J. H. Schröter & the ghost mountains of Venus
2021 November 30
What follows is the final paper by Richard Myer Baum, written shortly before his death in late 2017. In it we hear the distinctive voice of one of the most compelling historians of observational astronomy, much of whose work has graced the pages of the BAA Journal for more than half a century. Richard was a fine observer of the Moon and planets, and his enduring interest lay in the mysteries uncovered on our neighbouring worlds by observers of the past. Sometimes the findings of those telescopic explorers turned out to be illusory, as was the case in the present paper, but Richard’s great experience as an observer allowed him to understand the deceptions practised on the human eye and mind when working at the limits of perception. His accounts of the past were always based not only on insight and sound scholarship, but also upon a nuanced understanding of human fallibility.
He was also acutely aware of his own fallibility and would work upon draft after draft of his articles and books, often reluctant to release them into the realm of finality. For that reason this paper – on a topic over which he ruminated for many years and which he considered in several earlier articles and chapters – must be considered unfinished, since it is based upon drafts found on Richard’s computer after his death. In particular, it lacks the complete and carefully assembled apparatus of footnotes and references that was the hallmark of Richard’s thoroughness as a historian. However, some footnotes were found referring to the earlier pages of this article and these have been included here. Illustrations are drawn from Richard’s earlier writings on the subject, as well as from the archives of the Mercury & Venus Section.
For almost 300 years, from the early part of the seventeenth century to the dawn of the Space Age, there existed in the minds of some astronomers a vision of vast mountain ranges in the southern hemisphere of the planet Venus; of majestic ramparts and snow-clad summits, thrusting magnificently into the visual remit of Earth-based observers. Camille Flammarion (1842–1925), for instance, asserted with full confidence that ‘the surface of the planet is quite as uneven as that of the earth, and even more so; that there are there Andes, Cordilleras, Alps, and Pyrenees, and that the most elevated summits attain a height of 44,000 metres (27 miles). It has even been ascertained that the northern hemisphere is more mountainous than the southern.’1 It was a bold, rather flamboyant statement, echoed in part by Ellen Mary Clerke (1840–1906) in her exquisite booklet The Planet Venus (1893). In the chapter on the ‘Cytherean Alps’, as she described them, Clerke – sister of Agnes Mary Clerke (1842–1907), the well-known historian of astronomy – viewed the mountains as ‘high enough to protrude … above the cloud canopy of the planet. Their glaciated aspect would be accounted for both by their height and by their proximity to the cold hemisphere.’2 Scottish author and astronomer, Hector Copland Macpherson (1888–1956), though similarly disposed, thought the planet less mountainous than some authorities believed.3 Meditating on the plurality of inhabited worlds, Thomas Dick (1774–1857), a fellow Scot, speculated that ‘the existence of mountains on the planets is therefore a proof, or, at least, a strong presumptive evidence that they are habitable worlds; for a perfectly smooth globe could present no great variety of objects nor picturesque scenery, such as we behold in our world.’4 Nevertheless this probably owed more to the changing cultural attitudes to mountains then taking place in Europe, than to the aesthetics of other worlds. Ostensibly, the hypothesis seemed highly plausible. Even so, there were many dissenters as we shall presently see.
In a sense the ‘Himalayas of Venus’, as I long ago termed them,5 are a legacy of the revolution in science that transformed our concept of the Universe; a reminder of Galileo’s astonishing discoveries with the newly invented telescope, which transformed the heavenly bodies from the smooth polished spheres of Aristotelian cosmology into Earth-like worlds. This provided a platform for commentators to speculate, like the astronomer Jean le Rond d’Alembert (1717–1783) in his article ‘Planète’ in Diderot’s Encyclopédie, that ‘Since Saturn, Jupiter and their satellites, Mars, Venus and Mercury are opaque bodies which receive their light from the sun, which are covered by mountains and surrounded by a changing atmosphere, it seems to follow that these planets have lakes, have seas … in a word, that they are bodies resembling … the earth. Consequently, according to many philosophers, nothing prevents us from believing that the planets are inhabited.’6 Hence, it follows that the detection of irregularity in the profile of Venus prompted immediate comparison with the Moon.
