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Hunting the Astronomical Unit Aristarchus of Samos
De magnitudinibus, et distantiis solis, et lunae
(On the sizes and the distances of the Sun and the Moon)
Pisa:
Camillo Fanchischini,
1572.
Gift of the Burndy Library
In the third century B.C., the Greek mathematician Aristarchus reasoned that, if you measured the angle between the Sun and the Moon when the Moon was exactly half-full, you could determine the Earth-Sun distance geometrically. His method demanded great accuracy. Aristarchus, lacking such precision, concluded that the Sun was 4 million miles away. This is the first appearance of his work in print, translated into Latin by Federico Commandino.
Hunting the Astronomical Unit Edmond Halley
(1656-1742)
Catalogus stellarum australium
(Catalogue of the southern stars)
London:
T. James,
1679.
Gift of the Burndy Library
In 1676 the English astronomer Halley traveled to the remote South Atlantic island of St. Helena for one year to observe stars in the southern skies that were not visible in England. While there he witnessed the 1677 transit of Mercury and realized that transits might be useful for finding the Earth-Sun distance. He first published his idea in his catalogue of the stars located near the south celestial pole.
Hunting the Astronomical Unit Edmond Halley
(1656-1742)
"Methodus singularis qual Solis Parallaxis sive distantia a Terra (Remarkable method for determining the solar parallax of the distance from the Sun to the Earth)".
Philosophical Transactions of the Royal Society of London
London:
W. Innys,
1717.
Volume 29
Smithsonian Institution Libraries
After working on the problem for nearly forty years, Halley announced in this article the final details of his method for measuring the value of the AU from transit-of-Venus observations. By this time, Halley’s reputation was so great that astronomers took notice of his plea for scientists everywhere to make numerous observations of the transit of 1761. This article was the most important published work that brought about the surge of interest in the transit of Venus.
Hunting the Astronomical Unit Johannes Hevelius
(1611-1687)
Machinæ coelestis
(Celestial Machines)
Gdansk:
by the author,
1673.
Gift of the Burndy Library
The astronomers Johannes and Elisabetha Hevelius using their sextant at their private observatory in Gdansk.
17th-century astronomers used instruments like this to determine the parallax—the apparent shift of a planet’s position caused by observing it from different locations—of Mars in order to find a value for the Earth-Sun distance.
Hunting the Astronomical Unit James Giles
Orrery
London:
1740-80.
Courtesy of the National Museum of American History, Behring Center
The first orrery, or model that simulates the motions of the planets around the Sun, was made in 1713 for the Earl of Orrery. These models were made primarily as educational aids and conversation pieces. While the true size of the solar system was still a mystery at this time, by the early 1700s people basically understood its arrangement and relative proportions
Hunting the Astronomical Unit Guillaume Le Gentil de La Galaisiere
(1725-1792)
"De la conjunction inferieure de Venus avec le Soleil (On the inferior conjunction of Venus with the sun)".
Histoire de l' Academie Royale des sciences, annee 1753
Paris:
deI'Imprimerie Royale,
1757.
Smithsonian Institution Libraries
French astronomer Le Gentil, who worked at the royal observatory in Paris, downplayed the worth of observing transits of Mercury in determining the Earth-Sun distance. His article was critical in raising French support for observations of the transits of Venus in 1761 and 1769, using Delisle’s improved method. An inferior conjunction of Venus (when Venus passes between Earth and the Sun) happens every 584 days.
Mobilizing for the 1761 and 1769 Transits
Encyclopedie, ou Dictionnaire raisonne des sciences, des arts et des metiers
(Encyclopedia or descriptive dictionary of the sciences, arts, and trades, plate volume XII)
Paris:
Chez Panckoucke et al.,
1777.
Smithsonian Institution Libraries
The famous Encyclopédie was one of the great products of the French Enlightenment and one of the great achievements in the history of printing and publishing. This illustration shows a small telescope of the kind that might be used to observe the transit of Venus on an expedition. A piece of dark glass would be placed in front of it to reduce the intensity of the sunlight.
Mobilizing for the 1761 and 1769 Transits Johann Elert Bode
(1747-1826)
Deutliche Abhandlung … von dem bevorstehenden merkwürdigen Durchgang der Venus…
(Clear treatise on the forthcoming remarkable transit of Venus)
Hamburg:
Dieterich Anton Harmsen,
1769.
