows us that these occur at half-moons.

So now you know!! The easiest way is to use a torch, football, tennis ball and golf ball in a darkened room to model the processes. Finally, since the Moon rotates around the Earth every 28 days then the “bulge” moves 24/28 of an hour each day (51 minutes) so each day high tide moves forward by this time.

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