Mark I. Vuletic

Last updated 22 May 2008

Background

The creationist contention as posed to me in personal correspondence is that the rotation of the Earth has been slowing down at a rate of 30 seconds per century, and that extrapolating back 4.5-billion years from this would give us a rotation rate that would have disintegrated the planet. Thwaites and Awbrey (1982) discuss a creationist publication that alleges a 1 second per year slowing rate, from which it extrapolates massive deformation rather than disintegration.

Analysis

(i) Apparently, both of the rates given by the creationists above are wrong. According to Thwaites and Awbrey (1982), the rotation of the Earth is decreasing at a rate of 0.005 seconds per year every year.

(ii) Given the rate supplied by Thwaites and Awbrey, and assuming its constancy over the entire history of the Earth, we can calculate the length of a rotation on Earth 4.5 billion years ago. The manner in which Thwaites and Awbrey cast their calculations has proved confusing to my readers, so I will recast them in what I hope will be a clearer form, though the differences are merely in presentation and not in substance.

The first critical thing to understand is that the Earth's rotation rate and revolution rate are independent of one another: one revolution around the Sun has always taken about 31,536,000 seconds, regardless of the rotation rate.

The second critical thing to understand is what scientists mean when they claim that the Earth's rotation rate is slowing by 0.005 seconds per year every year. They do not mean that one year ago, a rotation took 0.005 seconds less than it does today. To express that, they would say that the rotation rate is slowing by "0.005 seconds per rotation per year." The actual rate given means that if the Earth today can complete 365.25 rotations in 31,536,000 seconds (that is, one revolution), then one year ago the Earth would have required only 31,535,999.995 seconds to complete the same number of rotations. Since this is 0.005 seconds short of a full revolution, that means that one year ago, there were slightly more than 365.25 rotations in a revolution. There was an extra tiny bit of a rotation equal to (0.005 s)(1 hr/ 3600 s)(1 rotation / 24 hrs), which is close to 600 millionths of a rotation: truly tiny.

The slowing rate means that every time the Earth makes a trip around the Sun, you can cram another extra such little bit of rotation into the trip. 4.5 billion years ago, of course, you had 4.5 billion extra such little bits in the year, which add up to 260.42 extra rotations. These are extra rotations, so the total number of rotations in a year all the way back then was the sum of the current 365.25 rotations and the extra 260.42 rotations: namely, 625.7 rotations per year.

Now, if, 4.5 billion years ago, there were 625.7 rotations in a year, and there are always 31,536,000 seconds in a year (because, recall, the rotation rate has no impact on the revolution rate), then we can figure out how many hours there were in a rotation back then, by dividing the 31,536,000 seconds by 625.7, and then converting the result into hours. In the end, this gives us 14.01 hours per rotation.

Thus (assuming, of course, the constancy of the slowing rate), a rotation on Earth 4.5 billion years ago would have been about 14 hours long. Through comparison with Jupiter's ten-hour rotations, Thwaites and Awbrey conclude a 14-hour rotation is insufficient to cause significant deformation (much less disintegration) of the Earth.

(iii) Chris Stassen has pointed out to me that given the slowing rate of 0.005 seconds per year every year, there would have been around 400 days per year in the Devonian Period, which corresponds nicely to the approximately 400 daily growth layers per year present in Devonian corals.

(iv) Stassen also points out that the assumption of a uniform slowing rate is uncertain. The rate becomes

much less accurate with increasing time (particularly back to near the origin of the Earth). There are still arguments over the forces which dominate the slowing, and how much stronger or weaker they would have been when integrating backwards in time. (Stassen, personal correspondence, 1997).

Stassen recommends as resources Thwaites and Awbrey 1982, Cazenave 1982, Bursa 1982, and Mignard 1982.

References

P. Brosche and J. Sunderman (eds.). 1982. Tidal Friction and the Earth's Rotation II. Berlin: Springer-Verlag.

M. Bursa. 1982. On some topical problems of the dynamics of the Earth-Moon system. In Brosche and Sunderman 1982:19-29.

A. Cazenave. 1982. Tidal friction parameters from satellite observations. In Broshce and Sunderman 1982:4-18.

F. Mignard. 1982. Long time integration of the Moon's orbit. In Brosche and Sunderman 1982:67-91.

W. Thwaites and F. Awbrey. 1982. As the world turns: can creationists keep time? Creation/Evolution IX:18-22.

Acknowledgements

Chris Stassen graciously provided me with help with this claim early on.