So the diagram on the left shows how the water on the earth gets pulled into a bulge one way by the moon’s gravity and into a bulge on the opposite side by the rotational force of the earth.
As the earth rotates on its axis once every 24 hours, if the moon stood still then the moon will be overhead and ‘underfoot’ once in every 24 hours giving (in most places) 2 high and 2 low tides a day, but as the moon is also orbiting in the same direction as the earth revolves it actually takes about 24 hours and 50 minutes for the moon to be in the same place overhead. So the period from one high tide to the next is about 12 hours 25 minutes.
When the sun, moon and earth all line up at new (as in the picture) or full moon then we get the highest (and indeed lowest) tides which are called ‘spring’ tides (nothing to do with the time of year!). In fact it takes a bit of time for the enormous mass of water to move, so the spring tide will actually occur a couple of days after the new (or full) moon, so the picture on the left is a simplification.Because of the tilt of the earth on its axis, at the equinox the sun appears to be over the equator and if there is a new or full moon at about the same time then all the forces contrive to create the highest of high tides and if the moon is also at perigee (closest to the earth in its orbit) then that will produce the biggest tides of all.
When the moon is at first or third quarter (you can find more on moon phases here) then the moon and sun are exerting forces from two different directions and the overall effect on the water is less, so the high tides are lower than average and the low tides are higher than the average and we call these low tides ‘neap’ tides.The word ‘neap’ is thought to have originated from the Middle English Word ‘neep’ meaning small.
Other Factors and Facts
Although the sun, the moon and the rotation of the earth are the major forces involved in creating the tides the local conditions such as the shoreline and the contour of the ocean floor also have an effect.Because of this not everywhere has 2 tides a day – there are some places that experience what is known as a double-high water (e.g. Southampton) or double-low water (e.g. Portland).The highest tides of all (17m) occur in Canada and after a long running dispute between the famous tides of the Bay of Fundy and those of Ungava Bay on the northern coast of Quebec, the Canadian Hydrographic Service has declared a draw.Most people think of the Mediterranean area as being without tides and it is true that the tides there are very small, and can often go unnoticed as atmospheric conditions and headwinds can attenuate the effect of these tides making them difficult to see.The very smallest tidal ranges (actually 0) are found in the Adriatic and South of Sicily.