Transits of Venus

Venus circles the Sun inside Earth's orbit, and each time it overtakes Earth it passes between us and the Sun, a moment called inferior conjunction. Usually it slips by above or below the solar disc, because its orbit is tilted against ours. NASA's transit catalogue, compiled by Fred Espenak, gives the condition: Venus's orbit crosses Earth's orbital plane in early June and early December, and only a conjunction falling on one of those crossings puts the planet in front of the Sun. Eight Earth years hold almost exactly thirteen orbits of Venus, so a transit is often repeated eight years later; but Venus reaches each rendezvous about 2.45 days early, and after a pair the alignment drifts out of reach until the opposite node comes round, 105.5 or 121.5 years on. The whole pattern repeats every 243 years.

What an observer sees is modest. In 2004 and 2012 Venus was about one thirty-second of the Sun's width and took more than six hours from first contact, when its edge touches the Sun, to fourth, when it leaves. The dimming is tiny. In 2004 the ACRIM 3 radiometer on NASA's ACRIMSAT satellite measured it: Glenn Schneider, Jay Pasachoff and Richard Willson report a fall of about 1.4 watts per square metre at mid-transit, close to 0.1 per cent, matching the area of the Sun that Venus covered.

Kepler's prediction came to nothing. His Rudolphine Tables of 1627 foresaw a transit in December 1631, and Pierre Gassendi prepared to watch from Paris, but, as Britain's HM Nautical Almanac Office tells it, the tables could not show that it would happen before sunrise in western Europe. Eight years later Jeremiah Horrocks, working from his own recalculation of Venus's motion, predicted the 1639 transit and caught it from Much Hoole, near Preston in Lancashire, shortly before sunset on 4 December (24 November in the Julian calendar England still used). The Almanac Office calls this the first successful observation of a transit of Venus and thinks the only other observer was his friend William Crabtree in Manchester, whom Horrocks had urged by letter to watch. Costantino Sigismondi's reconstruction adds that Horrocks found Venus far smaller than Kepler supposed and put the Sun's distance at the equivalent of about 95.7 million km.

Edmond Halley turned the spectacle into an instrument. In a 1716 paper in the Philosophical Transactions, reproduced on NASA's eclipse site in its 1809 translation, he recalled timing a transit of Mercury from St Helena and argued that Venus, much nearer Earth, would serve far better. Observers far apart in latitude see Venus cross the Sun along slightly different chords, so the transit lasts measurably different times for them; from those differences follows the solar parallax, the tiny angle Earth's radius makes as seen from the Sun, and with it the Sun's distance. Halley claimed it could be found to within its five-hundredth part. A note appended to the 1809 translation is sober: 1761 proved much less favourable than he expected, because the motion of Venus's node was poorly known, and he had added two angles that should have been subtracted.

The first test came on 6 June 1761, 26 May in Russia's Old Style calendar. Vladimir Shiltsev and Igor Nesterenko, writing in the Russian magazine Science First Hand in 2012, count more than 170 astronomers returning from 117 expeditions. The St Petersburg Academy staffed its own observatory and sent observers to Irkutsk and Selenginsk, while the French abbé Jean Chappe d'Auteroche observed from Tobolsk. The harvest was thin: Amédée Guillemin's Le Ciel (1877) blames stations whose transit durations differed too little and the practical difficulties of installing the observers.

Mikhail Lomonosov, at his house on the Fontanka in St Petersburg, cared more about physics than the clock. His report describes a two-lens telescope four and a half feet long, which the 2012 reconstruction team identifies as an early Dollond achromat, with a deliberately weak filter of lightly smoked glass. As Venus approached, the Sun's edge turned indistinct, as though smudged. At egress, when the planet's leading edge was about a tenth of its diameter from the limb, a bump of light rose on the Sun's edge ahead of it and then vanished. He reasoned that Venus 'is surrounded by a considerable atmosphere of air, such as (if not greater than) the one that pours around our terrestrial globe', the bump being sunlight refracted in it. Russian writers call the effect the Lomonosov phenomenon.

Whether he saw what he believed he saw became a live argument before the 2012 transit. Jay Pasachoff and William Sheehan, in the Journal of Astronomical History and Heritage in 2012, concluded that Lomonosov inferred the atmosphere from observations related to the black drop, an optical effect unconnected with Venus's air, and that the credit should go instead to several observers of 1761 and 1769 who may have seen the aureole genuinely, David Rittenhouse in 1769 perhaps with the strongest claim. Shiltsev answered in the same journal in 2014 that Lomonosov's claim remains the strongest, since he was the first to publish a comprehensive report and the first to explain the aureole in detail. The camps overlap more than their titles suggest: Shiltsev and Nesterenko accept that the smeared limbs at first and last contact most likely came from telescope imperfections and Earth's turbulent air, so the quarrel narrows to the thin glow and the bump. By their account the discovery was credited for over a century and a half to Johann Schröter and William Herschel, who in 1790 saw small bright horns on a half-lit Venus, and Soviet astronomers publicly restored Lomonosov's priority only in the 1950s.

