Until June 8, 2004, no one alive had seen a transit of Venus — a rare phenomenon in which we can see the silhouette of Venus as the planet passes across the face of the Sun. Only five such transits have been viewed before (in 1639, 1761, 1769, 1874, and 1882). Astronomers raced to the far corners of the Earth to see them, from sunny Tahiti to frigid Siberia.

Transits of Venus offered astronomers an opportunity to calculate the distance between the Earth and the Sun (called the astronomical unit or AU). Just as significantly, transit expeditions marked the first large-scale international scientific collaborations. Hounded by bad weather, missed opportunities, and even wars, astronomers doggedly pursued these natural events in the name of science.

In this web exhibit, you can see historical artifacts that document past transits of Venus.

Chasing the Goddess of Love Across the Sun is part of the Adler's Special Topics in the History of Astronomy series. It was curated by Anna Friedman (books) and Marvin Bolt (instruments) from the Adler's History of Astronomy Department. Special thanks are extended to Mr. and Mrs. Robert N. Gordon for the loan and support of their Lalande transit of Venus visibility map for purposes of the exhibition. All other items are from the Adler Collection.

Institutions of Astronomical Calculations Containing a Survey of the Solar System
London, 1773
QB 42 .M36 1773

This exquisite illustration, part map and part landscape scene, documents the part of the 1769 transit of Venus that could be seen from London. The center of the diagram shows the passage of Venus across the face of the Sun as it sets. Martin, an eighteenth century British astronomy popularizer, highlighted the significance of the transits in 1761 and 1769. Such texts demonstrate the public's interest in calculating the size of the solar system and the astronomical mathematics that made this possible.

Venus in sole visus (1639), In Johannes Hevelius Mercurius in sole
Gdansk, 1662
QB 41 .H4 1662 OVSZ

In the Rudolphine Tables, eminent astronomer Johannes Kepler predicted the transit of Venus in 1631, invisible to Europeans because it happened at night. However, he missed predicting the 1639 transit. Amateur astronomer Horrocks, at an impressively young age, noticed the omission with just enough time to spare before the event to view it. Horrocks and his friend William Crabtree appear to be the only humans to have viewed the 1639 transit. A self-made man born into an impoverished family, Horrocks's early brilliance presaged an important future in astronomy. Many colleagues mourned his premature death soon after the transit, which delayed the publication of his observations until several decades later.

This illustration, recreated from Horrocks's notes by the prominent Polish astronomer Hevelius, shows three positions of the planet Venus as it crosses the face of the Sun. Notice the two black and one white dot (the progression of Venus) in the lower left portion of the central circle (the Sun).

Voyage dans les Mers de l'Inde... a l'Occasion du Passage du Vénus, sur de Disque du Soleil
Paris, 1769
R1997.50

Le Gentil (as he is commonly known) traveled to India to view the 1761 transit of Venus for France. Unfortunately, the British ousted Le Gentil from the area. At sea during the transit, he had a perfect view, but because he could not determine his exact location, his observations were useless! Undaunted, Le Gentil decided to stay in the Indian Ocean region to await the next transit in 1769. He traveled far and wide from the Philippines to Madagascar and back to India. When the time came to pick a place to view the 1769 transit, he decided to return to India. After days of clear skies, June 3rd dawned cloudy, and Le Gentil missed out on observing the transit. Later he learned that his other possible viewing locations had perfect weather conditions. Upon returning to Europe, he found that his family had declared him dead and divided his estate.

As with many other transit expeditions, Le Gentil left a legacy that went beyond astronomy. The two volumes he published upon his return contain a wealth of geographical, botanical, zoological, archaeological, and ethnographic information, maps, and illustrations. This illustration shows two non-astronomical contributions: a map of the area around Manila in the Philippines and an illustration of seaweed.

Machina coelestis, volume 1
Gdansk, 1673
QB 85 .H4 1673

Before the development of solar filters that enabled direct observation of the Sun, astronomers used indirect means to safely view transits of Venus, sunspots, and eclipses. This apparatus, by the eminent Polish astronomer Hevelius, shows how the image of the Sun can be projected through a telescope on to a sheet of paper. An astronomer could then draw what he saw from life, as you see an astronomer doing in this illustration. Observers today can still use this viable option for viewing transits and other solar phenomena.

The Transit of Venus
London and New York, 1874
QB 509 .F6

The black drop effect prevented astronomers from obtaining the precision in observation and calculation that they desired. This optical effect causes the edge of Venus to distort, as if a tail or line connects it to the edge of the Sun. This visual problem complicated determining the exact time of Venus's contact with the edge of the Sun. These diagrams (top image), based on accounts of what the black drop effect looked like in 1769, illustrate a book about the transit of Venus published for popular audiences in 1874. Notice how Venus connects to the edge of the Sun in the top two illustrations.

