Tides: Does the Moon Pull the Sea?
The moon does pull the sea, and it is the pulling of the sea by the moon that makes the tides. In any great dock or port or harbor we see how the water rises and falls twice a day. We know that the tides, as we call them, with their ebb and flow, bring all this water to us and take it away again without ever stopping. The tides never stop because the earth never stops turning, and it is the turning of the earth that somehow makes the tides. Plainly, tides have something to do with days, for they always correspond. Long ages ago, before men knew that the earth turns, they saw, as they could not help seeing, that the tides were in some way influenced by the moon.
The tides always correspond in their ebb and flow to the rising and setting of the moon. The moon is made of matter, and so is the water of the sea. All matter pulls and is pulled toward all other matter everywhere. We call this gravitation. The whole earth and the whole moon are affected by this pull; but as part of the earth is ocean, and as water is not rigid, it is specially affected by gravitation. The water opposite the moon at any time is pulled up toward the moon. As the earth is turning all the time, this means that a mighty wave of water travels over all the oceans, day and night, in response to the pull of the moon. If the moon had oceans, there would be tides there, too, owing to the earth’s pull; and as the earth is very much bigger than the moon these tides would be enormous. But the moon has no oceans, though possibly it has ocean-beds long since dried up. All the moon does is simply to pull the water of the earth’s oceans toward it as the earth twists.
Let us suppose for a moment that the moon did not go around the earth, but simply moved through space with it. Then the moon would appear to rise and set, as it does now, only it would rise and set at regular hours every day. And so, at the same time every day, in any part of the world, there would be tides, as there are now.
The only difference between this and what actually happens is that the moon is moving round the earth, while the earth turns on herself. This makes the moon seem to rise and set about half an hour or so later every day. Tables of tides for certain places are computed far in advance, to aid mariners and others. You can learn from such tables at what time of day there will be high tides in New York Harbor, for instance, and how high the tides will be, a month from to-day, or even a year from to-day.
The sun causes tides on the earth as the moon does, and for exactly the same reason; but the power of gravitation lessens as the distance through which it acts increases. Though the sun is vastly bigger than the moon, it is so much farther away from the earth that its influence on the ocean is comparatively small; but it can be shown.
As the moon goes round the earth once a month, there will always be times when the moon and the sun are on the same side of the earth, and times when they are on opposite sides. When the sun and the moon are pulling on the same side of the earth or when they are opposite each other with the earth between them, they help each other; and the tides for a few days will be very high and very low as the water flows and ebbs. At another time in the month, when the moon and the sun are at right angles to each other in relation to the earth, they do not help each other.
The moon pulls the hardest, but it has to be content with pulling the water toward itself much less than at the other time, because the sun’s pull is not now with it.
