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Lalande's Rare Anticipatory Illustration of the June 1769 Transit of Venus

This large double-hemisphere world map was produced in Paris under the direction of astronomer Joseph-Jérôme de Lalande, utilizing a map of the world first published by Jean Lattre.   It was intended to accompany Lalande's 1764 Explication de la figure du passage de Venus, a treatise on the 1769 transit of Venus, the purpose of which was among other things to advocate the study of the transit of Venue as an important scientific event which would not occur again for over 100 years.

In format, the map is a classic 18th‑century double-hemisphere projection. The geographic content of the map itself is essentially that of a standard mid-18th-century world map. The tropics and polar circles are drawn, and the coastlines (e.g. of Nouvelle Hollande (Australia) and Terre Mére (the Antarctic Terres antarctiques) follow contemporary charts. Place-names and exploratory routes (Magellan, Tasman, etc.) appear along the southern continents. Lalande re-purposed an existing world-map plate (originally engraved about 1760) for this diagrammatic transit chart – in fact Lalande’s own notes call it an “improved version” of a 1757 Parisian Academy of Science map. 

Lalande’s Transit Annotations

Overprinted in sepia ink atop the map are Lalande’s detailed transit diagrams. The most striking feature the depiction of the transit drawn across both hemispheres, marking the apparent track of Venus during the transit. Around this are concentric arcs and fine dotted lines denoting zones of equal contact time as Venus traverses the Sun’s disk. Lalande explicitly describes the content: in a printed legend he notes that on this chart one sees “les momens de l’Entrée et de la Sortie de Vénus pour tous les Païs de la Terre” (the times of Venus’s ingress and egress for every country), including the effect of parallax, and highlighting those regions where observations would most usefully yield the Sun’s distance. In practice this means each curve corresponds to a particular contact time (all referenced to Paris’s meridian), and the differences due to Earth’s curvature are encoded geometrically.

To help readers interpret the overprint, Lalande added a color-coded key (lower left) in which lands shaded green are those from which only the ingress of Venus will be seen, yellow indicates only the egress, and red indicates regions where both entry and exit are visible (unshaded areas see no transit). An explanatory note printed near the title further promises that the “principles, method and calculations” behind the figure are given in detail in Book XI of my Astronomie, referring to Lalande’s own astronomy treatise. (Indeed, in Lalande’s Astronomie the eleventh chapter is devoted to the passages of Venus and Mercury across the Sun.) Thus, Lalande’s overprint transforms the decorative world map into a scientific chart of the Venus event: the curved ingress/egress path, the parallactic adjustment, and the epoch and location of contacts are all drawn with precision. 

In the lower left corner is a planisphere inset – a circular diagram of the Sun’s disk (“Disque du Soleil”). This small circle shows a yellow‑washed solar disk with marks for the positions of Venus at the times of contact as seen from Paris. A straight line from the solar center (marked C) goes through labeled Venus spots: one at “1h 25 du matin” (time of full entry at Paris) and another at “7h 33 du soir” (time of exit). The caption “Situation de Vénus par rapport au Vertical du Soleil au moment de l’Entrée totale à Paris” explains that this diagram depicts Venus’s relative position at the moment of full (total) entry over Paris. In short, this inset provides a geometrical check – showing how Venus’s path cuts the Sun’s face at the predicted times – complementing the world-map curves.

Overall, the piece reads as a sophisticated melding of cartography and astronomy. Its physical format – an engraved folding map sheet – was typical for atlas plates of the period, but its content is entirely devoted to the 1769 transit.  The printed result is a highly specialized observational chart: clearly a one-off scientific “diagram” rather than a general-purpose map.   

