In striking contrast to the vague shadings sometimes reported by visual observers are the prominent markings clearly seen on ultraviolet photographs of Venus. They were discovered at Mount Wilson Observatory by Frank Ross, a pioneer in the photography of the planets through monochromatic filters. During a favorable eastern elongation of Venus in June and July of 1927, he obtained a series of photographs of the planet through the 60-inch and 100-inch reflectors in six regions of the visible spectrum and in infrared and ultraviolet light.
Ross expected infrared to offer the greatest promise; it was already routinely used in aerial photography because of its ability to penetrate haze and give the clearest views of the Earth’s surface from aircraft at high altitudes. But, to Ross’s surprise, his infrared images of Venus proved to be every bit as bland as those taken in visible light.
However, the ultraviolet images showed distinct dark streaks and bands roughly perpendicular to the terminator, presumably owing to the presence of UV-absorbing materials in the planet’s upper cloud deck. These features also appeared on photographs taken in deep violet light (3800-4000 angstroms, at the threshold of perceptibility to the normal eye), but they showed much higher contrast on the ultraviolet plates (3400-3800 angstroms). On these they displayed about the same level of contrast as the dappled “seas” of the Moon show to the naked eye.
Ross, however, was unable to discern an obvious rotation period from his photographs. He ventured a very tentative estimate of about 30 days. His work was not immediately followed up, quite possibly because the notorious intractability of the problem made the subject unappealing.
Thirty years would pass before Ross’s discovery would be exploited – not by a professional astronomer but by a French amateur named Charles Boyer. Born at Toulouse in 1911, he experimented with wireless at an early age. This became the basis of his lifelong friendship with fellow ham radio enthusiast Henri Camichel, a professional astronomer at Pic du Midi Observatory in the French Pyrenees. Shortly before the Second World War, Camichel imparted to Boyer an interest in astronomy.
In France the gulf separating professional and amateur astronomers has not been as wide as in the United States. A number of great French planetary astronomers began their careers as amateurs, some never forsaking their amateur status. The great Eugene M. Antoniadi, for instance, always referred to himself simply as an “astronom voluntaire de l’Observatoire Meudon.” It was quite natural, therefore, that Camichel and Boyer continued their association by mail after the war, when Boyer embarked on a career in colonial Africa – first as Chief Magistrate at Dahomey (now Cotonu) and finally as President of the Bench at Brazzaville in the Congo, a position he held from 1955 until his retirement in 1963.
At Brazzaville, located only 4[degree sign] south of the equator, the planets were favorably placed for observation high in the sky, and the humid atmosphere was often exquisitely steady. Realizing his opportunity, Boyer constructed a 10-inch Newtonian reflector around a primary mirror made by renowned optician Jean Texereau. The resulting instrument was optically superb, but it rode atop a rather rudimentary altazimuth mounting. Boyer asked Camichel to suggest observing projects. It happened that Camichel was photographing Venus in the ultraviolet at the time, and he proposed that his friend also attempt to do so.
The advent of computer-controlled dual-axis drives for altazimuth mountings lay decades in the future, so Boyer’s telescope was ill suited to photographing the planets – a process requiring very steady tracking during exposures several seconds long. But the resourceful amateur cobbled together an ingenious device for moving his camera across the focal plane of his telescope at the correct speed using parts from a Meccano erector set.
In August and September of 1957, when the air was unusually dry at Brazzaville, he began to photograph Venus, which was then at evening elongation. He used Kodak Micro-File film, a high-contrast, relatively fine-grained emulsion that is painfully slow by today’s standards. Lacking a proper ultraviolet filter that blocked visible light, he made do with a blue-violet Wratten 34 filter that transmitted wavelengths shorter than 4500 angstroms.
Although the images in Boyer’s photographs were extremely small and aesthetically unappealing, he soon thought he could detect the return of the same dusky spot to the terminator at intervals of about four days. This feature made five returns between August 28th and September 16th. Alerted to the suspected four-day rotation period by Boyer, Camichel examined his own set of images. He, too, found evidence for a four-day period.
Boyer continued his observing campaign from Brazzaville until 1960. By this time he and Camichel had come to regard the four-day rotation of the upper atmosphere of Venus as “completely uncontestable.”
