This year, our Island Low Tide Celebration will be on Sunday, 23 June 2013, and of course, it will be at our most popular and most used park, Point Robinson. The tide this year will be at 3.4 feet below datum, one of the lowest of the entire year.
The significance of that very low tide is that we will be able to get way out on the mudflats, and see and study the animals that only can be seen at the lowest tides. In other places, and from other writers you can see more descriptions of the activities, the people who will provide them, and where at Point Robinson they will occur. For me, it is to provide a short and pithy descriptions of tides, and why they happen, as I have done every year that we have been having this celebration.
For thousands of years, and especially as long as fishermen have been going to sea from ports that have wide tidal ranges, people have been trying to figure out when the tidal extremes will occur. If you have been to any of the fishing ports in Brittany, in Western France, you will see, at low tide, fishing boats sitting on the mud flats, and then at high tide the water coming up all the way to the top of the stone jetties. The extreme of this is at Mont Ste. Michelle, where the tide comes in with frightening velocity. Cars used to park on the mud flats at low tide, their owners did not pay attention to when the tidal race would come in, and would return just in time to see their vehicles engulfed-or to be more precise, embayed, as the Mont sits in a bay.
So ever-observant humans finally made a connection between the relative positions of the earth, moon, and sun, and in later years the underwater terrain, the hydrography, of the locale. We do not know exactly when this connection was made, but it certainly was long before the synthesis of the laws of gravity, including the work of Sir Isaac Newton. A little Newtonian arcania here-Sir Isaac held the Lucasian Chair of Physics at Cambridge University. He was a member of Trinity College, and his rooms in Trinity are still being used. The Lucasian Chair still is endowed, and is presently occupied by Professor Stephen Hawking.
So the next step was to see that when the sun, moon and the earth are all lined up on the same side of the earth, or in conjunction, the sun and moon exert the maximum pull on the earth, and in particular anything movable-and the 70% of the earth that is water is also moveable. Just recall the last boat trip you took! This pull is the pull of gravity, and it is gravity, coupled with the centrifugal force exerted by all rotating bodies that keeps the whole system in balance. So it was clear that when these bodies are in conjunction, the pull is strongest, and when the sun and the moon are on opposite sides of the earth, or in opposition, the pull is weakest, and the tides are at their smallest value above or below datum.
There are several subtlties to this, and as one of my instructors at the Coast Guard Academy would often say, "beyond the scope of this course". But one of them is worth adding here, and that is the terrain, both above the surface, and below the surface, that direct the water as it ebbs and floods. It is because of this terrain, for example, that we have the extreme tides in the Bay of Fundy , both as to tidal range and as to speed of the tidal current. For our local usage, the tidal current off of Point Robinson is seldom more than 3 knots, and that is when the tide is on the ebb, or going out, because it is additive to the river currents coming out at the bottom of the Sound.
So come to see us for this, our latest Low Tide Celebration. I will be in the lighthouse giving information about our own lighthouse, and lighthouses world wide, in general. I look forward to seeing you then.