The Transit of Venus
How the 1761 and 1769 Expeditions Measured the Solar System
- 3 chapters
- 14m
- Astronomy
- Free · no sign-up
These expeditions required careful planning and long voyages. Captain James Cook traveled to Tahiti in 1769, while other astronomers went to places like Siberia and India. Each expedition aimed to time Venus's passage across the Sun's disk. The observations would help calculate the astronomical unit - the distance from Earth to the Sun.
The 2004 transit provided a modern example of how these events continue to interest scientists. These historical missions showed how international cooperation could advance human understanding of space. Anyone interested in scientific exploration and astronomy will find this story compelling.
Listen
-
Read this chapter
Overview
A transit of Venus happens when the planet moves directly across the face of the Sun, appearing as a small black dot drifting over the solar disk. These events follow a rare pattern, with two transits eight years apart in December, then a long gap of 121.5 years, followed by another pair in June after 105.5 years. The cycle shifts slightly over time, advancing about two days every 243 years. The most recent pair occurred on 8 June 2004 and 5–6 June 2012, and the next will be in December 2117 and 2125. Each transit lasts around six hours. Scientists used these rare alignments to calculate the distance from Earth to Venus, which helped determine the size of the entire solar system. The 2012 transit also improved methods for finding planets beyond our own.
Conjunctions
When Venus and Earth align with the Sun, a transit occurs—Venus crossing directly across the Sun's face. This happens only when Venus reaches conjunction with the Sun while passing through Earth's orbital plane. The orbits of Venus and Earth are tilted slightly relative to each other, making such alignments rare. Transits follow a pattern repeating every 243 years, though exact timing shifts over time due to the mismatch in their orbital periods. In this cycle, pairs occur eight years apart, separated by long gaps—121.5 years, then 105.5 years. The current sequence lasts until 2846, when it will shift to a new pattern. These transits are part of a larger, stable 243-year cycle, but the number and timing of individual transits vary. Some series have ended, others begun, like the one starting in AD 1631 and reaching its next transit in 2117.
History of observation of the transits
Ancient observers from India, Greece, Egypt, Babylon, and China tracked Venus and recorded its movements, but there's no proof they saw a transit. Pythagoras is said to have figured out that the morning and evening stars were the same planet. Some scholars think a fresco at the Maya site of Mayapan might show a transit from the 12th or 13th century. The Persian thinker Avicenna claimed to have seen Venus pass across the Sun, although he didn’t say when. A transit did occur on 24 May 1032, but modern experts question whether he could have observed it from where he was. He may have mistaken a sunspot for Venus. Still, he used his alleged observation to argue that Venus, in the geocentric model, came before the Sun in the order of celestial spheres.
1631 and 1639 transits
In 1627, Johannes Kepler predicted a Venus transit but his calculations were too imprecise for European visibility. The first recorded observation came in 1639 when English astronomer Jeremiah Horrocks watched from Carr House in Much Hoole, near Preston, with friend William Crabtree observing from Broughton. Horrocks corrected Kepler's orbit calculations and realized transits occur in pairs eight years apart, predicting the 1639 transit. Though uncertain of exact timing, he guessed it would start around 3 p.m. Using a telescope to project the Sun's image onto paper, he safely observed without damaging his eyes. Clouds cleared just as the transit began around 3:15 p.m., and he estimated Venus's size and Earth-Sun distance—about 59.4 million miles. His findings weren't published until 1661, after his death.
