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First Light: Early Accounts of the Ashen Light 1643–1800

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Part of the Astronomers' Universe book series (ASTRONOM)


In the generation after Galileo’s pioneering discovery of Venus’ phases, early adopters of astronomical telescopes were disappointed when their instruments revealed essentially nothing more about the planet. It remained an impenetrable mystery, a dazzlingly white disc marred by the false rainbow of chromatic aberration. What was Venus? Was it just a mirror of the sunlight that illuminated it? What was its nature that made it so reflective? Imperfections in the glass from which telescope optics were then made further distorted planetary images, yielding spurious flares, shadows, and haloes. What was real, and what was not?

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Fig. 3.1
Fig. 3.2
Fig. 3.3
Fig. 3.4
Fig. 3.5


  1. 1.

    Francesco Fontana (c. 1580–1656) was an Italian lawyer and an astronomer.

  2. 2.

    Shown here as Fig. 3.2.

  3. 3.

    Riccioli here describes the observed colors of chromatic aberration in an uncorrected refracting telescope.

  4. 4.

    An oath; compare with ‘by Jove’ in English.

  5. 5.

    Almagestum novum, Vol. 1, pp. 484–85. Translated by the author from the original Latin.

  6. 6.

    The “first” and “last” phases of the Moon indicated here are the waxing crescent and waning crescent, respectively.

  7. 7.

    An archaic term for inferior conjunction, describing the situation in which Venus appears closest to the Sun and is nearest Earth during its orbit.

  8. 8.

    Dominique François Jean Arago (1786–1853) was a French polymath and, in later life, President of the Executive Power Commission of the French Second Republic. For 44 days in the summer of 1848, he served as the French head of state.

  9. 9.

    An obsolete term for the crescent phases of the Moon and planets, derived from the Latin word falx, referring to the shape of a sickle or scythe.

  10. 10.

    See Chap. 5 for a full exploration of this phenomenon.

  11. 11.

    The German astronomer Albert Marth (1828–1897) determined the date of the eclipse to which Derham refers here as May 3, 1715, N.S. (footnote on page 111 in The Astronomical Register Vol. 14, No. 161, May 1876). The date of this eclipse helps constrain the time period in which Derham made his first Ashen Light observation “some years ago.”

  12. 12.

    Recounted by Eduard Schönfeld, 1866, Astronomische Nachrichten, 1586, 27.

  13. 13.

    Here Kirch refers to the distorting effect of turbulence in the Earth’s atmosphere, through which light from astronomical objects passes before reaching the telescope.

  14. 14.

    Kirch correctly understood what he saw as an effect of the eye-brain combination. Just as bright stars are apparently slightly larger to the naked eye than faint stars, the brightness of the lit part of the crescent Moon in relation to the strong contrast with the much lower-intensity night side, including earthshine, leads the brain to believe that the day side is disproportionately large compared to the size of the rest of the disc.

  15. 15.

    Here Safarik means that the telescope Mayer used was not corrected for chromatic aberration, typical of certain small telescopes used to make astronomical timing measurements.

  16. 16.

    1762, Observationes Veneris Gryphiswaldenses, Greifswald: A. F. Röse, 19. The observation is recounted in Johann Hieronymus Schröter, 1811, Beobachtungen des grossen Kometen von 1807, Göttingen, Appendix, 74.

  17. 17.

    “As Venus approaches within 15 of the Sun, faint luminous arcs creep out from the needlelike cusps to extend the slender crescent beyond its geometric limit. … These arcs continue to advance around the dark rim of the planet until finally at inferior conjunction they encircle Venus with a ring of faint bluish light. The effect is at once dramatic and spectacular.” (R. Baum, Journal of the British Astronomical Association, Vol. 105, No. 5, p. 216, October 1995). Baum warned that the observation of this phenomenon through a telescope is literally so dangerous, due to the inevitable proximity of the Sun, that it “is not for the inexperienced.” (emphasis in the original).

  18. 18.

    This appears to be Johann Hieronymus Schröter, based on Herschel’s reference in another footnote to Beiträge Zu Den Neuesten Astronomischen Entdeckungen (1788) by Schröter and Johann Elert Bode, and Schröter’s Selenotopographische Fragmente (1791). Schröter himself first reported seeing the Ashen Light during observations of Venus on May 21, 1793, according to Schorr [64]. Herschel here makes a point that his observations did not qualitatively agree with Schröter’s.

  19. 19.

    Safarik cites his source as “Berliner astronomisches Jahrbuch fur 1793, p. 188,” but there is no mention of Venus on that page. Rather, it appears that Safarik quoted the wrong Jahrbuch. A publication for a given year documented events of the 3 years previous; the correct reference is the same page number in the Jahrbuch for 1796.


  1. Safarik, A. (1874). On the visibility of the dark side of Venus. Report of the British Association for the Advancement of Science, 404.

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  2. Schorr, F. (1875). Der Venusmond: Und Die Untersuchungen Uber Die Fruheren Beobachtungen Dieses Mondes. Braunschweig: Friedrich Vieweg and Son.

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Barentine, J.C. (2021). First Light: Early Accounts of the Ashen Light 1643–1800. In: Mystery of the Ashen Light of Venus. Astronomers' Universe. Springer, Cham.

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