How are high and low tides connected?
The tide is caused by the gravity pull of both the moon and the sun; the latter accounting for about one third of it. When the moon and the sun are on the same side (new moon) or opposite side (full moon) they are working together and the tide is said to be the spring tide; the highest and lowest level. But it is all together more complex than that. When calculating the tide, we use many different coefficients, not only the position of the moon and sun, but also their declination, etc. In fact, it is not unusual to use 20 or 30 different coefficients to do a proper tide calculation. In the old days, that was done by a clockwork mechanism lifting a dial for each coefficient. Today it is done by computer, of course. I have, myself, implemented such a function in a maritime GPS program I developed some years ago. Because of that, there are many spring tides. Some have a period as long as four and half years. It is in such an extremely high tide that I sailed, years ago, in the harbour of