1761 transit
In 1663, James Gregory suggested using a transit of Mercury to calculate the solar parallax, but Edmond Halley's attempt in 1677 from Saint Helena was hindered by limited observations, including one from Richard Towneley in Burnley and another from Gallet at Avignon. Halley proposed a better method using Venus transits, noting that the next one would occur in 1761—on 6 June N.S., 26 May O.S. Astronomers from Britain and France, including John Winthrop, Jean-Baptiste Chappe d'Auteroche, Alexandre-Gui Pingré, Guillaume Le Gentil, Jeremiah Dixon, and Charles Mason, traveled to places like Newfoundland, Siberia, Madagascar, and the Cape of Good Hope to observe it. Most saw at least part of the transit, though Nevil Maskelyne and Robert Waddington were less successful on Saint Helena, despite using their voyage to test the lunar-distance method for finding longitude. The planet was thought to have an atmosphere before 1761, but it wasn't considered detectable during a transit until Mikhail Lomonosov observed a luminous arc around Venus's black disc in 1761 and inferred its presence. This observation was later confirmed by experiments using 18th-century telescopes in 2012.
1769 transit
For the 1769 transit, scientists traveled across the globe. Czech astronomer Christian Mayer observed the event in Saint Petersburg with Anders Johan Lexell, while other Russian Academy members went to eight locations under Stepan Rumovsky's coordination. In the UK, King George III had the King's Observatory built near Richmond Lodge so he and Astronomer Royal Stephen Demainbray could watch. Maximilian Hell and János Sajnovics went to Vardø, Norway; Wales and Joseph Dymond traveled to Hudson Bay. In Philadelphia, David Rittenhouse led a committee that erected temporary observatories. Dr. Benjamin West's group observed from Providence, Rhode Island. Captain James Cook and Charles Green watched from Tahiti, at a spot still called Point Venus. D'Auteroche went to San José del Cabo in New Spain with Spanish astronomers Vicente de Doz and Salvador de Medina, but died of yellow fever shortly after observing. Only nine of twenty-eight in his party survived. Le Gentil spent over eight years trying to observe the transits, was declared dead abroad, lost his wife and possessions, then returned to regain his seat in the French Academy and remarried. Under Royal Society influence, Ruđer Bošković traveled to Istanbul but arrived too late. In 1771, Jérôme Lalande calculated the astronomical unit as 153 ± 1 million kilometers using data from both transits, though the black drop effect limited precision. Hell published his result in 1770 at 151.7 million kilometers. Lalande later challenged Hell's work but retracted his criticism in 1778.
1874 and 1882 transits
The transit of Venus in 1874 drew three expeditions to the Kerguelen Archipelago—Germany, the United Kingdom, and the United States—working to refine the measurement of the astronomical unit. American astronomer Simon Newcomb later combined data from the last four transits, including these observations, and calculated a value of 149.59 ± 0.31 million kilometres. That figure stood as one of the most precise estimates of the solar system’s scale at the time. The transit of 1882 followed, continuing efforts to map the cosmos with growing accuracy.
2004 and 2012 transits
During the 2004 transit, organisations led by the European Southern Observatory brought together amateur astronomers and students to calculate Earth's distance from the Sun, arriving at a value of 149,608,708 ± 11,835 kilometres, just 0.007% off the accepted figure. Scientists also used the event to practice measuring how much light dimmed as Venus passed in front of the Sun—techniques later applied to discovering exoplanets. The 2012 transit offered fresh opportunities for exoplanet research and was captured from space, with photos taken by NASA astronaut Don Pettit aboard the International Space Station. That transit happened during a peak in the Sun's 11-year cycle, giving astronomers a chance to test methods for spotting planets around variable stars. Observations of Venus's atmosphere from Earth-based telescopes and the Venus Express mission provided new insights into its climate, while spectrographic data helped compare it to atmospheres of exoplanets. The Hubble Space Telescope even used the Moon as a mirror to study the composition of Venus's atmosphere.
-
Read this chapter
Early life
Jean-Baptiste Chappe d'Auteroche was born in Auvergne, a region of central France, into a family with administrative connections. Not much is known about his early years, but he entered the priesthood, likely as a Jesuit, and turned to the study of astronomy. He became assistant astronomer at the Royal Observatory and was admitted to the Royal Academy of Sciences on 14 January 1759.