Courtesy of the U.S. Naval Observatory Library
In preparation for the 1769 transit, the German astronomer Bode produced this short work providing details on the previous transit observations and a map of places where the 1769 transit would be visible. In the pink area, the transit would be visible in its entirety. Green indicates where observers would see only the beginning of the transit, while yellow shows where they would see only the end.
Mobilizing for the 1761 and 1769 Transits John Singleton Copley
(1738-1815)
Portrait John Withrop
c. 1773.
Courtesy of the Harvard University Portrait Collection, Gift of the executors of the Estate of John Winthrop, grandson, 1894
Winthrop was Hollis Professor of Mathematics and Natural Philosophy at Harvard University. Copley, the preeminent painter in Boston at the time, captured the importance of Winthrop’s interest in the transit of Venus. Winthrop is shown next to the telescope with which he may have viewed the 1769 transit. He is pointing to a diagram of the transit (compare it to the one in his book) and the landscape corresponds to that of his 1761 observing site (“Venus’s Hill”) in Newfoundland.
Mobilizing for the 1761 and 1769 Transits George Adams
Gregorian reflecting telescope
London:
c. 1750-1800.
Courtesy of the National Museum of American History, Behring Center
Unlike the telescope at the gallery’s entrance, this telescope uses mirrors, not lenses, to gather the light from stars and planets. It was originally developed by James Gregory in 1663, but improvements made it practical around 1740. Reflecting telescopes could be made much smaller than the lens-based telescopes of the time. Their smaller size made them easier to take on distant expeditions to see the 1761 and 1769 transits of Venus.
Mobilizing for the 1761 and 1769 Transits Nevil Maskelyne
(1731-1811)
Instructions relative to the observations of the ensuing transit of the planet Venus. . .
London:
W. Richardson and S. Clark,
1768.
Gift of the Burndy Library
Maskelyne, an English astronomer, traveled to St. Helena in the South Atlantic to observe the 1761 transit, but was cursed with overcast skies. Seven years later, Maskelyne, by this time the Astronomer Royal, wrote to encourage people with the proper equipment to observe the 1769 transit, providing detailed instructions in how to make an observation of real scientific value.
Mobilizing for the 1761 and 1769 Transits Edward Scriven
(1775-1841)
Portrait of Rev. Nevil Maskelyne, Astronomer Royal from 1765 to 1811
(from a portrait by Van de Puyl)
Gift of the Burndy Library
Maskelyne, an English astronomer, traveled to St. Helena in the South Atlantic to observe the 1761 transit, but was cursed with overcast skies. Seven years later, Maskelyne, by this time the Astronomer Royal, wrote to encourage people with the proper equipment to observe the 1769 transit, providing detailed instructions in how to make an observation of real scientific value.
Mobilizing for the 1761 and 1769 Transits Benjamin West
(1730-1813)
New-England almanack … for the year of our Lord Christ, 1769
Boston:
Mein and Fleeming,
[1768].
Gift of Ronald S. Wilkinson
Since the 1769 transit of Venus was expected to be visible in the American colonies, there was a great deal of interest in seeing the rare event. The noted New England almanac producer Benjamin West included an extensive essay on the transit in his 1769 volume. It includes a diagram showing the path that Venus would take across the Sun’s disk.
The Perils of Chasing Venus Artist Unknown
Portrait of Jean Chappe d’Auteroche
For the 1769 transit, Jean Chappe d’Auteroche led a party to the southern tip of Baja California. They observed the transit under ideal conditions, but a few days later an epidemic struck the area, killing three-fourths of the local population and everyone in the expedition except for Chappe and Pauly, his engineer. Chappe lingered for a short while but died before leaving Baja. Pauly managed to get back to France with all of the observation records
The Perils of Chasing Venus Guillaume Le Gentil de La Galaisiere
(1725-1792)
Voyage dans les mers de l'Inde
Paris:
deI'Imprimerie Royale,
1779.
Courtesy of the U.S. Naval Observatory Library
Guillaume Le Gentil de la Galaisière traveled to India to observe the 1761 transit from the French colony of Pondicherry. Before he arrived, the British captured the colony, forcing Le Gentil to return to the French island of Mauritius. He did not get there in time to observe the transit. Rather than return home, he studied the Indian Ocean cultures, planning to watch the 1769 transit from Manila in the Philippines. But he was ordered back to Pondicherry, again under French control.