Forbes also produced a map (bottom image) showing where the 1874 transit would be visible.

Il passagio di Venere sul Sole...osservato a Muddapur nel Bengala
Palermo, 1875
R2000.18

Many books written about transit of Venus expeditions highlight the exotic places to which astronomers traveled. This book, published by an Italian team, depicts the hut in which the astronomers lived while in Muddapur, India.

Map of the Transit of Venus From Transactions of the American Philosophical Society
Philadelphia, 1769
QB 52 .T73

Rittenhouse, one of the most important early American astronomers, viewed the transit of Venus in 1769 from Norriton, Pennsylvania. The top map illustrates his observations of the path of Venus. The accompanying text describes his account of the internal contact. He writes that just before this moment, as Venus began to cross the Sun's edge, a pyramid shape appeared around the edge of the planet. He illustrated this apparition in the inset (Fig. 1) at the top left of the map.

Rittenhouse's colleague Ewing produced a similar map (bottom image) included in the same publication. Notice similarities between the two maps.

The 1769 transit of Venus marked the first major American participation in the international astronomy community. Only one American astronomer viewed the 1761 transit of Venus—John Winthrop traveled to Newfoundland to see it. The 1769 transit, however, could be seen over much of America enabling many more astronomers to observe this rare event. Notice, however, that the circles representing stages of the transit end about two-thirds of the way across the Sun. Unfortunately, in the Eastern part of America, the Sun set before the transit ended.

Expositio de transitu veneris ante discum solis
Saint Petersburg, 1769
QB 511 .M39 1769

Astronomers at particular locations on Earth view a transit from slightly different vantage points, creating distinct angles and chord lengths for the transit paths that they observe from their individual locations. These paths also differ from ones that would be observed from the center of the Earth. The variations in these paths, combined with the known distance between different viewing locations, allows astronomers to construct triangles that help them to calculate the distance from Earth to Venus, and from it the Astronomical Unit (the AU, the Earth-Sun distance).

Astronomy explained upon Sir Isaac Newton's Principle...
London, c. 1764-1773; Edinburgh, c. 1811-1841 (edited by Brewster)

James Ferguson wrote one of the first popular textbooks for astronomy. First published in 1756, later editions included material on the 1761 transit of Venus.

The top illustration shows four different observed paths of the transit of 1761 from London, Bencoolen (in Indonesia), Ganges (in India), and St. Helena Island off the West coast of Africa. Notice the path of the transit in a band across the lower part of the Sun. The brown, red, and yellow circles that represent Venus reflect an average path. The lines represent the different observed paths. The inset diagram of the Earth shows how the latitudes of these places differ.

The center map shows readers areas of visibility for the 1761 transit. Notice that the central area with cross-hatched lines represents where the transit could be seen in its entirety. Other areas marked with single rows of lines indicate where observers could see only the beginning or end of the transit. Notice also that in 1761 Antarctica had not yet been charted.

The bottom diagram, similar to Mayer, shows some of the geometrical principles behind calculating the AU. It also shows the path that Venus took across the Sun during the 1761 transit.

Essays on Astronomy...and a Dissertation on the Approaching Transits of Venus
London and New York, 1872
QB 51. P96 1872

Proctor wrote a popular book about transits of Venus and also included the material in this general astronomy book. He used many different types of maps to show visibility areas for the 1874 transit. This ingenious map represents fifteen stages of the Earth's rotation in front of the Sun during the transit of Venus. Notice that the continent of Australia is visible throughout all the stages. This illustrates that the transit could be viewed in its entirety from anywhere on that continent.

The Coelestial Atlas, or a New Ephemeris
London, 1761
AY 751 .W4 1761

An ephemeris, similar to an almanac, lists planetary positions throughout the year as well as other types of celestial events. This particular copy from 1761 discusses the transit of Venus in that year in the section on eclipses. Notice the small map on the right page. It shows the path that Venus would take across the lower portion of the Sun.

Popular Astronomy: A General Description of the Heavens
French translation by J. Ellard Gore
London, 1894
R2002.01

The French author Flammarion wrote astronomy books for the general public. In his description of transits of Venus, he includes a stylized illustration of Venus passing across the edge of the Sun. Compare this illustration with the one in Forbes that shows the visual distortion the actually happens.

Histoire des astres: astronomie pour tous
Paris, 1874
R2000.03

Many popular astronomy books contained information about transits of Venus. This French book, published just in time for the 1874 transit, attempts to explain how someone can compute the AU by a geometrical process of triangulation. See the description of this process in Mayer.