Tidal Friction and the Moon as a Failed Scientific Clock

By Brian Giedd

Many attempts have been made to determine the age of the earth and other celestial bodies. Some of these attempts have proven very promising, and some have not. One such not so successful attempt was made by George Darwin, the son of the famous naturalist Charles Darwin. George Darwin was a physicist and an astronomer, and that would explain why he was so interested in ideas such as the formation of the moon and the effects of tidal friction upon the earth and the moon. This paper will give a short background on George Darwin and his work, and will then focus on the successes and failures of his proposed model for dating the age of the earth.

For many people, George Darwin may be considered to be the father of modern geophysics, and he contributed much to the study of the origin of the moon. He also made extensive studies on the tides of the earth. He concentrated especially on solid body tides. Solid body tides are the gravitational effects that large bodies have on one another (The Talk.Origins Archive 1999). For example, the moons gravitational field causes the diameter of the earth to be about 4 meters greater at the equator (Odenwald 1995). Darwin proposed that these tidal effects on the spherical shape of the earth have changed as the moon has moved farther away in its orbit. Darwin's proposed idea for the creation of the moon came to be known as the "fission theory" of lunar formation. This theory basically stated that the moon broke away from the earth due to its rapid rotation. From this he also calculated that it would have taken at least 56 million years for the moon to reach its current distance from the earth. Even George Darwin himself was willing to admit that his idea of lunar formation was little more than a guess.

Darwin calculated an age for the earth based upon the same principles that he used in estimating the time that it had taken for the moon to reach its current distance from the earth. He used the idea that the tidal forces between the earth and the moon cause energy to be removed from the earth in the form of heat. This loss of heat energy would cause a decrease in the rate of the earth's rotation, and the slowing of the rotation of the earth on its axis could be used as a clock to make an estimate for the age of the earth. Using this clock, Darwin estimated that the age of the earth-moon system must be at least 56 million years (Dalrymple: 39). The starting point for Darwin's clock was the formation of the moon. This does not allow for a completely accurate date for the age of the earth, but the presence of the moon is necessary for the effects of tidal friction that Darwin used to make his calculations. The final condition of such a clock would only arrive when there is no longer a stable earth-moon system. The rate of the clock that Darwin was using would merely be derived from the calculations of the decreased rate of the earth's rotation. This could be determined by such criteria as the length of days and the cycles of the moon phases. During the time period in which Darwin was living, his ideas were considered to be a reasonable scientific exercise.

There were, however, some problems with Darwin's proposed age for the earth. First of all, even at the time, Darwin was willing to admit that his proposed ages for the moon and the earth were more of a lower limit in comparison to the actual ages. Today it is accepted that such a method as Darwin was using to date the earth is more of an exercise in futility than a reasonable scientific exercise. Darwin was able to calculate an age for the earth by looking at the effects of tidal friction. One of the measurable affects of tidal friction that he attempted to use was heat loss. Just as with any other type of friction, tidal friction releases energy in the form of heat, and as with other types of friction, tidal friction also leads to a decrease in velocity, or a slowing in the rate of rotation in this case. When all factors are considered, the rate of heat loss cannot be used as an accurate measurement for the fact that the earth is not simply cooling at a steady rate. We know that the heat loss that the earth undergoes is not due simply to the convection that was once thought to account for heat loss. Therefore, the exercise of calculating the age of the earth by determining the rate at which the rotation is slowing is simply too unreliable.

Another main problem that arose from Darwin's tidal friction age estimates for the earth and the moon is that some young-earth creationists now attempt to use information such as the rate of the moon's recession from the earth to prove that the earth is young. They state that the moon is receding at a few inches every year (The Secular Web Library 2002). The truth of the matter is that the moon is receding from the earth. The recession of the moon is caused by the tidal breaking that takes place between the earth and the moon. Tidal breaking causes the moon to accelerate in its orbit and therefore expand the orbit. Based upon measurable data, scientists have calculated that the moon is receding at a rate of 3.8cm/year (Pogge 2005). These measurements are taken by beaming laser rays at reflector arrays that have been positioned on the moon (Pogge 2005). At this rate, some young-earth scientists claim that the moon would have been so close a mere million years ago that the tides would have drowned the creatures on earth. It is said that based upon the rate of the moon's recession that it cannot be older than 10,000 years. Some young earth creationists claim that "The moon is already far too close to the earth. It is now know that, due to tidal friction, it is gradually moving farther away from us. Based on the rate of recession, the moon cannot be very old. If it were even 20,000 to 30,000 years old, it would at some earlier time have been so close-it would have fallen into our planet!" (Pathlights 2005).

It is safe to say that Darwin had good intentions in attempting to date the earth, and for the time period that he lived in, it probably did seem like a reasonable theory. Now, however, we are able to see that no significant age for the earth or the moon could be attained by such a method as observation of tidal friction. It is also interesting to note that the arguments that Darwin proposed for an earth that was at least 60 million years old are now being used by young earth creationists in an attempt to prove that the earth is no more than a few thousand years old.

  1. Dalrymple, G. Brent. 2004. Ancient Earth, Ancient Skies (The Age of the Earth and its Cosmic Surroundings). Stanford University Press, 247 p.
  2. Odenwald, Sten. 1995. Ask the Astronomer. http://www.astronomycafe.net/qadir/q277.html. Accessed: November 14, 2005.
  3. Pathlights. 2005. The Age of the Earth. http://www.pathlights.com/ce_encyclopedia/05agee2.htm. Accessed: November 14, 2005.
  4. Pogge, Richard. 2005. Lecture 20: Tides. http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit4/tides.html. Last Updated: October 19, 2005. Accessed: November 14, 2005.
  5. The Secular Web Library. 2002. Young Earth Proof #5. http://www.infidels.org/library/modern/dave_matson/young-earth/specific_arguments/moon_recede.html. Last Updated: September 13, 2002. Accessed: November 14, 2005.
  6. The Talk.Origins Archive. 1999. The Recession of the Moon. http://www.talkorigins.org/faqs/moonrec.html. Last Updated: December 2005. Accessed: November 14, 2005.