Dynamical, biological and anthropic consequences of equal lunar and solar angular radii

Abstract

The nearly equal lunar and solar angular sizes as subtended at the Earth is generally regarded as a coincidence. This is, however, an incidental consequence of the tidal forces from these bodies being comparable. Comparable magnitudes implies strong temporal modulation, as the forcing frequencies are nearly but not precisely equal. We suggest that on the basis of palaeogeographic reconstructions, in the Devonian period, when the first tetrapods appeared on land, a large tidal range would accompany these modulated tides. This would have been conducive to the formation of a network of isolated tidal pools, lending support to A. S. Romer's classic idea that the evaporation of shallow pools was an evolutionary impetus for the development of chiridian limbs in aquatic tetrapodomorphs. Romer saw this as the reason for the existence of limbs, but strong selection pressure for terrestrial navigation would have been present even if the limbs were aquatic in origin. Since even a modest difference in the Moon's angular size relative to the Sun's would lead to a qualitatively different tidal modulation, the fact that we live on a planet with a Sun and Moon of close apparent size is not entirely coincidental: it may have an anthropic basis.

Is There a Tectonically Driven Supertidal Cycle?

“…K1, in contrast, will be resonant only once in the current cycle, at 100 Myr. This is in agreement with results for the late Silurian (430 Ma), which showed more energetic tides than during the Early Devonian (400 Ma; H. Byrne, personal communication, 2017;Balbus, 2014). Pangea, the previous supercontinent, formed around 330 Myr ago and started breaking up some 180 Myr ago.…”

“…Herold et al (2012) and Green and Huber (2013) show that the changed location of abyssal tidal dissipation during the Eocene (55 Ma) can explain the reduced meridional temperature Geophysical Research Letters 10.1002/2017GL076695 gradients seen in the proxy record for sea surface temperature that coupled climate models have struggled to reproduce (see Herold et al, 2012, for a summary). Furthermore, the reduced tidal dissipation during the Mesozoic and Cenozoic eras reported by Green et al (2017) had implications for lunar recession rates and hence for interpreting cyclostratigraphy and long-term climate cycles (Waltham, 2015), for the evolution of the Earth-Moon system (Green et al, 2017), and for the evolution of life (Balbus, 2014).…”

The Tides They Are A‐Changin': A Comprehensive Review of Past and Future Nonastronomical Changes in Tides, Their Driving Mechanisms, and Future Implications

“…The reason for the overall weak tide is simply that the large horizontal scale of the ocean external to the supercontinent is out of resonance and thus not supporting a large tide. Consequently, there is no forcing in the deep ocean for large tides in local or regional basins either, although local resonances can of course still occur and lead to amplifications of up to a factor 10 (e.g., Balbus, for some theoretical considerations). As continents drift apart, basins may go in and out of resonance, thus enabling large local and regional tides.…”

Colloquium: Physical constraints for the evolution of life on exoplanets

“…The biological ramifications of tidal modulations arising from the presence of two planetary companions were discussed byBalbus (2014) andLingam and Loeb (2018e).…”