While Manila had beautiful weather during the transit, it rained in Pondicherry. Le Gentil missed both transits. When he returned home in 1771, he learned that he had been declared dead. After many expensive legal battles, Le Gentil managed to prove he was actually alive.
To the Ends of the Earth Samuel Dunn
(d. 1794)
"A Determination of the exact Moments of Time when the Planet Venus was at external and internal contace with the Sun's Limb…,".
Philosophical transactions of the Royal Society of London
London:
Lockyer Davis,
1771.
Volume LX (1770)
Smithsonian Institution Libraries
Dunn’s observations reveal why accurate measurements of the exact moment of contact were impossible. His drawings show how the edge of Venus appeared to smear instead of remaining sharply defined as it approached the Sun. This phenomenon, known as the black drop effect, played havoc with the transit observer’s measurements.
To the Ends of the Earth Charles Green
; Cook, James (1728-1779)
"Observations made … at King George's Island [Tahiti] in the South Sea".
Philosophical Transactions of the Royal Society of London, vol. LXI. For the year 1771
London:
Lockyer Davis …,
1772.
Volume LXI (1771)
Perhaps the most famous observer of the transit of Venus was Captain James Cook. The primary purpose of his first voyage to Tahiti was to witness the 1769 transit. Cook was experienced in nautical astronomy and received additional training from Green, the expedition astronomer. This article shows Cook’s drawings of the transit. In them, you can clearly see the black drop problem.
To the Ends of the Earth John Hawkesworth
(1715?-1773)
account of the voyages undertaken … for making discoveries in the Southern Hemisphere…
London:
W. Strahan and T. Cadell,
1773.
Vol. II
Gift of the Burndy Library
Hawkesworth’s well-known accounts of Cook’s voyages helped publicize a new era of exploration. This account of the first voyage includes a map of Tahiti (or “Otaheite”). At the top of the main island, you can see “Point Venus,” where Cook and Green observed the 1769 transit of Venus.
When Captain Samuel Wallis of the Swallow returned to England in 1768 and told everyone about the island of Tahiti, many agreed that it would be the best place to see the 1769 transit.
To the Ends of the Earth Charles Mason
(1730-1787)
; Dixon, Jeremiah d. 1777
"Observations made at the the Cape of Good Hope".
Philosophical transactions of the Royal Society of London
London:
L. Davis and C. Reymers,
1762.
Volume LII (1761)
Smithsonian Institution Libraries
Before the English astronomers Mason and Dixon surveyed the boundary between Maryland and Pennsylvania (the Mason-Dixon Line), they were sent to Sumatra to observe the 1761 transit of Venus. England was at war with France. On the voyage a French ship attacked them, killing some of the crew. Quite shaken, Mason and Dixon were persuaded to continue but, running short of time, they had to stop in South Africa, where they had good observing conditions.
To the Ends of the Earth David Rittenhouse
(1732-1796)
; and others
"An account of the Transit of Venus…".
Transactions of the American Philosophical Society
Philadelphia:
R. Aitken & Son,
1789.
vol. I, 2d ed. Corrected
The noted American astronomer, surveyor, and clockmaker Rittenhouse was one of many colonists who observed the 1769 transit of Venus. Members of the newly founded American Philosophical Society in Philadelphia observed the transit at Rittenhouse’s home in nearby Norriton. They purchased astronomical instruments with money from the Pennsylvania governor and provincial assembly, and published their observations in the first volume of their Society’s journal.
To the Ends of the Earth John Winthrop
(1714-1779)
Relation of a voyage from Boston to Newfoundland, for the observation of the transit of Benus, June 6, 1761
Boston:
Edes and Gill …,
1761.
Courtesy of the U.S. Naval Observatory Library
In contrast with the extensive and often dangerous French and British expeditions, the thirteen-day voyage of the party led by Winthrop, a Harvard mathematics professor, to St. John’s, Newfoundland, could be considered a pleasant excursion. His party named the hill where they set up their observatory “Venus’s Hill.” Winthrop’s rare little book describes how they could see only the end of the transit, although their weather was quite clear.
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