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Synchronizing to the Moon

April 11, 2016

It's hard to ignore the Moon. The main problem with the plot of the "Planet of the Apes" movie is that the time-traveling astronauts should have recognized the Moon after a single day and known that they were back on Earth. Our huge, solitary Moon with its recognizable features is unique in our Solar System, and such solitary moons are likely rare elsewhere in the galaxy. Lunar eclipse of November 9, 2003 (Oliver Stein)

Lunar eclipse of November 9, 2003, caused by Earth's shadow.

The regularity of total eclipses of the Sun caused by the Moon's passage between the Earth and the Sun is one reason for man's initial interest in science.

(Photo by Oliver Stein, via Wikimedia Commons.)

The large size of our Moon compared to that of its host planet is an anomaly. The leading theory of the origin of the Moon is that the Earth was impacted by a large body, a planetesimal called Theia, about 4.5 billion years ago (Theia was the mother of Selene, the goddess associated with the Moon in Greek mythology). The debris from this collision reassembled into a smaller Earth and the Moon. This theory still has a few gaps, so there's still no scientific consensus on the matter.

As I wrote in two previous articles (
General Lunacy, August 9, 2013, and Moonstruck, April 16, 2015) the Moon has considerable affect on the Earth, most more scientific than the transformation of werewolves at the time of the full moon. The existence of the Moon might be the reason why life exists on Earth.[1]

Charles Darwin thought that life on Earth started with a chemical soup in a "warm little pond."[2] In a letter to his close friend, Joseph Dalton Hooker, February 1, 1871, Darwin wrote,
"It is often said that all the conditions for the first production of a living organism are now present, which could ever have been present.— But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,—light, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter wd be instantly devoured, or absorbed, which would not have been the case before living creatures were formed."[2]

The lunar tides are thought to have provided the means of concentrating ocean chemicals in littoral ponds to form Darwin's soup.[1] Tidal flow would have stocked and restocked the ponds with chemical compounds, and our rapid tidal cycle would have aided in the formation of polymer precursors to nucleic acids.[1]

Aside from creating tides, the Moon has an influence on terrestrial life because of its cycle of night time illumination over a period of the lunar month. Studies have shown lunar rhythms in aquatic animals;[3] in particular, a 1958 study showed evidence for lunar periodicity in the breeding of the scallop,[4] and the numbers of several species of flying insect, Ephemeroptera (Mayfly), Trichoptera (Caddis fly), and Diptera (true fly), showed periodic fluctuations that correlated with the phase of the Moon.[5]


An 1896 illustration of a Mayfly.

Mayflies are are an evolutionary backwater, and they exhibit traits likely present in the first flying insects. Aristotle and Pliny the Elder, both of whom get frequent mention in this blog, noted the brief lives of adult mayflies.

(Via Wikimedia Commons.)

Lunar affects are not absent from humans. The obvious example is the period of the human menstrual cycle, shown to have a mean of 29.5 days, or exactly one lunar month.[6] A 2013 study has shown that people sleep poorly around the time of the full moon,[7] possibly a relict of our ancestors' biological clock. In our past, humans needed to be more alert for predators when the Moon was full. At the time of the full moon, the time to fall asleep was found to increase by five minutes, and the duration of sleep, as assessed by electroencephalography, was reduced by twenty minutes.[7]

Some things, however, have been incorrectly thought to be under the Moon's influence. There's an old wives' tale of more births at the time of the full Moon, but a 1979 study of 8,142 natural births (those not induced by drugs or Caesarean sections), at the University of California, Los Angeles hospital, showed no correlation with lunar phase.[8] Other supposed correlations with such things as automobile accidents and hospital admissions have been disproved.[9]

While we think of tides in terms of water, there's an atmospheric tide as well, a small amplitude effect called the lunar gravitational semidiurnal atmospheric tide.[10] Scientists from the University of Washington (Seattle, Washington) reasoned that this tide, which results in a slight volume change in the atmosphere, will modulate relative humidity and rainfall. They detected such an effect through examination of a 15 year global precipitation data set acquired in the Tropical Rainfall Measuring Mission.[10-11]

Says Tsubasa Kohyama, a University of Washington doctoral student in atmospheric sciences who co-authored the study, "As far as I know, this is the first study to convincingly connect the tidal force of the moon with rainfall."[11] Kohyama noticed a slight oscillation in the air pressure while doing another study, and he and University of Washington John (Michael) Wallace, were able to demonstrate the validity of the phenomenon after two years of study.[11]

The change in air pressure caused by the phase of the Moon was first detected in 1847. A concurrent phenomenon, a change in temperature from volume changes of the atmosphere, was shown in 1932. The University of Washington scientists quantified the air pressure effect in an earlier paper (see figure).[12] The air pressure is higher when the Moon is overhead, or underfoot.

lunar semidiurnal tide (Kohyama and Wallace)

Amplitude of the lunar semidiurnal tide, as recreated with a regression of data in a period of a half lunar synodic cycle (14.765294 days). Positive deviations are shown in red/orange, negative in blue.

