The earths axis of rotation

The earths axis of rotation
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Fig 5.4 – The earth axis or rotation is tilted, so any given point on the surface will pass through two different sized tidal bulges.

The earth axis of rotation is not vertical and is actually tilted at 23.5°. This means that the tidal bulges created by the moons gravity are not symmetrically positioned on the earth (Fig 5.4). As the earth revolves on its axis of rotation, any position on the earths surface will pass through two different sized tidal bulges. In effect this means that the tides we experience each day have slightly different level. We will see that one of the high tides is a little higher than the other when we look at the tide tables further on in this module.

Of course this disparity between the daily high tides caused by the tilt of the earth is also enhanced and diminished by the fact that there is a greater gravitational pull on the side of earth nearest the moon.

The gravitational pull of the sun

The sun also has a gravitational pull and effect on the tides. The sun is massive and has a huge gravitational effect but is diminished due its distance from us. When the sun, the moon and the earth are in line with each other, and gravitational pull of the sun and moon are complimentary and we have relatively high tidal ranges called spring tides and when earth is at right angles to the moon and sun we have reduced tidal ranges called neap tides.