The 2012 transit offered a test. Alexandre Koukarine, Igor Nesterenko, Yuri Petrunin and Shiltsev re-enacted the observation with eighteenth-century refractors. Koukarine, at Lick Observatory in California, used a 4.5-foot Dollond achromat and a filter passing about 1/6,000 of the light, and saw bright whiskers curling round Venus and then the full arc. Their paper concludes that Lomonosov's telescope was fully adequate to detect the arc if his technique was followed. In 2014 Igor and Alfiya Nesterenko argued in Priroda that the 1761 figures are sketches of an image projected behind the eyepiece, with Sun and Venus in proportion to within 0.5 per cent. Lomonosov's own telescope, they note, was lost during the Second World War; in 2014 they had begun assembling a close copy, meant to be handed on for the transit of 2117.

The 1769 transit, on 3 and 4 June, went better. Lieutenant James Cook and the astronomer Charles Green, on a voyage sponsored by the Royal Society, observed from Point Venus on Tahiti under a cloudless sky. Maximilian Hell watched from Vardø in Arctic Norway, and Pulkovo's history records that Catherine II herself took part in the Russian observations. Chappe d'Auteroche went to Baja California and died there, of yellow fever according to the Nautical Almanac Office and of typhus according to Shiltsev and Nesterenko. Guillaume Le Gentil was clouded out at Pondicherry; from Manila, his first choice, he would have seen it. The timings still scattered. Guillemin gives the solar parallax from 1769 as between 8.50 and 8.88 arcseconds, roughly 155 to 148 million km for the Sun's distance, and Johann Encke's reanalysis as 8.57 arcseconds (about 153.5 million km), long adopted and later found too small.

Why could skilled observers with good clocks do no better? Cook named the problem. His journal for 3 June 1769, quoted by Pasachoff and Sheehan, describes 'an Atmosphere or dusky shade round the body of the planet which very much disturbed the times of the Contacts'. Near second and third contact the black disc seems to cling to the Sun's edge by a dark ligament, the black drop, and the instant of contact smears across many seconds. It was long blamed on Venus's atmosphere, and still is in places: announcing the 2012 transit, the Special Astrophysical Observatory of the Russian Academy of Sciences explained the dark bridge that way. Glenn Schneider, Pasachoff and Leon Golub separated its main contributors with NASA's TRACE spacecraft at the 1999 transit of Mercury, a planet without an atmosphere: the telescope's blurring of the image combined with the steep darkening of the Sun towards its edge. Espenak concludes that timings of the accuracy Halley needed are impossible because of seeing and diffraction, though 1761 and 1769 still gave astronomers their first good value for the Sun's distance.

By 1874 photography, better clocks and telegraphed longitudes promised the precision the eighteenth century had lacked, and the Almanac Office counts expeditions from a dozen countries, sent from Vladivostok and Peking to Kerguelen and New Zealand. Pulkovo Observatory's history describes a commission under Otto Struve that equipped every station from Pulkovo, including two Dollond telescopes that had served in 1769; 27 stations were chosen, 25 of them in Russia. At Simon Newcomb's request Lieutenant Mikhail Onatsevich joined the Americans at Vladivostok with a six-inch refractor, and Karl Struve, the Russian envoy in Japan, took part in the observations there (with Austrian and Mexican parties, says the Pulkovo history, though the Japanese accounts name no Austrian one). On the day the weather at every Russian station was unsettled, though Bernhard Hasselberg's reduction of the photographs taken at Posyet harbour was published in 1877.

Japan hosted three of the 1874 expeditions, before Tokyo University or the Tokyo Astronomical Observatory existed. The National Astronomical Observatory of Japan tells how the Meiji government asked David Murray, the American adviser to the Ministry of Education, to explain; his memorandum, 'Venus crossing the Sun', made the case for helping them. The United States observed from Ōhirayama in Nagasaki, now Hoshitoriyama; Jules Janssen's French party from Konpira-yama in Nagasaki, with a detachment on Suwayama in Kobe; and Mexico, which by NAOJ's account sent parties to Japan alone, from Nogeyama and Yamate in Yokohama under Francisco Díaz Covarrubias. The government strung a telegraph line from the Yokohama telegraph office to the Mexican station, letting it exchange signals with the American and French parties for longitude and send results to Greenwich. The weather split the outcome. At Nagasaki it prevented a full observation, says the Astronomical Society of Japan, though Nagasaki Prefecture's citation says Janssen's party of six succeeded; Yokohama was clear, and the French detachment at Kobe photographed the transit.

