Anyone lucky enough to be in the right place at the right time early next month will be in for a very rare treat – a sight of Venus passing across the face of the Sun. These fleeting astronomical events, known as “transits of Venus”, occur in pairs eight years apart, separated by gaps of more than a century. The fascination of these transits lies not only in their extreme rarity, but also in that they were once used to solve one of the biggest questions in astronomy: the distance between the Earth and the Sun. Yet one study of a particular transit of Venus remains controversial to this day.

It concerns the transit of 26 May 1761, as observed by the Russian polymath Mikhail Lomonosov. Using his four-and-a-half foot refracting telescope in St Petersburg, he claimed to have seen Venus’s atmosphere for the first time. “Venus”, he wrote shortly after the event, “is surrounded by a significant air atmosphere, similar to (if not even greater than) that which bathes our terrestrial globe.” Although we now know that Venus does indeed have an atmosphere, of the several dozen other people who saw the transit that day, none drew that explicit conclusion. Moreover, several current scientists still deny him the discovery (see “Venus: it’s now or never” by Jay Pasachoff).

The controversy is starkly displayed in the biography of Lomonosov published in 1937 by the chemist and historian Boris Menshutkin. Translated into English in 1952 by the US chemist Tenney Davis as Russia’s Lomonosov: Chemist, Courtier, Physicist, Poet, the book celebrates Lomonosov’s statement as a magnificent discovery. However, a footnote by the translator states equally authoritatively – the translator having consulted a US astronomer – that what Lomonosov saw had “nothing to do with the presence or absence of a Venus atmosphere and can in no case be regarded as a proof of its existence”.

So why does controversy still surround the discovery of Venus’s atmosphere more than 250 years later? What, if anything, can be done to resolve it? And what does this controversy tell us about the nature of scientific discovery itself?

Blisters and radiances

A polymath who contributed to many fields, including astronomy, Lomonosov (1711–1765) was the first Russian-born academician at the Russian Academy of Sciences. By the time of the 1761 transit, having notched up some 17 years’ experience making observations and inventing scientific instruments, he viewed the event from his house, a large dwelling with its own observatory.

“Having waited for Venus to enter on the Sun for about 40 min beyond the time prescribed in the ephemerides,” Lomonosov wrote, “[I] finally saw at last that the Sun’s edge at the expected entry had become indistinct and somewhat effaced, although before [it] had been very clear everywhere.” Thinking he was suffering from eye fatigue, Lomonosov looked away briefly, before returning to the eyepiece to find a small black spot joining the disc of Venus and the Sun. This was just after the moment of “first contact”, when the leading edge of Venus first touches the edge, or “limb”, of the Sun.

At second contact, when Venus was now entirely within the Sun and its trailing edge crossed the solar edge, Lomonosov noted that these two points were separated by “a hair-thin bright radiance”. Some six hours later as Venus’s leading edge approached the solar edge on its way outward from the Sun – what is dubbed third contact – the Russian wrote that “a blister appeared at the edge of the Sun, which became more pronounced as Venus moved closer to complete exit”. Not long afterwards, he saw fourth contact as “the blister disappeared, and Venus suddenly appeared with no edge”.

To support his conclusion that he had observed Venus’s atmosphere, Lomonosov cited the “loss of clearness in the [previously] tidy solar edge” at first contact, which he presumed to be caused by the “oncoming Venusian atmosphere”; as well as the “blister” at the third contact, which was, he said, caused by “refraction of solar rays in the atmosphere of Venus”. Lomonosov’s report, which also contained diagrams, was published in Russian on 17 July 1761 and in German a month later. Although other observers of the event mentioned a light radiance or aureole around Venus, none proposed a mechanism or explanation. So where is the controversy? Lomonosov said he had seen Venus’s atmosphere, described what he saw, produced a presumed mechanism, and published. Furthermore – spoiler alert – we know that Venus has an atmosphere.

Aureoles and artefacts

The controversy is sparked by two things: first, by how Venus’s atmosphere looks to contemporary Earth-bound instruments; and second, by what we now know about observational artefacts.

Historians know well the danger of “Whig history” – of judging the past from the perspective of the present. But science history can be different. Because laws and properties of nature are invariant, later observations can sometimes decisively correct previous ones. There are plenty of examples of wrong ideas being put right, not least Enrico Fermi’s detection of what he thought were transuranic elements in 1934, and Percival Lowell’s alleged detection of canals on Mars in the late 19th century. In the case of Venus, Jay Pasachoff from Williams College and his collaborator, the independent scholar William Sheehan, have written an article for the March/ April 2012 issue of the Journal of Astronomical History and Heritage (in press) pointing out that what we now know about the appearance of the refracted image of the Sun by Venus’s atmosphere, based on high-resolution observations of the 2004 transit, seems inconsistent with what Lomonosov reported in 1761.