(Fig. 1 of Ref. 12, licensed under a Creative Commons Attribution-NonCommercial-NoDerivs License.)[12]

When the Moon is overhead, Earth's atmosphere will bulge towards it, there will be more atmosphere, so the weight of the atmosphere, its pressure, will be higher.[11] Higher pressure increases the temperature of the air; and, since warmer air can hold more moisture, the relative humidity is less so there's less chance of precipitation.[11] Using data from the joint NASA/Japan Aerospace Exploration Agency Tropical Rainfall Measuring Mission, Kohyama and Wallace were able to show that rainfall is slightly lighter when the moon is high in the sky.[11]

This is a very small effect, just about 1%, so it's not really noticeable.[11] Says Kohyama, "No one should carry an umbrella just because the moon is rising."[11] This finding would be a good test for climate models to see whether they're sensitive enough to reproduce this lunar effect.[11] This research was funded by the National Science Foundation and some Japanese agencies.[11]

Moon's affect on rainfall

Moon's affect on rainfall.

The upper graph shows the air pressure, the middle graph shows the rate of change in air pressure, and the bottom graph shows the rainfall difference from the average. The change is less than one ten thousandth of an inch per hour.

(Tsubasa Kohyama/University of Washington image.)


  1. Bruce Dorminey, "Without the Moon, Would There Be Life on Earth?" Scientific American, April 21, 2009.
  2. Alison Pearn, "Darwin's "warm little pond," The Darwin Project, February 15, 2012.
  3. R. M. McDowall, "Lunar Rhythms in Aquatic Animals A General Review," Tuatara, vol. 17, no. 3 (December, 1969), pp. 133-143.
  4. James Mason, "A possible lunar periodicity in the breeding of the scallop, Pecten maximus (L.)," Journal of Natural History, ser. 13, vol. 1, no. 9 (1958), pp. 601-602.
  5. W. Danthanarayana, "Lunar Periodicity of Insect Flight and Migration," Proceedings in Life Sciences (1986), pp. 88-119.
  6. Winnifred B. Cutler, "Lunar and Menstrual Phase Locking," American Journal of Obstetrics and Gynecology, vol. 137 (1980), p. 834.
  7. Christian Cajochen, Songül Altanay-Ekici, Mirjam Münch, Sylvia Frey, Vera Knoblauch and Anna Wirz-Justice, "Evidence that the Lunar Cycle Influences Human Sleep," Current Biology, vol. 23, no. 15 (August 5, 2013), pp. 1485-1488, July 25, 2013, DOI:10.1016/j.cub.2013.06.029.
  8. G.O. Abell and B. Greenspan, "Human births and the phase of the moon [Letter]," New England Journal of Medicine, vol. 300 (January 11, 1979), p. 96, DOI: 10.1056/NEJM197901113000223.
  9. Stuart Wolpert, "Stop blaming the moon, says UCLA scientist - Study highlights flaws in earlier research on hospital admissions and the lunar cycle," UCLA Press Release, March 30, 2015.
  10. Tsubasa Kohyama and John M. Wallace, "Rainfall variations induced by the lunar gravitational atmospheric tide and their implications for the relationship between tropical rainfall and humidity," Geophys. Res. Lett., vol. 43 (January 30, 2016), DOI: 10.1002/2015GL067342. This is an open access article with a PDF file here.
  11. Phase of the moon affects amount of rainfall, University of Washington Press Release, January 29, 2016.
  12. Tsubasa Kohyama and John M. Wallace, "Lunar gravitational atmospheric tide, surface to 50 km in a global, gridded data set," Geophysical Research Letters, vol. 41, no. 23 (December 16, 2014), pp. 8660-8665, DOI: 10.1002/2014GL060818. This is an open access publication with a PDF file available here.

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