Todd MacLean, age 11 of Calgary, Alberta, Canada, for his question:
How are the tides controlled?
When we are very young, we wonder what makes the tides wash up and down the beaches. Later in life, we watch their regular rhythms and grasp the fact that someting must keep them within bounds. We wonder what stops an excessive high tide from sloshing the whole ocean over the land. Actually this cannot happen because the limits of the tides are controlled by the same mighty forces that create them.
The highest high tide can go so far and no farther before it must turn back to the ocean. Actually it has only three hours to advance and do its worst. Then it must begin to inch back to the level of low tide. Where there are funnel shaped estuaries, a high tide can advance in a mighty wall of water. But after three hours, it must retreat. The only thing that may delay it is a furious gale wind but this pile up of sea water on the shore is not related to the tides.
We know, of course, that the tides are caused by the gravitational forces of the moon and the sun. Together, they exert a tremendous pull on the fluid water in the enormous worldwide ocean. Naturally, the massive sun exerts more pull than the moon, but gravity diminishes with distance. The little moon exerts far more tidal pull on the earth because it is 400 times closer. It pulls on a global scale, forcing the world oceans to heave up and down as a single unit.
Since the earth spins on its axis, the moon appears to swing around the globe every 24 hours or so. As it passes overhead, it heaves a massive bulge of water above average sea level. This bulge of high tidal water trails the passing moon ¬and creates troughs of low water ahead and behind. The low tidal troughs cause a back¬lash of bulging water to pile up on the far side of the globe. Hence, as the moon passes overhead, it leads a parade of four tides high low, high low. This daily parade brings two high tides to every shore. One is caused by the moon passing over¬head, the other by the heaving backlash as the moon passes over the opposite side of the globe.
The tides are controlled and limited mainly by the earth's rotation. As it spins on its axis, first one side then another faces the moon. But rotation never ceases. Every 24 hours or so the moon passes overhead, leading its parade of changing tides around the globe. Each rising must subside as it passes on its way.
Other factors modify the general pattern. The moon's orbit is not a perfect circle and higher high tides occur when it comes closest. Twice during the lunar month, the moon lines us up with the sun. Then the sun and moon pull together and our high tides are higher. But even the highest high tides must turn back after three hours because its time limit is controlled by the rotating earth.