Tidal Slowdown, Coral Growth,

and the Age of the Earth

The earth is slowing down. It is not rotating as fast today as it was in the past. This slowdown is caused by the tides which the moon raises on the earth. However, before we explain this slowdown, we need to discuss how tides are formed.

All physical objects attract one another through the force of gravity. This force depends upon the mass of each object and upon the distance between the objects. The closer they are to each other, the more strongly they attract. In the case of the moon and the earth, the moon pulls the ocean water on the nearer side of the earth more strongly than it pulls the rest of the earth, so the water forms a bulge. The opposite occurs on the far side of the earth. Here the water experiences the least pull because it is farthest from the moon. The rest of the earth is pulled away from the water, forming another bulge of water pointing away from the moon, though the bulge is not quite as large as the first one. The moon holds these bulges (more or less) in place while the earth rotates beneath them. The net effect is the tides we experience at the seashore.

Tides slow down the earth's rotation speed because of friction between the earth and the water under which it rotates. The effect is very small - a slowdown rate of about 0.0002 seconds per day per year. This means that as each year goes by, each day of the year lasts 2 hundred-thousandths of a second longer. Thus the length of the day changes by about 20 seconds every million years.

Since the earth's rotation is slowing down, it took less time in the past for the earth to rotate on its axis than it does today. But the time for one complete rotation is the length of a day. So if the days were shorter in the past, then there were once more days in the year than there are now. This, of course, assumes that the length of the year has not changed. This is a reasonable assumption, since the year is the time (measured in unchanging units) it takes for the earth to go once around the sun, and there is no known mechanism to make any measurable changes in this period over a few billion years.

If we assume that the rate of slowing of the earth's rotation has been constant, we can calculate the number of days in a year at various times in the past (Hayward, 1985, p. 95). Suppose we want to know how many days made up a year in the Devonian period, estimated to have been some 400 million years ago. Each day was 20 sec shorter per million years x 400 million years = 8,000 seconds shorter. This means each day was only 21.8 hours long then, as opposed to 24 hours per day now. Since a year is 8799 hours long (24 hours/day x 365.25 days/year, using modern-length days) and this length has not changed, we can calculate the number of ancient days in a Devonian year by dividing 8766 hours/year by 21.8 hours/day, to get about 400 days/year. A similar calculation for the Pennsylvanian period, beginning about 280 million years ago, gives 22.4 hours/day, or 390 days in the Pennsylvanian year.

The reason for choosing the Devonian and Pennsylvanian periods is that we can check to see if these calculations correspond to reality. In certain modern corals and shellfish, we find growth-bands that indicate yearly, monthly, and even daily growth, rather like the annual rings that trees produce. By counting these bands, we can determine how long a particular coral or shellfish lived just as we can for a tree by counting its rings. We can also see that there are about thirty daily bands per month and about 365 daily bands per year for modern corals and shellfish. But careful analysis of the growth-bands of fossil corals and shellfish from the Devonian and Pennsylvanian has confirmed that years in these periods contained more days than years do now, and that the number of days per year for both these periods is remarkably close to the values calculated above.

This correlation between theory and observation is striking. After all, three different modes of dating are used here, and they all correlate with each other. The fossils are dated by the rock layers in which they are found, which dating ultimately depends on radiometric methods (decay rates of radioactive elements). The growth bands in the fossils are biological in origin, depending on the response of the organism to daily, monthly and yearly changes in environment (light, weather, and temperature). The earth's slowdown is an astronomical phenomenon. The three processes upon which the dates depend - radioactivity, biological growth, and tidal friction - are independent processes, yet all three combine to form a coherent, natural picture of what is happening.

This is surely not mere coincidence. The Lord himself seems to be providing sincere seekers and careful investigators with valid evidence that the earth is old.

Dr. Perry G. Phillips

Materials for this tract were developed from:

Hayward, Alan. 1985. Creation and Evolution: the Facts and the Fallacies. Triangle Books. London.