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Mystery of the Ashen Light of Venus pp 125–158Cite as

Venus as a Knowable World: Chasing the Ashen Light into the Space Age 1900–1980

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Part of the Astronomers' Universe book series (ASTRONOM)

Abstract

At the start of the twentieth century, astronomers had reached something of an impasse in terms of understanding the Ashen Light. Although the available documentary evidence suggests that it was then widely viewed as a real phenomenon, there was nothing close to a consensus as to what caused it. The status quo was ironically frustrated by the advance of technology: bigger and better telescopes offered no more information to visual observers about what was going on, and the photographic process failed to yield incontrovertible proof of the Ashen Light’s existence. The sense of frustration persisted for decades, and a certain malaise about understanding Venus failed to resolve when the first spacecraft to visit the planet sent back mostly featureless images of its bland and impenetrable upper cloud deck. Even the smallest hoped-for windows to surface views through the clouds remained stubbornly shut, and humanity’s knowledge of the deeper atmosphere was limited to information encoded into the infrared light and radio waves that leaked through the otherwise opaque clouds.

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Notes

  1. 1.

    In fairness, photographic emulsions of the time were largely unsuited to the task of capturing the emission of faint light except as deployed on the largest telescopes then available. If the Ashen Light were as faint as many eyewitness reports suggest, it was hopeless to expect that the “slow” emulsions then available could have recorded it. For an early account of the history of astronomical photography, see Daniel Norman, 1938, The Development of Astronomical Photography, Osiris, 5, 560–594.

  2. 2.

    It’s unclear why Newcomb thought that the Ashen Light was never seen against a dark night sky, for any cursory examination of the literature in his time would have turned up many such accounts. But Newcomb seemed inclined to disbelieve the physical reality of the Ashen Light, introducing it as an “illusion to which the sight of even good observers may be subject.”

  3. 3.

    1909, Astronomical Curiosities Facts and Fallacies, London: Chatto and Windus, 27.

  4. 4.

    1932, La ‘Lumière Cendrée’ de Venus en 1932. L’Astronomie, 46, 370. This appears to be the first unambiguous reference in the scientific literature to the “negative” Ashen Light.

  5. 5.

    As in the case of Simon Newcomb’s pronouncement of the “fact that it [the Ashen Light] is not seen … after the end of twilight in the evening,” it’s unclear why Danjon claimed something that was controvertible by literature references available in his time, unless he either deliberately ignored observations incongruent with his hypothesis or he was simply unaware of those accounts.

  6. 6.

    For an early history of digital imaging in astronomy, including an image that purports to be the first of an astronomical nature ever made with a proper digital imaging device, see J. Janesick and M. Blouke, 1987, Sky on a Chip: The Fabulous CCD, Sky and Telescope, 74, 238–242.

  7. 7.

    1927, Journal of the British Astronomical Assocation, 38, 64–66. His Scottish colleague, Henry McEwen, made a brief mention of the observation earlier in 1927 in JBAA, 37, 345, in which he wrote that Saxton “saw the whole disc, not as a ring, but as a very faint pearly whiteness or a suspicion of mistiness against the blue sky.”

  8. 8.

    1964, Quarterly Journal of the Royal Astronomical Society, 5, 295.

  9. 9.

    Alexander, A. F. 1964, Journal of the British Astronomical Association, 74, 122–124.

  10. 10.

    Heath to Baum in a letter of November 27, 1953, quoted in Baum [4], 191.

  11. 11.

    1956, The Strolling Astronomer, 10, 30–32.

  12. 12.

    “Walter H. Haas (1917–2015)” (April 8, 2015) https://www.skyandtelescope.com/astronomy-news/walter-haas-1917-2015-04072015/.

  13. 13.

    1950, La Visibilidad del Hemisferio Oscuro de Venus, Urania, 223–224.

  14. 14.

    Haas attributed the tale to “Mr. C.B. Stephenson, a graduate student at the Yerkes Observatory, and director of the Mercury Section of the ALPO” from a personal letter “in which he describes that Dr. A.B. Meinel has successfully achieved photographs within the narrow crescent of Venus, which was darker than the surrounding sky.” Stephenson specifically noted in his communication that Meinel’s photograph was taken “with ultraviolet light.” This probably involved use of a near-ultraviolet filter passing wavelengths a little shorter than 400 nanometers; below about 300 nanometers the Earth’s atmosphere becomes completely opaque to ultraviolet light due to absorption by ozone (O3) molecules. To date, it appears there is no other known data on this phenomenon in the ultraviolet region of the spectrum.

  15. 15.

    Haas here quotes Thomas William Webb, 1917, Celestial Objects for Common Telescopes (Sixth Edition), Vol. I, 74–76. “Noble” is Captain William Noble (1828–1905), an officer in the British Army who later retired to Sussex and built a private observatory, contributing to several English astronomical organizations over a half-century.

  16. 16.

    1934, La Lumière Cendrée de Vénus. L’Astronomie, 48, 289.

  17. 17.

    Citing J. Payer, 1927, Revue d’Optique, 6, 73.

  18. 18.

    “Secondary spectrum” refers to the colored fringes around objects induced by poor chromatic aberration correction in the objective lenses of refracting telescopes.

  19. 19.

    Payer, 293.

  20. 20.