Summary of Lalande’s 1764 Explanation of the 1769 Transit of Venus

In 1764, Joseph Jérôme de Lalande published a short treatise entitled Figure du passage de Venus sur le disque du soleil Par M.de la Lande in Paris. In this substantial astronomical study, Lalande provides a detailed explanation and justification for the preparation, observation, and calculation of the transit of Venus across the Sun, scheduled for June 3, 1769. Building upon his experience with the 1761 transit and subsequent refinements, Lalande compiles the theoretical and practical foundations necessary to predict and analyze the event with unprecedented accuracy.

Scientific Importance of the Transit

Lalande opens by emphasizing the extraordinary importance of the 1769 transit. This will be the last such event visible to the current generation, as the next transits will not occur until 1874 and 2004. More significantly, this occasion offers an improved opportunity for measuring the astronomical unit (AU)—the mean distance between the Earth and the Sun—through coordinated global observations. The 1761 transit had been hampered by poor weather and the disruptions of war, whereas the 1769 event is expected to yield more reliable data due to improved preparations and broader international cooperation.

The Use of Geographic Mapping and the Figure

The treatise is structured around a specially designed map that divides the Earth into colored zones corresponding to the phases of visibility of the transit: green for regions that will see only the beginning, yellow for those that will see only the end, red for zones observing both entry and exit, and uncolored areas for those unable to view the event at all. Circles of illumination and circles indicating the times of Venus’s entry and exit across the solar disk are overlaid on a double-hemisphere map to show where and when observers should expect to record the contact points.

Lalande provides detailed descriptions of these zones, carefully explaining which geographic regions will be best positioned to observe the event and what parts of the passage will be visible from each. He notes, for instance, that much of northern Europe and parts of Asia will be able to observe the entire transit during continuous daylight, due to the summer solstice's long twilight. Meanwhile, much of Africa and parts of the western Atlantic will miss the event entirely due to night-time conditions.

Observational Strategies and the Parallax Method

One of the key contributions of this document is its explanation of how the transit can be used to calculate the solar parallax. By comparing precise timings of Venus’s contact with the solar disk from widely separated locations on Earth—particularly from northern latitudes (such as Torneå in Lapland) and southern oceanic islands (like the hypothetical Quiros islands)—astronomers can determine how much Venus’s apparent position shifts due to Earth’s curvature. From this, one can deduce the absolute distances between Earth, Venus, and the Sun.

Lalande stresses the importance of synchronized observation and minimal error, arguing that even a discrepancy of 15 seconds in recording contact times can significantly affect the precision of the results. He urges observers to work in teams and distribute their efforts strategically to mitigate the risk of bad weather and observational uncertainty.

Calculations and Ephemeris Corrections

In the latter part of the treatise, Lalande presents his corrected astronomical tables and the methods by which he refined the predicted timings. Drawing on his own 1761 observations and incorporating corrections for solar and planetary longitudes (including adjustments for nutation), he recalculates the timing of Venus’s conjunction with the Sun. He compares these predictions to earlier models, particularly those of Halley, and improves them with new data.

Among the critical numerical results Lalande provides are the expected times of first and second contacts (entry), greatest transit (midpoint), and third and fourth contacts (exit), all corrected for the center of Earth and for specific observing locations, such as Paris. He also supplies the apparent diameters of the Sun and Venus, the relative velocities and orbital inclinations, and deduces the solar and Venusian parallaxes.

Final Conclusions

Lalande concludes that accurate observation of the 1769 transit, particularly from remote but optimally situated locations, will enable astronomers to resolve longstanding uncertainties about the scale of the solar system. He calls upon the international scientific community to recognize the rarity and value of the event and to make every effort to observe it with rigor and precision.

Rarity

The map and Explication de la figure du passage de Venus, are both exceptional rarities.  

The map has not appeared at auction or in a dealer catalogue since 2004 (Jonathan Potter, Catalog 23).  OCLC locates 3 copies of the map (Bibliotheque National de France, Bibliotheque National & Universite de Strasbourg, and Sachsische Landsbibliothek and National Library of Australia).

No auction or dealer catalog listing for the Explication appears in RBH.