THE EARTH, MOON AND THE SUN
The Severn Bore, so-called, is a phenomenon of the tides, sufficient to be ridden, as here by a surfer at Minsterworth
© Lesmalvern
What Did the Moon ever Do for Us? Dr Stuart Blackie We owe a lot to the Moon. Oliver Morton in his book The Moon: A History for the Future has proposed that if the Moon did not circle our planet, the axis about which the Earth rotates each day would have been unstable. The extreme scenarios would be either to straighten or to dramatically tilt. The first scenario might mean that we end up like Mercury, a planet with no moon, whose axis of rotation tilts a tiny 2 degrees. Were this to have been the case, we would never have experienced our seasons. The second extreme is that we might roll over like Uranus and rotate on our side with one pole pointing at the sun and half our world stuck in a prolonged, if not perpetual, night-and-ice age. Of course, this is hypothetical. But there is no doubt that the Moon and, to a lesser extent, the Sun are responsible for the ocean tides and these tides are thought to have been of enormous significance in the development of life on earth. The range of tides shows enormous differences over the earth as a whole, but in the Devonian era, because the moon was closer, the tides were greater. However, even now on the Atlantic coast of North America there are important variations. There is a rise of only a foot or two at the Florida Keys, but at Cape Cod Bay the spring tide range is 11 – 12 feet and at other places it is higher. In this country, the Severn is noted for its impressive tidal race. As day after day the great tides ebb and flow, the rocky shores are visibly marked by stripes of colour parallel to the sea’s edge. These bands or zones are composed of living things. They reflect the states of the tide, and the length of time that a particular level of the shore is uncovered. It is this which determines, in large measure, what can live there. At the lowest of the spring tides there is a zone exposed only during the last 8
hour of their fall – that sub-tide world where all the rock is painted a deep rose hue by the lime-secreting seaweeds that encrust it and where the gleaming brown ribbons of large kelps lie exposed on the rocks. Above this, towards the low tide mark the Irish moss spreads its low cushioning growth. Above this are seaweeds. But the most conspicuous zone extends to the upper line of the high tides. On an open shore with moderate heavy surf, the rocks are whitened by the crowded millions of barnacles. Above this, snails evolving towards land existence browse on vegetation or hide in seams and crevices in rocks. The hardiest species live in the upper zone. Some of the world’s most ancient plants, the blue-green algae thought to have originated aeons ago in the sea, have emerged from it to form dark tracings on the rocks above the high tide line, a black zone visible on rocky shores in all parts of the world. Above the high-water mark of neap tides is a band, the splash zone, that is more earth than sea. It is inhabited chiefly by pioneering species which have gone far along the road towards land life, because they can endure separation from the sea for many hours or days. With only minor variations, this pattern of life exists in all parts of the world. The differences from place to place are related usually to the force of the surf. This scenario, which continues to be played out today, is the basis of a credible theory to explain how life, which is thought to have arisen first in the oceans, could have begun to colonise dry land. Last June we commemorated 50 years since we first set foot on the Moon. We are already eyeing up its commercial potential but, before we recklessly exploit our neighbour, perhaps we should think about treating it with respect. Oremus
March 2020