Distance from the Earth to the Sun
In the mid-18th century, astronomers understood the movements of planets well, but not the actual size of the Solar System. To figure that out, they needed to measure the distance between Earth and Venus. That could be done by watching transits of Venus—when Venus passes across the face of the Sun, looking like a small black dot. The next such transit wouldn’t happen until 1874, after 1769. Because of how rare these events are, scientists planned an international effort to observe them from as many distant locations as possible. Even with the Seven Years’ War going on, astronomers were granted safe passage so they could travel and make their observations under the direction of learned societies.
Transit of Venus 1761
Jean-Baptiste Chappe d'Auteroche was sent to Tobolsk in Siberia to observe the transit of Venus on 6 June 1761. The trip was tough, and he arrived with little time to spare, but he did manage to record a lunar eclipse on 18 May, which helped determine Tobolsk's longitude. That spring, floods on the Tobol and Irtysh rivers had been especially bad, and some local peasants blamed him for meddling with the Sun; he had to be protected by armed Cossacks to make his observations. The weather was clear, though, and he successfully watched the entire transit. He published his results from Saint Petersburg in Mémoire du passage de Vénus sur le soleil, and didn't return to France until 1763. A few years later, in 1768, he released Voyage en Sibérie, a travel account that offered little praise for Russia. Soon after, an anonymous rebuttal appeared, widely believed to be written by Catherine the Great—or possibly by Count Andrey Petrovich Shuvalov.
Longitude measurement
The challenge of measuring longitude had puzzled astronomers for decades, and by the 1760s, marine chronometers were growing more reliable. In 1764, Jean-Baptiste Chappe d'Auteroche was chosen to test a new timepiece crafted by the Swiss maker Ferdinand Berthoud. He joined physicist Henri-Louis Duhamel du Monceau aboard the corvette L'Hirondelle to put the instrument through its trials at sea.
Transit of Venus 1769
On 3 June 1769, Jean-Baptiste Chappe d'Auteroche made his way to the Mission of San José del Cabo at the southern tip of the Baja California peninsula in what is now Mexico. The journey and the transit observation itself were unremarkable. Earlier, from 1762 to 1768, he had reported seeing a halo around Venus, but during this transit, he documented the black drop effect. As the expedition packed to leave, an outbreak of fever—possibly yellow fever—struck the area. Chappe chose to stay and tend to the sick, but he became infected and died on 1 August. Only one member of the group returned to Paris with his observations and notes. His account, Voyage en Californie, was later published posthumously by his colleague César Cassini de Thury. The manuscript remains at the Library of Paris Observatory and is accessible online.
Legacy
Jean-Baptiste Chappe d'Auteroche’s work helped solve one of astronomy’s biggest puzzles: how far away the sun really was. His nephew, Claude Chappe, was deeply influenced by reading Voyage en Sibérie, and together with his brothers, went on to build the first optical telegraph system using semaphores and telescopes. An asteroid discovered in 1996, 14961 d'Auteroche, bears his name. In 1994, a crater on the Moon was also named in his honor.
-
Read this chapter
Overview
In June 2004, people around the world watched a transit of Venus, the first such event since 1882, when no one alive had seen one before. The European Southern Observatory, the European Association for Astronomy Education, the Institut de Mécanique Céleste et de Calcul des Éphémérides, and the Observatoire de Paris joined forces with the Czech Academy of Sciences to launch the VT-2004 project. Nearly 1,000 school classes and 2,763 participants from around the globe took part in measuring the astronomical unit, calculating it as 149,608,708 kilometers ± 11,835 kilometers—only 0.007% off from the accepted value.
Visibility
The transit of Venus in 2004 could be seen from Europe, most of Asia, and almost all of Africa. In easternmost Asia and Australia, observers were able to witness the start before sunset. At the end of the event, people in westernmost Africa, eastern North America, and much of South America saw it after sunrise. The transit did not appear at all from certain regions including western North America, southern South America, Hawaii, or New Zealand. A map to the right illustrates exactly which areas had visibility of the event.
Read
Free to download, keep and share. For general information only — not professional medical, legal or financial advice. Please consult a qualified professional.