This concept is much more complicated than most people imagine and this section hopes to take the audience through it step by step, adding complications as we go along.


2.3.1: Simple Model

Imagine the whole earth to be covered by a shallow, uniform ocean, and that both the Earth and Moon are standing still.


Fig 17: Earth and Moon

In Fig.11, we have a high tide at point A, and another at point B, with low tides at C and D. What happens is that the Moon’s gravitational pull will cause the water to heap up at A, where the force is strongest. The land is not affected as much as land is harder to move than water. There is also a second high tide at B. This is because being furthest away from the force of the Moon’s gravity, water there is comparatively slow (compared to C and D) in flowing towards the pull at A.


Explanation of simple model

  1. The Moon is standing still.
  2. Point A, closest to Moon, is subjected to greater accelerating force than the average.
  3. Water bunches up around it, prevented from leaving the solid surface by Earth’s gravity.
  4. Results in high tide.
  5. Reverse occurs at B.
  6. Acceleration is less than the average, so water in the region will tend to get ‘left behind’, so bulging away and causing a similar high tide.


Next, assume that the Earth and Moon are keeping the same distance away from one another, but that the Earth is spinning once in 24 hours. The water-heaps (i.e. the high tides), will keep ‘under’ the Moon instead of spinning with the Earth. Thus each bulge will seem to sweep right round the Earth once in 24 hours, and every region has two high and low tides.


However, there are more complications:

1.      The Moon is not stationary; it moves along in its orbit so that the water-heaps shift slowly as they follow the moon around.

2.      High tides at any particular place will be 50 minutes later each day on average because the time it takes the Moon to orbit the Earth varies.

3.      The two high tides are also not equal. This is known as ‘diurnal inequality’ of the tides. (Will leave out confusing details for now)

4.      Also since the seas are of various shapes and depths, and the Earth does not have a shallow uniform ocean, local effects are important for the tides. For example in the Bay of Fundy in Nova Scotia, and along some coastal areas in South America, the tidal range is 50 feet or so, while in other places it is less than 12 inches.

5.      There is a lag-time and the highest tide follows the moon after an interval, which varies according to local conditions. The interval is greatest for shallow coastal areas.

6.      Another complication is the changing distance of the moon. Near perigee, when the Moon is closest to the Earth, the pull is stronger than at apogee, when it is furthest, tides are higher, giving a difference of about 20%.