What Causes the Tides



Tides are mainly caused by the gravitational effect of the Moon and the Sun.  Without the influence of these heavenly bodies the sea level would even out and we wouldn't need Tide Tables.  

The Moon being our closest neighbour has the greatest effect whilst the Sun, although far larger is much further away and has a lesser effect.

If the Moon and Sun were fixed in the same relative positions, the tide times and heights would also remain fixed, but they don't.  The Moon is in orbit around the Earth and takes about 28 days to complete each orbit.  The Earth is also rotating with each cycle taking 24 hours.  

When the Moon and Sun are in alignment with the Earth the combined forces work together to create Spring Tides (high high tides and low low tides), when the Moon and Sun are at right angles to the Earth the lessened effects create Neap Tides (high low tides and low high tides).

Causes of tides



The diagram below shows the Earth rotating within a body of water, the shape of which has been drawn out by the gravitational effects of the Moon and Sun.  It shows an observer at point A at High Water, as the Earth rotates the observer will notice the sea level falling to Low Water just over 6 hours later.  After nearly 12.5 hours the tide will have risen to High Water again, the cycle continues going through the next Low Water at just over 18.5 hours and back to High Water again at about 24 hours and 50 minutes.   

Tidal cylcle over 24 hours