The places survive. Janssen's sandstone monument, inscribed in French, stands behind the Konpira shrine; Nagasaki Prefecture listed it as a historic site on 13 July 1960, and an observing pier found 24 m to the east in 1993 was added in 1995 because it also helps locate the reference point used to fix Japan's first geodetic origin. Kobe keeps a granite column in Suwayama Park, and Yokohama has 1974 centenary monuments and, in a private garden, the Nogeyama telescope pier. The Astronomical Society of Japan named the three sites Japan Astronomical Heritage for fiscal 2021, a decision of 10 January 2022, recalling that the historian Saitō Kuniji called the visitors 'the black ships of science'. Across the sea, a Chinese naked-eye sunspot record of 9 December 1874 probably shows the transit itself, misread as a sunspot, Hisashi Hayakawa and colleagues argued in 2019.

Photography was the century's great hope, and the aureole turned up on at least one plate. Guillemin reproduced, with Janssen's permission, a photographic proof of the 1874 transit showing a faint luminous ring round the black disc. His text says the proof was obtained at Kobe and his preface calls it a proof of the transit 'observed by him at Kobe'; the Japanese heritage citation agrees on Kobe for the photographs but places Janssen himself at Nagasaki and credits Kobe to his detachment.

The second transit of the pair, on 6 December 1882, was visible in full from South and Central America and eastern North America. The United States sent eight expeditions and France ten, while Russia and Austria, according to the Almanac Office, stayed out, unconvinced that a transit was still the best way to measure the solar parallax. In 1890 Newcomb combined the 1874 and 1882 observations into a parallax of 8.79 arcseconds, which Sigismondi quotes as 8.794 plus or minus 0.018 beside today's adopted 8.794148: a Sun about 149.6 million km away. In 2012 the International Astronomical Union redefined the astronomical unit as a conventional length of exactly 149,597,870,700 m, citing the accuracy of modern ranging.

Nobody alive had seen a transit of Venus when 2004 approached, as Schneider, Pasachoff and Golub noted in 2003. The European Southern Observatory and its partners made it a public campaign, VT-2004, to repeat Halley's measurement: 1,510 observers sent 4,550 contact timings, and the analysis in Paris gave 149,608,708 km with an uncertainty of 11,835 km, 0.007 per cent above the radar value. The science moved to the atmosphere. Paolo Tanga and colleagues describe their 2004 measurements as the first photometric analysis of the aureole, relating its brightness along the limb to the scale height of the south polar atmosphere and the height of the cloud tops. Pascal Hedelt's team, with the Vacuum Tower Telescope on Tenerife, picked out carbon dioxide absorption in Venus's upper atmosphere, a rehearsal for reading the air of planets around other stars.

On 5 and 6 June 2012 NASA's Solar Dynamics Observatory recorded the transit in high definition. Cyril Pere, Tanga and colleagues point out that it was the first transit to occur with a spacecraft in orbit around Venus: ESA's Venus Express observed the planet during the event. Hinode, the solar observatory of JAXA and the National Astronomical Observatory of Japan, caught the aureole. ISAS released Solar Optical Telescope images from around 07:30 Japan time on 6 June, just before second contact, in which sunlight refracted by Venus's atmosphere makes the planet's edge shine like a ring of light, and Hinode's X-ray telescope watched Venus approach against the corona. Fabio Reale and colleagues used Hinode and SDO images to measure Venus's radius: about 80 km above the solid body in visible light, where clouds and haze are opaque, and more than 70 km larger again in extreme ultraviolet and soft X-rays, marking the densest layers of the ionosphere. Pere's team modelled the aureole in SDO images against Venus Express data and derived a haze scale height of 4.8 plus or minus 0.5 km above 80 km.

The Hubble Space Telescope cannot point at the Sun, so its observers used the Moon as a mirror. Program 12537, 'Venus observed as an extrasolar planet', led by David Ehrenreich of the Université de Grenoble, aimed to catch the sliver of reflected sunlight that had passed through Venus's atmosphere, about one part in 100,000 according to NASA's preview, as a test for reading an Earth-sized exoplanet's air. The MAST archive shows that it ran: STIS spectra of a lunar target labelled MOON-HIPPARCHUS-C from 15:32 UT on 5 June to 10:58 UT on 6 June 2012, with ACS and WFC3 images alongside. The widely reproduced picture of the crater Tycho is a rehearsal image from 11 January 2012. A search on 26 September 2026 found no refereed paper reporting what the transit data showed.

The next pair falls on 10 to 11 December 2117 and 8 December 2125, both at the December node. Venus keeps passing close in the meantime: at its passage between Earth and the Sun on 24 October 2026, JPL's Horizons ephemeris puts it about 6.5 degrees south of the Sun's centre, more than ten times the Sun's apparent width, so that passage misses the disc (Venus in the sky follows it). Mercury crosses the Sun about thirteen or fourteen times a century, Espenak notes, so it is the Venus event that stays rare, and the copy of Lomonosov's telescope the Nesterenkos mean to leave behind waits for 2117.