    Modern values for these quantities vary from those Barbier assumed. In 2017, Christopher Kyba, Andrej Mohar, and Thomas Posch (How bright is moonlight? Astronomy & Geophysics, 58(1), 1.31–1.32) quoted a value of the maximum possible horizontal photopic illuminance due to full moonlight as ∼0.3 lux, or about 33,000 times less intense than direct sunlight. To find the corresponding luminance, or surface brightness, of the Moon, we need to know the solid angle it subtends. On average, the Moon has an angular diameter of about 31 minutes of arc, corresponding to a solid angle of ∼ 6.4 × 10−5 steradian. This implies that the brightest possible full Moon has a luminance of about 5000 candelas per square meter, which is roughly twice as large as Barbier’s value. Meanwhile, the Earthshine is observed to have surface brightnesses ranging from 13 to 16 magnitudes per square arcsecond, or 0.04–0.7 candela per square meter depending on the lunar phase angle and the Earth’s weather-dependent albedo (Montañés-Rodríguez, P., Pallé, E., & Goode, P. R. 2007. Measurements of the Surface Brightness of the Earthshine with Applications to Calibrate Lunar Flashes. The Astronomical Journal, 134(3), 1145–1149). This is anywhere from 8 to 150 times smaller than Barbier’s number. Given the modern values of the luminance of the full Moon and the Earthshine, their ratio is on the order of 7,000 to 125,000.

  21. 21.

    This result is reasonably close to a modern value. Leinert et al. (1998. The 1997 reference of diffuse night sky brightness. Astronomy and Astrophysics Supplement Series, 127(1), 1–99) cite a brightness at 500 nm for the zodiacal light 40∘ from the Sun along the ecliptic of 925 in a unit called S 10, where one such unit equals the brightness equivalent to that of one tenth-magnitude star of solar spectral type per square degree of sky. Given that the zodiacal light is scattered sunlight, it has a spectrum like that of the Sun and an effective temperature of 5800 kelvins; using the temperature-dependent conversion between magnitudes per square arcsecond and luminance given by Bará et al. (2020. Magnitude to luminance conversions and visual brightness of the night sky. Monthly Notices of the Royal Astronomical Society, 493(2), 2429–2437), the zodiacal light at this position on the sky has a surface brightness of about 9.7 × 10−8 candela per square centimeter, or a factor of four higher than Barbier’s value.

  22. 22.

    Aggregating literature data published between 1955 and 1996, Kimura and Mann (1998. Brightness of the solar F-corona. Earth, Planets and Space, 50(6–7), 493–499) found that the surface brightness of the solar corona 2∘ from the Sun in its equatorial plane is about 4 × 10−10 that of the surface brightness of the Sun itself. This is still some 5,000 times higher than the surface brightness of the night sky, so it’s arguable whether Barbier’s characterization of the corona at this elongation as “practically nothing” is reasonable.

  23. 23.

    1935, Recherche de la Lumière Cendrée de Vénus. L’Astronomie, 49, 264–268.

  24. 24.

    Barbier, D., & Schlumberger, R. 1936. Recherche de la Lumière Cendrée de Vénus Pendant la Conjonction Inférieure de 1935. L’Astronomie, 50, 27–34.

  25. 25.

    “Bleaching” here refers to the destruction of cone opsins, light-sensitive proteins in the retinal cone cells of the eye. At high, persistent light levels, bleaching of photopigments in the cone cells leads to a decrease in sensitivity to light while the photopigments regenerate. As Barbier suggests by his comment, the persistent, focused light of the Venus crescent as seen through the eyepiece of a telescope can create after-images on the retina if the eye position suddenly changes after several seconds of continuous observing. He thought this must explain why observers saw dark shapes adjacent to the crescent under such circumstances, which in this hypothesis result from a photochemical process in the retina and not in the atmosphere of Venus.

  26. 26.

    Barbier’s co-author, Rene Schlumberger, in a separate commentary (pp. 32–34), dismissed chromatic aberration as a cause of supposed Ashen Light observations and proposed the contrast effect as an alternative, noting that “…observing Vega, by day, our instrument has never shown us the violet halo, due to the secondary spectrum, while at night it is quite apparent.”

  27. 27.

    1960, Journal of the Royal Astronomical Society of Canada, 54, 99.

  28. 28.

    A unit of surface brightness, the Rayleigh is defined as follows: one Rayleigh corresponds to a rate of light emission of 106 photons per square centimeter of column per second. A ‘column’ in this context is integrated along the line of sight such that a three-dimensional object, like a cloud of emitting material, is collapsed down into a two-dimensional cross section—hence “per square centimeter.” This accounts for the fact that we view three-dimensional light emitters, assuming they remain transparent to radiation throughout, in projection.

  29. 29.

    The development of studies of Venus, Venus (ed. D.M. Hunten, L. Colin, T.M. Donahue, and V.I. Moroz) Tucson: University of Arizona Press, 8.

  30. 30.

    1968, Journal of the British Astronomical Association, 79(1), 52.

References

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  1. International Dark-Sky Association, Tucson, AZ, USA

    John C. Barentine

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Barentine, J.C. (2021). Venus as a Knowable World: Chasing the Ashen Light into the Space Age 1900–1980. In: Mystery of the Ashen Light of Venus. Astronomers' Universe. Springer, Cham. https://doi.org/10.1007/978-3-030-72715-4_6

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