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What Is the Light? Historical Explanations of the Ashen Light

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Before diving into the possible reasons for why something on the night side of Venus may be glowing, it’s worth reviewing the evidence from the historical record to date.

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Fig. 5.1
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  1. 1.

    1859, Die Erleuchtung des Planeten Venus durch die Erde, Freiburg: Wagner. The figure of 480 times is quoted in Rheinauer, 1862, Grundzüge der Photometrie, Halle: H. W. Schmidt, 56; see also C. V. Zenger, 1863, On the visibility of the dark side of Venus, Monthly Notices of the Royal Astronomical Society, 43, 331. Of Rheinauer himself, very little is preserved in the historical record. He lived in Offenburg, then in the Grand Duchy of Baden and now a town in Baden-Württemberg, Germany, and was active in the mid-nineteenth century. He taught physics at the Offenburg Gymnasium and dabbled in astronomy.

  2. 2.

    Astronomical magnitudes assign a number to objects like stars according to their brightness. The magnitude system is unfamiliar to most people in part because it is logarithmic in nature: one integer step in the system indicates a difference in brightness of one power of the base number 2.512, chosen such that the system characterizes the human perception of the brightnesses of stars well. The brightest star in the night sky, Sirius, has an apparent visual magnitude of about − 1.5; Rheinauer’s comparison here, at magnitude +14, is some 1.6 million times fainter to the eye.

  3. 3.

    Both Henry McEwen and R. E. Pressman reported observations of the Ashen Light in December 1939, some 200 days before the following inferior conjunction of June 26, 1940, and 380 days after the preceding inferior conjunction of November 20, 1938.

  4. 4.

    Napier assumed the detection thresholds based on the pioneering laboratory studies of Blackwell (1946, Contrast Thresholds of the Human Eye, Journal of the Optical Society of America 36(11), 624–643) and Middleton (1957, “Vision through the Atmosphere” in Geophysik II / Geophysics II. Handbuch der Physik / Encyclopedia of Physics, Vol. 10 / 48. Springer: Berlin, Heidelberg). For reference, a natural, unpolluted night sky on Earth has an average luminance of about 1.71 × 10−4 cd/m2, or 22 magnitudes per square arcsecond.

  5. 5.

    Johann Heinrich von Mädler (1794–1874) was a German astronomer who, together with Wilhelm Wolff Beer (1797–1850), published Mappa Selenographica, the first scientifically accurate map of the Moon, in four volumes between 1834 and 1836. For a more complete account of what von Mädler reported in 1833, see R. Baum, 1988, Historical Note: The Maedler Phenomenon, Journal of the British Astronomical Association, 98(6), 293.

  6. 6.

    The north magnetic pole is now moving nearly 50 km a year relative to the Earth’s rotational north pole; since its formal identification in 1831, the north magnetic pole has accumulated a displacement of about 2,000 km from the Boothia Peninsula in the Canadian far north to a position high in the Arctic Sea. The magnetic pole’s rapid motion and a decades-long observed decrease of the intensity of the Earth’s magnetic field have led some to speculate that the planet is about to undergo a reversal of its intrinsic magnetic field direction. These geomagnetic reversals have occurred randomly some 183 times over the last 83 million years; see, e.g., S. C. Cande and D. V. Kent, 1995, Revised calibration of the geomagnetic polarity timescale for the late Cretaceous and Cenozoic, Journal of Geophysical Research, 100, 6093–6095.

  7. 7.

    For recent work on this idea, see S. A. Jacobson, et al., 2017, Formation, stratification and mixing of the cores of Earth and Venus, Earth and Planetary Science Letters, 474, 375–386.

  8. 8.

    J. G. Luhmann, 1991, Induced magnetic fields at the surface of Venus inferred from pioneer Venus orbiter near-periapsis measurements, Journal of Geophysical Research, 96, 18831–18840.

  9. 9.

    T. L. Zhang, et al., 2012, Magnetic Reconnection in the Near Venusian Magnetotail, Science, 336(6081), 567–570.

  10. 10.

    He expanded on these ideas in “Über die Sichtbarkeit der dunkeln Halbkugel des Planeten Venus,” from Sitzungsberichte der Böhmischen Gesellschaft der Wissenschaften in Prag, 243.

  11. 11.

    1795, On the Nature and Construction of the Sun and Fixed Stars, Philosophical Transactions of the Royal Society of London, 85, 51.

  12. 12.

    Known colloquially as ‘foxfire,’ a faint blue-green light can be produced by certain species of fungi growing on decaying wood. This type of bioluminescence results from luciferase, a type of oxidative enzyme that emits light when it reacts chemically with luciferin, a small-molecule substrate. Not uncommon in the biological world, a luciferase variant is what powers the familiar glow of fireflies.

  13. 13.

    D. R. Griffin, R. Hubbard, and G. Wald. 1947. The sensitivity of the human eye to infra-red radiation. Journal of the Optical Society of America 37(7), 546–554.

  14. 14.

    V. G. Dmitriev, et al. 1979. Nonlinear perception of infra-red radiation in the 800–1355 nm range with human eye. Soviet Journal of Quantum Electronics, 9(4), 475–479.

  15. 15.

    PNAS, 111, 50, E5445–E5454.

  16. 16.

    Recorded in the Journal of the British Astronomical Association, 64, 60–62, ed. E. A. Beet and D. A. Campbell.

  17. 17.

    Sheehan (1988) makes a strong case that the apparent “spokes” Lowell claimed to have seen on Venus may have been nothing more than defects in Lowell’s eyes, obliquely illuminated and seen in shadow against the bright disc.

  18. 18.

    Quoting Percival Lowell, 1910, in The Evolution of Worlds, New York: MacMillan, 80.

  19. 19.

    A Handbook of Descriptive and Practical Astronomy, Oxford University Press (4th ed.), 1, 101. Chambers (1841–1915) was an English barrister and author who switched to law after initially studying engineering. The 600-page first edition of his Handbook was published when Chambers was only 19 years old, undergoing an expansion to three volumes in subsequent editions.

  20. 20.

    The Reappearance of Venus Observed 8 October 2015, American Astronomical Society Meeting #231, abstract 144.11.

  21. 21.

    1914, La Planète Vénus en 1913, L’Astronomie, 28, 221–232.

  22. 22.

    In order to explain the observed pattern of light that accounts for the phenomenon, Danjon means that each discrete point of light along the crescent becomes the source of its own violet halo.

  23. 23.

    1934, Sur la Pretendue Lumière Cendrée de Venus, L’Astronomie, 48, 370–372. Friedrich von Hahn reported that the Ashen Light effect was reduced in angular extent and considerably fainter as the Venus crescent became very thin around inferior conjunction, consistent with Danjon’s explanation. See von Hahn’s 1793 drawings illustrating this effect in Fig. 3.5.


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  3. Sheehan, W. (1988). Planets and perception: telescopic views and interpretations, 1609–1909. Tucson: University of Arizona Press.

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Barentine, J.C. (2021). What Is the Light? Historical Explanations of the Ashen Light. In: Mystery of the Ashen Light of Venus. Astronomers' Universe. Springer, Cham.

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