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T I D E S   -   are primarily caused by the Moon and the Sun.

   That's the simple answer - there is a lot more to it than that. We will try and give a more explicit answer, but since we don't have bookfuls of space to spare, it won't be the total answer.

   Very basically the Earth revolves round the Sun, and the Moon revolves round the Earth - that is the basic model for the explanations. Other planetary bodies have such a miniscule part in all of this that we can ignore them.

   The reason they orbit round each other is gravity which Mr Newton created some laws about. The Sun exerts gravity on the Earth and tries to pull it in; the Earth has a sideways force which tries to drag it away from the Sun and into space. The reason one continues to orbit the other is a balance of those two forces called the resultant force. The analogy for the Earth and Moon is the same and that keeps things simple - if only.
   Newton's famous law of gravitation states that the force is proportional to the mass of the Moon and inversely proportional to the square of the distance between them.

   There is however something of a complication in that we are not considering just Sun and Earth or just Earth and Moon, we are considering all three together which kind of makes things a little more complicated.

   If you consider the major obvious facts you have two:-
      (1) The Earth orbits the Sun every year, and itself every 24 hours, which is called a solar day, and there are 365 days in a year, so every year we start the cycle again - not that simple though because it is closer to 365¼ days per year which is why every 4th year we add the odd ¼ s together to give an extra day which is of course our leap year.
   Even that is not exact and every so often a leap second has to to be added to adjust that small but increasing inaccuracy
      (2) The Moon seemingly orbits the Earth every day except it is more like 24 hours 50.47 minutes which is called a lunar day.

   Something that may interest somebody out there is how the Earth and Moon do truly interact. Most people consider that the Earth spins on its own axis and the Moon on its own axis. Whilst that is to a theoretical degree true, the Earth is exerting a force on the Moon and the Moon on the Earth - however, the Earth is more massive with higher gravity than the Moon so they interact with each other and do in fact have a common centre of mass which is not the axis of the Earth but a point inside the Earths crust, (fig.1). If you consider two Earths rotating together with equal mass and gravity the common centre would be exactly half way between them, (fig.2). This is a binary system which is something you more commonly hear about regarding 2 stars rotating together.
   You can of course apply the same analogy to Sun and Earth where you find the centre of mass is actually in the Sun but not at it's centre.

   A further complication is that the Earth's axis of rotation is inclined 23.45 with respect to the plane of Earth's orbit about the Sun, so as the year progresses we move through the seasons with the Sun appearing higher or lower in the sky.

   Getting back to the subject of tides, as the Moon orbits the Earth it pulls the oceans up at the closest point and gives a high tide, the furthest point on the Earth has the least pull and in essence rises further from the surface. At the sides the effect is minimal and you have low tides. Whilst the Moon is pulling up the Earth is pulling back which gives a near balance, which is why the oceans rise and fall in the order of metres and don't fly off into space.
   That said we have to consider the effect of the Sun. Whilst it is immensely more massive than the Moon, it is also a lot further away and does in fact exert a force that is 46% of the Moons force, ie., less than half the effect of the moon.
   Since the Sun and Moon are not synchronised we find that the effect on the Earth from both Sun and Moon varies resulting in varying tides. Orbits are not circular and the Earth and Moon are rarely at nearest or furthest points at the same time as Earth and Sun. These are some of many factors which make tide calculation much more complex than might be imagined.

   If the Sun and Moon are in the same direction the force is the greatest and we have a 'high' high tide and a 'low' low tide. This is known as a 'Spring tide' (fig.3) and the Moon is 'New'. NB.'Spring' tides happen all year, not just in the spring.
   When they are at 90° to each other we have a 'low' high tide with a 'high' low tide. This is known as a 'Neap tide' and the Moon is in its 'first' (fig.4) or 'last quarter' (fig.6).
   If they are on opposite sides of the Earth they compliment each other and we again get a 'high' high tide and a 'low' low tide. This again is known as a 'Spring tide' (fig.5), but the Moon is 'Full'.

   Leaving aside Sun and Moon further consideration has to be given to the effect of our own Earth on the tides. If it were a perfect sphere covered only in water of a constant depth the tidal pattern would be relatively well balanced with a bulge towards the moon, and a bulge opposite, and no other complications. We have however an Earth which has oceans, seas, channels etc., of massively differing depths in amongst lumps of land of massively differing shape and size.
   All of that makes local tidal prediction exceedingly more complex than the perfect global model. A typical tide would be something like that depicted, (fig.7) with the high tide being roughly 12 hours 25 minutes after a low and the next low after a further roughly 12hr 25mins.

   At this point we can get very local and consider what happens at Swanage which is the major coastal town in the Isle of Purbeck.
   Swanage is founded around Swanage Bay, but effectively in the English Channel. The tide is funnelled as it passes round Kent and spread as it moves west to the Isle of Wight where it is 'split' each side of the Isle of Wight. Having so done it 'rejoins' aroundabout the Needles, but since it has covered different distances and been subjected to different sea-floor conditions and depths there is a lag on the north side.
   This is a very simplistic explanation, there are many variables that change the tidal pattern around the UK and the rest of the world besides the south coast of England.
   There is also loss of tidal energy on the north side of the Isle of Wight courtesy of the Solent. This results in there being a double tide where the leading high starts to drop and is then caught up with by the trailing high.

   If for instance you were standing on Swanage beach you would expect the tide to flow up the beach for the high tide, then ebb down the beach for a while and then hover or flow again, although not to such a high point, and then ebb down to the low tide. (fig.8)
   Given this sequence of high tide and then a second tide at Swanage, there are places where the sequence is reversed due to different local conditions.

   This is not the only anomaly along the south coast of England, but it is one of the most interesting, we think.

   A popular misconception with tides is that it is the whole body of water flowing along whilst it is in fact a rise and fall at the surface which does produce movement back and forth with the tide. The deeper the body of water the less the apparent movement at that depth, and at greater depth you reach a state of almost zero movement. When you get to the sea floor of course, which cannot move, no movement at all.
   Very simplistically the tide flows up the beach and ebbs back down it. Less simplistically as waves move into shallow water the wave energy is concentrated in lessening depth which causes them to steepen, and when the slope at the crest becomes sufficiently steep the wave breaks with the shape of the breaking wave dependent on the slope of the bottom. If the slope is sufficiently steep the wave may not actually break but impact and dissipate as 'white water'.
   Many wave phenomena are actually not tidal effects but weather effects such as 'white horses' caused by the wind clipping the top of the wave. There are of course concentrations of flow and ebb around and over places like Old Harry Rocks and Peveril Shelf which can prove most hazardous.






 

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