November 2017 - Sun, Moon Time and Tide
Members of the Horndean & District U3A met for their regular monthly meeting on Friday 3rd November 2017 at Merchistoun Hall. They listened to a talk by Prof Mike Whittle titled "Sun, Moon Time & Tide".
At the start of Mike's presentation his very first comment was "Everything I thought I knew about the tide turned out to be wrong!"
Mike covered why the tide rises and falls, why twice each day, timing, spring & neap. During one month the tide rises and falls 60 times and in a year 700 times and it's complicated. Mike showed many diagrams etc which explained the complexity of the subject throughout his presentation.
The mean tide was based on information collected over a period of time at Newlyn in Cornwall based on height of land and
lowest water level. The moon causes the tides and the sun modifies it. The moon's tidal force is 2.2 times greater than the sun and the moon is the driving force.
The tide rises and falls owing to the oceans bulging out on each side of the earth. The moon pulls these bulges towards it and they follow the movement of the moon as it orbits the earth The rotation of the earth, moon and sun cause a centrifugal force with the highest tide on the side farthest away from the moon.
The height of the tide is affected by many different things as well as the moon and sun, like atmospheric pressure, wind, weather and changes daily. The tide comes in slower than it goes out. When sun and moon line up it causes tidal forces which added together give high tides, known as spring tides (nothing to do with the spring season). When they are not lined up it causes neap tides which are not as dramatic as the spring tides. The greatest force when the sun and moon line up are in late September and March; the lowest are June and December. Every 18 years approximately there are super tides as in 2015. Each tide takes 12 hours and 25 minutes with a slip of 50 minutes per day. Pattern repeats every two weeks depending on bulges around the earth.
The tidal waves start in the centre of the oceans of just a few centimetres high and move at 80 kilometres an hour gaining height and depending on depth of water and coastlines create the size of the waves. The tidal range is different to high and low tides and creates low tides in the Mediterranean and Caribbean. The Med is subject to oscillations and they can also happen in oceans. The Solent area has a double high tide caused by oscillations and reflections in the tidal wave as it runs up the English Channel. Mike assured the members that the two high tides are not caused by the Isle of Wight.
The amount of energy that tides can create for electricity etc is enormous; very good but expensive.
Mike talked easily about this subject and the amount of information he imparted was mesmerising. There was hardly a murmur from the members throughout his talk. Several questions were asked of Mike and he was thanked by Geoff Southgate on behalf of everyone. Members are looking forward to Mike coming back for another of his talks sometime in the future.
Diane Stoner
Speaker Coordinator