Yet for all that, there was a problem. What had been reported amounted to little more than a mirage, for no one had actually seen mountains on Venus: only recurrent anomalies in the profile of the planet displayed as minute bright irregularities, either on the disc or as protuberances at the limb of the planet. As François Arago (1786–1853), the sagacious director of the Paris Observatory, explained: ‘Let us suppose the planet to be without any asperities, to be perfectly smooth, its crescent will always be terminated by two exactly parallel and very acute points. Let us assume, on the contrary, that Venus is covered with mountains. Their interception of the rays of the solar light will occasionally prevent one or other of the horns, or both at once, from being regularly formed; the crescent will no longer appear completely symmetrical; the horns will not be always pointed, always similar to each other; they will appear truncated. This is the mode in which the phenomenon actually appears as we have already seen by the drawings given by MM. Beer and Mӓdler.’7 These anomalies would vary in shape and position, exactly as they might be expected to do when their cause is considered. ‘Now a plain or sea, now a high table-land between light and darkness; now valleys, now mountain peaks would diversify the seeming figure of the boundary’,8 interposed Richard Anthony Proctor (1837–1888). The irregularity would vary in its disposition in accordance with the diurnal rotation of the planet. Arago concluded that the planet Venus, then, is not a smooth body, in reiteration of the opinion expressed by the French Academy of Sciences in 1700. It all seemed very plausible to the savants of those days.
The mountains are now beyond history and, like the phantom planet Vulcan, the canali of Mars and the apocryphal satellite of Venus, are remembered only as relics of emotional prejudice from an innocent and less critical era. It may thus be asked: why write about non-existent entities which even in their own time many assigned to the shadow land of dreams, where memory and imagination, not strata of rock are the absolute arbiters of truth and value?
However, such an approach is justified. As a ‘star’, Venus is incomparable. Like the sirens of mythology, it beguiles the eye and conditions us to expect appearances of striking aspect. Yet as an object of telescopic enquiry it tests the most experienced of observers, confounding all with a brilliance that ‘dazzles the sight and exaggerates every imperfection of the telescope’,9 to cite John Herschel. Deception aside, repeat appearances of the irregularities, usually of the same form and in the same location, seemed reason enough to deny fiction. Ultimately fiction became fact and an integral, if eccentric, part of the history of solar-system exploration, one that enjoyed a lifespan somewhat longer than the gossamer wisps of Mars. More importantly, the episode showcases the relative reliabilities of theory and observation, as the cosmologist Hermann Bondi demonstrated in his landmark paper ‘Fact and inference in theory and in observation’.10
Textured with details that deserve more than the few words they usually attract, the hypothesis enables us to address the history of planetary astronomy from a novel perspective, obtaining insights which lead to a deeper understanding of the observational practices of the past, and helps to explain how for three centuries or so, this tournament of shadows, these mountains of the mind, stubbornly retained a hold on the imagination to become an intriguing tradition of popular descriptive astronomy.
As time passes, memory grows dim. A hundred years from now, even history majors will not have heard of the Venus mountains. But we are not at that point yet, and it is well to remember the older ideas that helped establish our knowledge of the planet. Today’s myth was once a presumed reality. Importantly, it is rich in detail relevant to the present, for who can say how much of our current thinking will not be thrown onto the scrap heap in times to come?
The mountains discovered
Effects consistent with the supposition of inequalities of surface level on Venus were apparently first described by the Neapolitan lawyer and amateur astronomer Francesco Fontana (1585–1656) around 1643. Tito Livio Burattini (1617–1681), in Poland, obtained a similar impression in 1666. A jagged terminator implanted the idea in Fontana’s mind. Burattini was more circumspect, offering no detail or inferences other than making a comparison with the Moon.11 Jesuit professor of philosophy and astronomy at Bologna, Giovanni Battista Riccioli (1598–1671), commented on the ‘irregularly horned aspect’ of the planet in his Almagestum Novum, a comprehensive treatise on astronomy published in Bologna in 1651.
