Recession of the Moon

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“”This is yet another example of a phantom problem. YECs love to raise these, and try to give the impression that mainstream scientists ("evolutionists") are floundering around in a desperate struggle to salvage their belief in long ages, and avoid the obvious conclusion that there's no answer except the YEC one. But usually the problem, if it ever existed, has been solved many years ago[.]

—Mike Hore, old Earth creationist

Young earth creationists (YECs) often claim that the recession of the moon, at its current rate of recession (how fast it moves away from the Earth), would have been too close to the Earth to survive at some point in its scientifically accepted 4.5 billion year lifespan, which indicates a young universe.

Some of these claims may stem from early scientific confusion. For example, Slichter 1963 (using a simpler Earth-moon model) found that the Moon couldn't have receded from Earth for more than 1.4–2.3 billion years. Later research in the 1970-80s (discussed below) improved this model and found that recession could indeed have occurred for 4.5 bn years. Yet even the very first version of this argument, by Thomas Barnes in 1982, ignored this relevant research, preceding to cite Slichter. And creationists, over 30 years on, still claim scientific failure. If so little research is done on this relatively simple subject, creationist scientific honesty in general is seriously undermined.

Ultimately, research shows that the recession of the moon is consistent with a 4.5 billion year age.

Fixed-rate variant

Figure 1, from AIG.

Several creationists claim that the Earth and moon would have smashed together relatively quickly at the current rate of lunar recession.

First, the facts:

1. The moon is currently an average 38,440,000,000 cm (384,400 km) away from the center of the Earth.
2. The moon recedes (moves further away) at a hefty 3.82±0.07 cm per year. This occurs because (see Figure 1) The moon causes tides on Earth we are familiar with. Earth, in turn, causes much greater tides in the moon. The tidal friction has a heating effect of which the energy is lost to space. The loss of angular momentum to tidal friction is why the solar system has expanded since formation and why the moon recedes from Earth. This is the mechanism by which the ratios of orbital periods and velocities of the planets are maintained according to the mass-distance relationships between them, as provided for by Newton's law of universal gravitation. Velocities decrease with radial distance from the sun; orbital periods increase with radial distance from the sun. Cyclicity of the solar system suggests that the current rate of the lunar recession may be very different from what it has been at other times.
3. The Roche limit is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal forces exceed the second body's self-gravitation. The distance depends on the properties of the two celestial bodies in question. In the Earth-Moon system's case, it is anywhere from roughly 9,500 km to 18,500 km from the center of Earth, depending on the assumptions made. The calculations cited in this article assume an Earth-Moon system Roche limit of 18,400 km.

The ironically named EvolutionFacts claims the moon cannot be more than 30,000 years old (and its rebuttal):

Scientists have discovered two interesting facts: (1) the moon is already far too close to the earth, and (2) it is gradually moving farther away from us. This is called recession of the moon. Due to tidal friction, the moon is slowly spiraling outward away from planet earth! Based on the rate at which the moon is receding from us, the earth and the moon cannot be very old. This is an important point and in no way can be controverted. The present rate of recession clearly indicates a young age for the earth-moon system. If the moon were older, even 20 to 30,000 years old, it would at that earlier time have been so close that it would have fallen into the earth!

If we assume that this rate was constant for the past 30,000 years (like EvolutionFacts), the moon would have moved 114,600 cm, or about 0.000298% of the total distance and 0.000313% of the distance to the Roche limit. EvolutionFacts' claim is almost entirely incorrect.

Kent Hovind claims the moon cannot be more than 3,000,000 years old:

The Moon is receding a few inches each year. Less than a million years ago the Moon would have been so close that the tides would have drowned everyone twice a day. Less than 2 or 3 million years ago the Moon would have been inside the Roche limit and, thus, destroyed.

If we assume that this rate was constant for the past 3,000,000 (6 mil) years (like Hovind), the moon would have moved 11,460,000 cm, or about 0.0298% of the total distance and 0.0313% of the distance to the Roche limit. Hovind's claim is almost entirely incorrect.

W. T. Brown claims that the moon cannot be 4,600,000,000 (4.6 bn) years old:

Since 1754, observations of the moon's orbit have indicated that it is receding from the earth. As tidal friction gradually slows the earth's spin, the laws of physics require the moon to recede from the earth. However, the moon should have moved from near the earth's surface to its present distance in several billion years less time than the 4.6 billion year age that evolutionists assume for the earth and moon.

If we assume that this rate was constant for about 4,600,000,000 (4.6 bn) years (the accepted age of the Earth and the moon), the moon would have moved about 17,572,000,000 cm, or about 45.7% of the total distance and about 48.0% of the distance to the Roche limit.

And finally, if we assume that this rate was constant, it would take about 10,000,000,000 (10 bn) years for the moon to touch Earth.

Thus, it is definitively proven that, if we assume that the rate is constant, then this is no evidence for Young Earth creationism.

Backwards-extrapolation variant

ICR's Thomas Barnes stated first in 1982 and then again in 1984:

The present speed of recession of the moon is known. If one multiplies this recession speed by the presumed evolutionary age, the moon would be much farther away from the earth than it is, even if it had started from the earth. It could not have been receding for anything like the age demanded by the doctrine of evolution. There is as yet no tenable alternative explanation that will yield an evolutionary age of 4 billion years or more for the moon.

Barnes is correct: as noted above, if we assume that this rate was constant, it would take about 10,000,000,000 (10 bn) years for the moon to touch Earth. Barnes then quote mines Slichter 1963:

Dr. Louis B. Slichter, Professor of Geophysics at Massachusetts Institute of Technology[,] treats this problem in great detail and concludes that "the time scale of the earth-moon system still presents a major problem."

