9 NAVIGATION TIDES


In the previous section our course went directly from point to point as if the sea was fixed and static. No allowances were made for tidal stream which, for many of us, would mean that if we blindly steered the compass course we would be swept aside and would miss our destination.


How tides occur


Not all of the world’s seas are significantly tidal but, in those that are, they become a potent issue for navigators. Tides are caused by gravitational forces – more specifically, the gravitational pull of the moon and sun. Although the sun’s mass makes for the mightier gravitational force, it’s much further away so its effect is about equal to the moon’s.


Spring tides


Neap tides


The word ‘diurnal’ means once a day. As the Earth spins on its axis, those waters directly under the moon are attracted upwards – creating a diurnal tide. But, hang on, you say – surely there are two tides per day? And you would be right. The reason for this is that there’s a balancing effect on the other side of the globe – another, smaller bulge in the water matching the bulge under the moon. So, the reality is that we have two tides a day – semi-diurnal, in fact. The sun’s involvement affects the size of each tide in approximately two-week cycles. When both moon and sun line up, or when the sun and moon are on opposite sides of the Earth you get high ‘spring’ tides. And when they are at 90° to each other you get the lesser ‘neaps’.


64


RYA Seamanship for Sea Anglers


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