How are tides monitored ?

Tides are fantastically important. They are controlled by the gravity of the moon rotating the earth on a 28 day cycle, pulling the water round the world. If you are a sailor you need tide tables for two very important reasons. The first is that the water may not be deep enough to get in and out of harbour or, say, across a sand bank, until certain times of day. Another reason is that to save time and power we want to sail with the outgoing tide if it is going our way - to sail against the tide can mean actually going backwards - I have seen large sailing boats beating to windward against the tide on the western approaches to the Solent going literally backwards, (though they had forward speed through the water). Bad planning! Since it is a 28 day cycle and there are 13 lots of 28 days in a year the tides vary daily as to when it is high tide and low tide, More than that we have neap tides when the highs and lows are less and spring tides when they are greatest. These depend on the phases of the moon. On the ecology front they wash into and through seaweed aerating the plants and sea life and stir up sediments to clean the bottom of the beach and inlets and estuaries and refresh pools that remain on the beach for other plant and animal life. This is one very good reason why tidal barriers will be a disaster for coastal life since this will be lost to power generation. The same thing applies to wave power and, on land, wind power - there will be a large ecological cost to any sustained or practical use of energy that is in fact not "renewable" - we will be messing with a delicate balance of nature that will do far more damage than carbon ever could (if it was doing any damage now, which I doubt) Tides are not so much monitored as predicted but dangerous high tides can happen at the highest spring tide, with the wind onshore and low air pressure - it all helps to push the water further up the beach and cause damage and flooding.

The correct answer is a. we know this because n (hap) + n (hap) = 2n (dip).

Try and make the question more clear : )