Furthermore, modern observations have shed light on artefacts – products of instrument and observing imperfections rather than of the phenomenon being observed – that interfere with its perception. By far the most disruptive of these is the “black-drop effect”, which was first noticed by astronomers during the 1761 transit while trying to time the precise moment of second and third contacts. Nearly all observers saw, at second contact, a strange dark column or bridge linking Venus to the Sun, its precise appearance varying widely from one observation to another. Sometimes even being seen when Venus was as far as 3 arcseconds inside the solar disc, the bridge often thickened to make Venus’s disc look like an elongated droplet – hence the name. Another black drop affected the third contact, as Venus’s silhouette began to exit the solar disc. This effect – which was ascribed to a combination of factors, including the inherent optical limit of the telescope, physiological factors in the retina and atmospheric conditions – reduced the expected precision of 18th-century measurements of the Earth–Sun distance by about two orders of magnitude.

Notes from history

In space observations of the 1999 Mercury transit, Pasachoff and Schneider noted a black drop and found that an important contribution came from a hitherto largely overlooked effect: the extreme drop-off in brightness or “limb-darkening” at the Sun’s edge. In their article in the Journal of Astronomical History and Heritage, Pasachoff and Sheehan argue that Lomonosov observed not Venus’s atmosphere, but probably an artefact partly caused by this limb-darkening effect. Moreover, they suggest that Lomonosov was predisposed to assume that an atmosphere existed on Venus because of his belief in the plurality of worlds; in other words, because he was already convinced of the existence of an atmosphere, he may have been looking for effects that might reveal it. “Lomonosov arrived at the correct conclusion but on the basis of a fallacious argument,” they write.

The task of reconstructing what Lomonosov saw is extremely difficult because the words are ambiguous – and of course originally in Russian and German. It is like trying to recreate a photograph based on someone’s 250-year-old description in another language and then coming to conclusions regarding not one but several fine details about the item photographed. The Russian word translated into English as “blister”, for instance, is pupyr, which also means pimple. How can we confidently connect it with either an aureole or the notoriously variable black drop? What about the “hair-thin bright radiance”? Is that an aureole produced by the atmosphere or – as Pasachoff and Sheehan say – the first bit of solar disc visible limbward of Venus’s silhouette when the black-drop effect ended?

Lomonosov reported unusual effects at all four contacts, and argued that effects at the first, third and fourth contacts were caused by Venus’s atmosphere. Lomonosov’s description of the third contact seems to provide the firmest support – although Pasachoff and Sheehan dispute this, pointing out that in 2004 they saw the atmosphere for 20 min after the exit of the leading edge, and that Lomonosov writes that Venus had “no edge”.

Re-enactments?

Historians have discovered the danger of being overly confident about declaring that an experimentalist from an earlier era could not have seen a certain phenomenon, for such claims often short-change the abilities of antecedent scientists. A notorious cautionary tale involves the historian of science Alexandre Koyré, who in the 1950s confidently asserted that Galileo could not have deduced his law of falling bodies using such an inexact method as rolling balls down inclined planes and timing them with water clocks; surely Galileo must have deduced the laws first and cooked the data! This view was famously destroyed in 1961 by Thomas Settle, then a Cornell University history of science graduate student, who recreated the experiment in his room and managed to obtain precise enough data to deduce the law.

The rarity of Venus’s transits makes repetition problematic, but next month’s transit – the last until 2117 – seems to provide an outstanding if rare opportunity: pull out Lomonosov’s telescope and look.

However, this is not as simple as it sounds. First, we do not know which telescope Lomonosov used; we might only be able to deploy one of a similar kind. Second, there would be the non-trivial problem of securing permission to use a valuable and delicate historical instrument. Third, atmospheric conditions on transit day may not be identical. Finally, there’s the matter of experimental skill. Experiments are not automatic, and often involve pushing instruments to their limits. Knowing how such instruments can be trusted under which conditions is a mark of experimental skill, and often a function of how much an experimentalist has worked with the instrument. More skilled experimentalists can notice things that others miss with better equipment. A 21st-century observer using an 18th-century instrument is likely to lack this kind of skill.

The critical point

During the 1950s, one important fuel for passions surrounding the dispute over Lomonosov’s priority was Cold War rivalry; Western historians and scientists often attributed Soviet claims involving achievements of Soviet scientists as being propaganda. Such attitudes, however, are now long gone. But the issue still generates passions because the inability to resolve this issue points out the disturbing fact that some historical questions – and even some scientific ones – have hard-to-decide or ambiguous aspects.

Sometimes this is because the discoveries evolve in phases over an extended time; examples include the discoveries of oxygen and of dark energy. Who discovered Venus’s atmosphere would seem much easier to decide, for it has to do with what one astronomer observed at a precise instant looking at a specific event through a particular telescope. Yet because of the ambiguities of language, and of the impossibility of repeating the relevant conditions and deploying the necessary skills, this too may remain impossible to decide upon.

Teams of astronomers, both professional and amateur, hope to use antique telescopes to observe the coming transit – “experimental archaeologists”, as Sheehan calls them. He himself is planning to use a 19th-century 168 mm Brashear refractor at Mt Wilson, while others at Lowell Observatory plan to use the 152 mm Clark refractor that Percival Lowell took to Japan in 1892. What they find may shed some additional light on what earlier observers may have seen. Even so, the question of what Lomonosov saw is likely to remain forever open-ended.