In his Astro-Theology, a record of celestial observations first published in 1714, the English cleric William Derham (1657–1735) spoke of ‘Roughness on the Concave Part of the enlighten’d Edge (such as we see in the New Moon)’ as he was ‘Viewing Venus with Mr. Huygens’s Glass divers Nights, when near her Perigee, and much horned’.12 Observing Venus in daylight close to inferior conjunction in 1700 August, Philippe de la Hire (1640–1718) described ‘… greater inequalities in the termination of light in Venus, than in the moon’,13 which induced the Paris Academy to conclude ‘that planet to have higher mountains’. One of the last advocates of note was the French astronomer E. L. Trouvelot (1827–1895). From an extensive series of observations made between 1876 and 1892, he concluded the south cusp cap was very broken upland, studded with numerous high peaks.14 In 1912, William Henry Pickering (1858–1938) also called attention to the bluntness of the south cusp and referred to the mountain hypothesis as a possible explanation,15 while in 1928 F. E. Ross (1874–1960) referred to the subject in his epoch-making paper ‘Photographs of Venus’.16
However, it would be an unnecessary imposition to list the many and varied sightings to be found in the archive, quite apart from associated ideas and inferences. It is enough to say that where no maps exist, the coloured spectacles of imagination take over, a fact exemplified by the work and ideas of the exemplary Hanoverian astronomer Johann Hieronymus Schröter of Lilienthal.
Fact vs. fiction: the mountains affirmed
Unable to find sure anchorage anywhere on the apparent surface of Venus, Christiaan Huygens (1629–1695) let it be known he had:
often wonder’d that when I have viewed Venus at her nearest to the Earth, when she resembled an Half-moon, just beginning to have something like Horns, (…) she always appeared to me all over equally lucid, that I can’t say I observ’d so much as one spot in her, tho in Jupiter and Mars, which seem much less to us, they are very plainly perceived. For if Venus had any such thing as Sea and Land, the former must necessarily show much more obscure than the other, as any one may satisfy himself, that from a very high Mountain will but look down upon our Earth. I thought that perhaps the too brisk Light of Venus might be the occasion of this equal appearance; but when I used an Eye-glass that was smok’d for the purpose, it was still the same thing. What then, must Venus have no Sea, or do the Waters there reflect the Light more than ours do, or their Land less? or rather (which is most probable in my opinion) is not all that light we see reflected from an Atmosphere surrounding Venus, which being thicker and more solid than that in Mars or Jupiter, hinders our seeing anything of the Globe it self, and is at the same time capable of sending back the Rays that it receives from the Sun?
Melchior della Briga (1686–1749), reader in mathematics at the Florentine College, writing to Francesco Bianchini at Rome on 1726 September 7, metaphorically shook his head, asking where was the proof? How could the idea of a thick atmosphere be reconciled with the irregularities Philippe de la Hire had seen in the terminator when Venus was near inferior conjunction in August of 1700? Is it not true that in the Moon we regularly see what de la Hire saw on Venus? Had not the savants of the Paris Academy of Sciences taken this into account in their deliberations when they decided Venus has mountains higher than those of the Moon, and certainly more impressive than those of the Earth? Are we to say that so reliable an observer as de la Hire was mistaken? Does Huygens’ surmise provide sufficient grounds upon which to mount a contradiction?
Briga thought not. With no apparent opposition and backed by the authority of the Paris Academy, he pronounced in favour of mountains. He considered their alleged height exaggerated, but ascribed it to de la Hire’s use of a horizontal solar parallax of six arcseconds (the modern value is 8.794 arcseconds) and thought the true value would emerge when, as predicted by Edmond Halley (1656–1742), a fresh determination of the solar parallax would be possible after the transit of Venus in 1761.
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