Boyer had taken the precaution of depositing a sealed envelope describing his discovery with the French Academy of Sciences in 1957. Not everyone was able to see the pattern in his tiny images, however. The eminent French planetary observer Audouin Dollfus recounted in 1992: “I examined the images carefully. They did not seem to me completely convincing at the time.” But with Camichel’s unflagging support, Boyer persisted. The four-day rotation became his idee fixe – in fact, he did no other astronomical work of importance before his death in 1989. His first published article, coauthored by Camichel, appeared in the popular magazine L’Astronomie in 1960, followed by papers in the prestigious journals Annales d’Astrophysique and the Comptes Rendus de l’Academie des Sciences. They failed to attract much attention or to elicit many comments.
Indeed, recognition of Boyer’s discovery was agonizingly slow. Even today, the details of the improbable story are little known outside of France. (A recent book in English makes the following bald statement about the prespacecraft era: “Strangely enough, the best results of all at that period came from visual work by the French observers, who recorded a characteristic Y-shaped dark feature centered on the equator.” This is, of course, untrue; they did not use visual means.)
In the meantime, radio astronomers in the United States and the Soviet Union bounced radar impulses off Venus’s solid surface in 1962. These revealed the very slow, 243-day retrograde (backward) rotation of the planet’s solid body. This finding seemed utterly incompatible with the four-day rotation rate of the planet’s upper atmosphere. How could the Venusian cloudtops rotate 60 times faster than the underlying surface? Shortly after the announcement of the radar results, Boyer and Camichel submitted a paper on the four-day rotation to Icarus, the leading international journal of planetary science. One of the journal’s referees, a young Harvard astronomer named Carl Sagan, rejected it on the grounds that “the four-day rotation is theoretically impossible, and shows how foolish the work of the inexperienced amateur can be.”
Following his retirement in 1963 Boyer himself frequently worked beside his professional colleagues at Pic du Midi. They were able to follow the motions of the UV markings continuously for periods of up to six hours. Among these were the now-famous Y- and y-shaped markings.
In 1964 yet another French astronomer, Bernard Guinot, employed a sensitive technique known as interference spectroscopy to measure Doppler shifts very precisely. From these he determined the radial velocities of various points on the limb of Venus. His data also suggested that the cloud canopy circulates every 4.3 days.
But final, irrefutable proof of Boyer’s discovery did not come until 1974. The Mariner 10 spacecraft, encountering Venus in February of that year, imaged the planet in the ultraviolet during its approach. When these images were combined into a movie sequence, the four-day retrograde rotation of the upper atmosphere was dramatically confirmed. Dollfus recalls that when he showed Boyer a copy of this film, Boyer reacted with la belle indifference; it contained no surprise, since he already knew the result.
So how did Boyer manage to solve one of planetary astronomy’s oldest and most enduring mysteries – a mystery that had defeated the best efforts of generations of astronomers, dating back to the time of the elder Cassini at Paris Observatory in the reign of Louis XIV?
Boyer was very clear in his purpose and methodical in his observing program, and he used an excellent telescope in a favorable climate. But even this does not completely explain his remarkable achievement, for three decades earlier Ross had employed far more powerful instruments that recorded far more detail. Here, ironically, even Boyer’s apparent deficits worked to his advantage. As Dollfus later explained: “Lack of resolution in this case helped, by making the true picture of what was happening clearer. On images of Venus taken with larger instruments, such as those by Ross and our own at Pic du Midi, there were simply too many details; the sheer plethora of markings confused matters.” In the end, Boyer the magistrate made exceptionally judicious use of the meager facts at his disposal.
When Dollfus was asked if he thought Boyer had been justified in divining the four-day rotation period from his 1950s images or if Boyer had merely been lucky, he hesitated a few moments before responding: “Difficult to reply – in between. As an amateur, he had more freedom, was not tied to the same high standards of rigor that he would have been as a professional.”
Three centuries of intense study had produced a legacy pathetically barren of results. The field had been all but abandoned when a persistent and single-minded amateur made one of the last fundamental discoveries in ground-based planetary astronomy. One of Boyer’s dearest friends, the renowned astrophotographer Jean Dragesco, summed it up: “This case is unique in the history of planetology.”