However, assuming a constant rate for the moon's recession is flawed. (Surely creationists who whine so often about uniformitarianism should know this better than anyone else!) Slichter 1963 mentioned that the conflict between the Earth's age and the moon's recession age is resolved if the tidal torque that the Moon exerts on Earth was much less in the past.

Changing tidal torque

By 1982 (2 years before Barnes republished), physicists (Hansen, Finch) had shown that this is indeed the case, because the effectiveness of this tidal brake on the earth's rotation strongly depends on the configuration of the oceans. Hansen showed that the number, sizes, shapes, and locations of the continents and continental shelves hugely impact the Moon's tidal torque; compared to typical continental features in the past, today's continental features cause "anomalously high" tidal torques. Plate tectonics models confirm that roughly once every 0.5 bn years, the crust undergoes the supercontinent cycle, in which a single supercontinent breaks up and reforms. Single supercontinents result in much less tidal torque than Earth's present configuration of multiple smaller continents.

Brush 1983 summarized:

The present rate of tidal dissipation is anomalously high because the tidal force is close to a resonance in the response function of the oceans; a more realistic calculation shows that dissipation must have been much smaller in the past and that 4.5 billion years ago the moon was well outside the Roche limit, at a distance of at least thirty[-]eight earth radii [or ca. 240,000 km (24,000,000,000 cm)] (Hansen 1982; see also Finch 1982).

The 1993 astronomy textbook Astronomy Today gives a "moon's-closest-to-Earth-distance" estimate of 250,000 kilometers, which agrees very closely with Brush's figure. Thus, the "problem" disappears.

More recently, Bills et al. 1999 showed that accounting for factors such as the natural tidal modes of the oceans entirely resolves the problem (brackets in original):

To our mind, the dynamical solution of the "time scale" problem for the lunar orbit evolution has been solved by the ocean models presented by Hansen [1982], Webb [1982], Ooe et ai. [1990], and Kagan and Madova [1994]. These authors all find that ocean tide models, satisfying various forms of Laplace's hydrodynamic equations, generate significantly smaller torques in the distant past than implied by the present f-value (9). All of them find ages for the lunar orbit greater than 3 Gyr. A heuristic explanation, couched in terms of oceanic normal modes, is as follows: From work by Platztnan et al. [1981] and others, we know that the presentday ocean has a rich spectrum of normal modes, some having frequencies close to the main tidal spectralines. These modes grow increasingly complex (i.e., higher wavenumber) with increasing frequency. In the distant past when the Earth's faster rotation forced tidal frequencies higher, those normal modes with near-tidal frequencies would be spatially less well matched to the large-scale tidal forcing (a degree- 2 spherical harmonic), and hence they would be less easily excited. Note that this does not imply smaller ocean tidal heights--in fact, they were likely comparable or larger than present heights [Webb, 1982, Figure 5J--because the tidal potential was larger; rather the tidal adminance to that potential was reduced. In particular, the degree 2 part of the admittance, which completely quantifies the dissipation, was reduced. The torques were therefore correspondingly smaller than they would otherwise have been if the admittances had maintained their present-day values.

Thus, scientific consensus as early as the 1990s disagrees with information that creationists continue to publish today.

Slower earth rotation

See the main article on this topic: Rotation of the Earth

Creationists argue that, because the Earth rotated faster in the past, tidal torque would have been greater, and so the recession of the moon would also have been faster. (They also argue that this puts an upper age limit on the Earth far less than 4.6 billion years; this is entirely incorrect.) CreationWiki:

A faster rotation rate for the Earth causes the tidal bulges' lead on the Moon to be larger, and this increases the net tidal force, which causes the moon to recede faster.

While true, this effect is more than overridden by the past increased orbit speed of the moon, documented in the section below.

Lunar orbit speed

According to Kepler's laws, the lower the distance between Earth and Moon the less time it takes for the moon to orbit the Earth. This means that the Moon orbited much faster in ancient times if it was much closer to Earth. Therefore the frequency of the tides was lower, because the difference of orbit time and the time of the rotation of the earth was lower (even if rotation was faster, because the relative time change of earth rotation is less than relative orbit time change of the Moon). Energy dissipation and the drag force on the Moon are dependent on the tidal frequency and the tidal strength, but even if the tidal strength was larger due to the closer Moon, the effect of the lower tidal frequency prevailed. This simply means that the Moon receded even slower in ancient times. E.g. in the extreme case when Earth and Moon are corotating, i.e. the Moon is so close that it circles the Earth in exactly the same time as the Earth revolves, the frequency of the tides would be zero, as would energy dissipation and drag force. The Moon would not recede at all, although the tides (which would then rather be permanent deformations of the Earth, being always at the same place) would be very high because of the close Moon.^{[citation needed]}

Decreasing-recession-rate variant

“”It is perhaps humorously ironic that both DeYoung and Brown fail, because they are implicitly making an improper uniformitarian assumption (the constancy of dissipation and deformation), which evolutionists have learned to avoid.

—Tim Thompson at Talk Origins

In response to this updated research, creationists have updated their counterarguments. (Similarly, however, they tend to cite 20-year-old research.) Numerous creationists (such as AIG, CMI, CreationWiki, the Center for Scientific Creation) have a more technical explanation, in which lunar recession rates were higher in the past.

Creationist response: The Moon is Still Young, Malcolm Bowden

See also

• Rotation of the Earth (the Earth's spin is running down too fast)
• Lunar dust (there's not enough moon dust)
• Lunar formation (the moon couldn't have formed)
• Lunar radioactivity (the moon is too hot)
• Lunar atmosphere (there's too much moon gas)
• Lunar bukkake hypothesis (the moon got shot with water during the Flood)