In 1734, King Philip V of Spain asked Jorge Juan and fellow scientist Antonio de Ulloa to join the French Geodesic Mission organized by the French Academy of Sciences from Paris, under command of the astronomer Louis Godin, and fellow geographers Charles Marie de La Condamine and Pierre Bouguer.
The mission was to measure the length of a degree of meridian arc at the Equator in South America and to determine the roundness of the Earth.
On 26 May 1735, they left Cadiz in the company of the Marquess of Villagarcía, who had just been appointed Viceroy of Peru.
Jorge Juan was on board the ship El Conquistador and Antonio de Ulloa on the frigate Incendio.
The expedition traveled to Quito, in present-day Ecuador, and after nine years of careful study, determined that the Earth is not perfectly spherical but is oblate, i.e. flattened at the poles.
Juan also successfully measured the heights of the mountains of the Andes using a barometer.

Juan remained nine years in America, studying the political and social situation of the Spanish territories.
Among other things, he observed a new metal the miners called Platina de Pinto (little silver of the Pinto river).
On his return to Spain, King Ferdinand promoted him to captain.
For their work and discoveries in Peru, both he and de Ulloa were elected Fellows of the Royal Society, Juan y Santacilia in 1749.

The Marques de la Ensenada ordered Juan to travel to England secretly to study new naval construction methods and armament of the British.
In 1749, he travelled incognito under the name of Mr. Josues and learned all that he could in England (writing back in a numerical cipher), and brought back 50 British naval constructors.
In an ironic twist, many of them (including Matthew Mullan and Richard Rooth) went on to build the Spanish ships that would later fight in the American Revolutionary War and the battle of Trafalgar.
 He was, however, disappointed with the English naval construction methods and upon his return to Spain in 1750, he was placed in charge of Spanish naval construction, creating his own naval construction system (approved in 1752) and improving the shipyards (such as the ones in Cartagena, Cádiz, Ferrol, Havana) and armaments.
He implemented modern industrial division of labor among the different disciplines involved in naval construction such as dry-docks, shipyards, furnaces, rigging, and canvas manufacturing.

In 1757, he founded the Royal Astronomical Observatory of Madrid (Real Observatorio de Madrid).

He died in Madrid in 1773 at the age of 60.
 His reported symptoms, including muscle stiffness and seizures, were almost certainly the result of a cerebral amebic infection.
He was buried in the church of San Martín.

A Churruca-class destroyer of the Spanish Navy, the Jorge Juan, was named after him.
In 1734, King Philip V of Spain asked Jorge Juan and fellow scientist Antonio de Ulloa to join the French Geodesic Mission organized by the French Academy of Sciences from Paris, under command of the astronomer Louis Godin, and fellow geographers Charles Marie de La Condamine and Pierre Bouguer. The mission was to measure the length of a degree of meridian arc at the Equator in South America and to determine the roundness of the Earth. On 26 May 1735, they left Cadiz in the company of the Marquess of Villagarcía, who had just been appointed Viceroy of Peru. Jorge Juan was on board the ship El Conquistador and Antonio de Ulloa on the frigate Incendio. The expedition traveled to Quito, in present-day Ecuador, and after nine years of careful study, determined that the Earth is not perfectly spherical but is oblate, i.e. flattened at the poles. Juan also successfully measured the heights of the mountains of the Andes using a barometer.
Juan remained nine years in America, studying the political and social situation of the Spanish territories. Among other things, he observed a new metal the miners called Platina de Pinto (little silver of the Pinto river). On his return to Spain, King Ferdinand promoted him to captain. For their work and discoveries in Peru, both he and de Ulloa were elected Fellows of the Royal Society, Juan y Santacilia in 1749.
The Marques de la Ensenada ordered Juan to travel to England secretly to study new naval construction methods and armament of the British. In 1749, he travelled incognito under the name of Mr. Josues and learned all that he could in England (writing back in a numerical cipher), and brought back 50 British naval constructors. In an ironic twist, many of them (including Matthew Mullan and Richard Rooth) went on to build the Spanish ships that would later fight in the American Revolutionary War and the battle of Trafalgar.  He was, however, disappointed with the English naval construction methods and upon his return to Spain in 1750, he was placed in charge of Spanish naval construction, creating his own naval construction system (approved in 1752) and improving the shipyards (such as the ones in Cartagena, Cádiz, Ferrol, Havana) and armaments. He implemented modern industrial division of labor among the different disciplines involved in naval construction such as dry-docks, shipyards, furnaces, rigging, and canvas manufacturing.
In 1757, he founded the Royal Astronomical Observatory of Madrid (Real Observatorio de Madrid).
He died in Madrid in 1773 at the age of 60.  His reported symptoms, including muscle stiffness and seizures, were almost certainly the result of a cerebral amebic infection. He was buried in the church of San Martín.
A Churruca-class destroyer of the Spanish Navy, the Jorge Juan, was named after him.
Jorge Juan y Santacilia