Our Dynamic Earth and the Length of Day

A modern philosopher believes that the scientific game is basically endless.
He who decides one day that scientific statements do not call for any other tests,
and that they can be regarded as finally verified, retires from the game.

K.R Popper (1980) in The logic of Scientific Discovery


Earth and the Moon are interactive partners in our Earth’s geologic history. The Moon’s gravity, with the Sun’s assistance, causes Earth’s aqueous and terrestrial body tides. These gravitational forces create a tidal bulge that, as Earth rotates, maintains a position relative to the Moon that imposes a gravitational torque acting as a drag on Earth’s rotation. In effect, this tidal bulge extracts energy (Angular Momentum) from Earth, slows its rotation, and increases the length of day.

The Earth’s tidal bulge, largely controlled by the Moon, has mutual effects. It transfers dynamic energy to the Moon, currently causing it’s average orbital radius to increase at about 3.6 to 3.8 centimeters annually. Extrapolating backward, it is readily seen that earlier in geologic time, the Moon has been closer to the Earth. When in closer orbit, Lunar induced tides would have been higher and more intense; energy extraction from Earth and its transfer to Lunar orbit greater; and the rate of slowing in Earth rotation more rapid.

The Moon’s rate of recession from Earth and the number of its annual orbits were greater in the past.

As the Moon’s orbit expanded, it’s influence on Earth tides and the tidal torque (often incompletely described as tidal friction), diminished. The Earth and Moon are, thus, an inter-active pair.

It may, therefore, be reasonably inferred that earlier periods of Earth history featured more intense tides, a closer but more rapidly receding Moon, and most important, a shorter period in Earth’s (sidereal day) —- a day that slowly lengthens as the Earth’s rotation diminishes.

The importance of significant reduction in the Earth’s rate of rotation (increasing the length of day) probably dates back to the Archaean and cannot be emphasized too strongly. Biological “clocks” based on marine fossils, and the timing of lunar and solar eclipses extrapolated into the past, make it reasonable to postulate that the Earth had days of less than 10 hours in the Proterozoic and Archaean (two to three billion years BP; Before Present).

The work of numerous researchers based on a variety of marine “paleo-clocks” supports an apparent day length of 21.9 hours in the Devonian (370 Ma B.P.) and proportionately shortened days in other periods of the Paleozoic and Mesozoic. If these data are indicative of a generalized truth, it is incumbent upon Earth scientists to examine the probable effects that constant rotational deceleration would have on Earth’s tectonic history and to determine, with greater accuracy and understanding, the factors related to the Earth’s and the Moon’s interrelated dynamic history.

The most important factor related to Earth rotation is its effect on gravity. The value of gravity expressed as an acceleration factor “g” is a function of attraction between two masses and the distance between them. Earth’s effective net gravity,(g), varies by location, geometry (polar flattening), and the Earth’s rotation imposing a centrifugal force.

At the equator, net gravity is a combination of static gravity and centrifugal force (Fc). Centrifugal force is dependent upon rate of rotation, and acts in opposition to gravity. As Earth’s rotation rate declines, Fc diminishes and effective net gravity is increased. At higher rates of past rotation, the value of net g was substantially reduced by the subtraction of greater Fc. Earth’s length of day is, therefore, a significant factor in the value of net g.

At the Earth’s poles, the value of net g is greater than at the equator due to two factors:

1) Centrifugal force at the poles reduces to zero; and 2) the distance from the surface to the Earth’s center of mass is less at the poles than at the Equator due to oblateness.

For discussion, consider the quantitative effect on net g influenced by length of day, only for sites along the equator. As distances from the equator increase, these effects will , of course, remain applicable, but only in lesser values (function of the cosine^2 of angle of latitude).

The hypothesis proposed herein may be summarized as follows:

  • A) Lunar tidal torque (and that from the Sun) causes Earth’s rotation to slow down and its day to lengthen. The present rate of slowing is about .0017 second per century; or about 1.0 second in 58,800 years. This reduction in rate of rotation increases the weight of all crustal rocks in proportion to their density and Earth’s geometry.
  • B) Projected back three billion years, Earth may have had solar day lengths as short as 4 to 6 hours. This would have profoundly affected the Earth’s physical characteristics and all aspects of its environment.

C) As Earth’s rotation slows, the following occurs:

  1. Centrifugal force decreases and net gravity “g” increases.
  2. As gravity increases, the weight of Earth’s lithosphere, asthenosphere, and other materials increases (weight = mass x g) as a function of density.
  3. As the weight of Earth’s lithosphere and asthenosphere increase, Earth’s internal pressures increase.
  4. Increased internal pressure causes an increase in average Earth density, and a reduction in total internal volume.
  5. The reduction in total Earth volume, along with reduced oblateness (a lesser factor), both result in reducing the Earth’s total surface area.
  6. Crustal materials of different density (ocean basins and continents), adjust isostatically as the value of gravity increases.
  7. Influenced by isostatic adjustments, basaltic ocean basins tend to subside, while lighter granitic continental areas will subside less, hold steady, or elevate due to lateral migration of dense mobile mantle from below oceanic basins to beneath continental areas.
  8. Eustatic sea levels are affected either positively or negatively as the ocean’s waters adjust to areal geometry, the changing depth of the ocean basins, and the vertical motion of the continents.
  9. Crustal surface adjustments occur almost continuously in a tectonic environment dominated by compression, increasing net gravity, and probably plate motion (ie.drift ).

These unceasing forces act in concert to modify the Earth’s restless surface, and influence world climate. This has been the major factor controlling Earth’s geologic history and the evolution of life. Though greatly reduced, these interrelated mechanisms continue today.