Tides In Ancient Times

One thing that I am surprised Europeans in ancient times and the Middle Ages did not notice is that the distance span between high and low tides can be used to gain a fairly good approximation of how vast a body of water is in comparison with another. The extent of the Atlantic Ocean and what was on the other side was a complete mystery until about five hundred years ago. As far as I can tell, no one in Roman times or before had the slightest idea of the scope of the ocean, they just knew that it seemed to go on forever.

The Mediterranean Sea, in contrast, was very familiar. The Romans called it "Mare Nostrum" (our sea). Ships crossed it continuously as is famously illustrated in the Acts of the Apostles in the Bible. Tides are caused by the gravity of the moon and, to a lesser extent, the sun. There is a low tide and a high tide each day unless the arrangement of the local landscape plays tricks with the usual tidal pattern, as does the Isle of Wight on England's south coast.

The span between the water level of high and that of low tide depends, to a large extent, on the volume of water in the sea or ocean. The more water there is, the greater the span of the tide will logically be. The depth of the water is a factor too, but since all large bodies of water are shallow relative to their width, depth can be safely ignored here.

The Mediterranean Sea has relatively weak tides. This is simply because, despite it's role in human history, it is a limited body of water, globally speaking. The Atlantic Ocean, in contrast, has a great tidal range because it is a far bigger body of water.

My surprise is that apparently, no one thought to estimate how wide the Atlantic was by measuring the average tide span around the Mediterranean, doing the same along the European coast of the Atlantic and figuring that it would give a good estimate of the extent of the Atlantic Ocean in comparison with the Mediterranean. My feeling is that if Alexander's conquests had gone westward instead of eastward, one of the scholars of ancient Greece would probably have thought of this.