Few know that the slopes of the Moncayo and the heights of Tarazona were, for a few days of the summer of 1860, one of the great scientific laboratories in the world. Some of the most prestigious astronomers of the nineteenth century coincided there to observe an exceptional phenomenon: the total solar eclipse on July 18.
Among them was Urbain Le Verrier, director of the Paris Observatory and famous for having mathematically predicted the existence of the planet Neptune. Now, 166 years after that expedition, Tarazona pays tribute to the French scientist with a monolith installed at the La Luesa viewpoint, as part of the events organized on the occasion of the total eclipse that will cross again España on August 12, 2026.
THE MONCAYO, CHOSEN BY THE GREAT EUROPEAN OBSERVATORIES
The scientific importance of the eclipse of 1860 was reflected in the Report of the Spanish Expedition, published by the Gaceta de Madrid – predecessor of the current Official State Gazette (BOE) – on October 31 of that year.
In the nineteenth century, the Gaceta de Madrid was the official newspaper of the State, where laws, government provisions and also the memoirs and reports of scientific expeditions promoted or supported by public institutions were published. The publication of this document in its pages demonstrates the relevance that the Government gave to the expedition organized to observe the total eclipse of the Sun and makes the text one of the main official sources to know what happened in Moncayo and Tarazona.
In it, the astronomer Eduardo Novella, head of the Spanish commission, explains that the Madrid and Paris Observatories decided to join their expeditions in the Moncayo, aware that the phenomenon could provide answers to some of the great unknowns about the Sun. «Based on these considerations, the Observatories of Paris and Madrid agreed to meet their Commissions in the Moncayo…».
A privileged place for astronomical observation
It was not a casual choice. After recognising the mountain, Novella describes the Moncayo as a privileged place for astronomical observation thanks to «its immense horizon» and «the diaphanous atmosphere«, ideal conditions for contemplating the eclipse.
However, he also warned of the difficulties of working at an altitude of more than 2,000 metres, without water, shelter or firewood and exposed to strong winds. In fact, the booth built to house part of the instruments ended up collapsing before the eclipse.
THE FOG FORCED TO CHANGE PLANS
July 18 dawned with a thick fog covering the Moncayo. Scientists even thought they would lose the observation. «The fog that surrounded Moncayo was so dense and humid that we all agreed not to see the eclipse,» says Novella.
The fog forced the expedition to split up and move part of the observations to the heights of Tarazona
The solution was to divide the expedition. While some researchers remained next to the Moncayo Sanctuary with the heaviest instruments, the rest descended for several hours towards Tarazona in search of a clearer sky. The decision was providential.
According to the official report, the astronomers managed to settle «in the heights of Tarazona», where the wind had swept away the clouds and the sky allowed the eclipse to be seen practically in optimal conditions. It was there that Urbain Le Verrier made his observations.
THE MAN WHO DISCOVERED NEPTUNE
Le Verrier was already an international figure at the time.In 1846 he had revolutionized astronomy by calculating, only through mathematical operations, the existence of an unknown planet whose position was confirmed a few days later by the Berlin Observatory: Neptune.
Fourteen years later he led the French expedition to Moncayo. Novella’s memoir records his work during the eclipse: «M. Le Verrier observed the contacts and studied the arrangement of the red protuberances.»
The solar bulges actually belonged to the Sun and were not an optical effect
Those observations helped to confirm that the so-called solar prominences really belonged to the Sun and were not simple optical illusions, a question much debated by the scientific community of the time. Novella himself considers at the end of the report that this conclusion justified all the efforts made by the expedition.
much more than a place of observation
The expedition turned Tarazona into a real center of scientific operations. From the city, telegraphic communications with Madrid were coordinated, geographical measurements were made and work was organized to determine precisely the position from which the observations were made.
Collaboration of the Cabildo of Tarazona, the mayors of Tarazona, Añón, Litago and San Martín
Novella also wanted to record the support received by local institutions, expressly thanking the collaboration of the Cabildo de Tarazona, the mayors of Tarazona, Añón, Litago and San Martín, as well as numerous people who facilitated the work of the scientific commissions.
In addition, once the eclipse was over, the scientists returned to Tarazona to complete the geographical measurements and telegraphic operations that made it possible to accurately determine the longitude of the observation point with respect to Madrid, consolidating the city’s role as a logistical and scientific base for the expedition.
A TRIBUTE A CENTURY AND A HALF LATER
This historical episode is the one that the Tarazona City Council has wanted to recover with the installation of a monolith dedicated to Urbain Le Verrier at the La Luesa viewpoint.
The inauguration, scheduled for July 18, is part of the official program of the Total Solar Eclipse 2026, which also includes conferences, historical reenactments, exhibitions and informative activities aimed at recovering the scientific memory of the eclipse of 1860, one of the events that placed Moncayo and Tarazona at the center of international astronomy for a few days.
More than a century and a half later, that episode looks up to the sky again. The tribute to Le Verrier not only recalls the figure of the astronomer who predicted the existence of Neptune, but also a little-known page in the history of Aragon: that of a summer in which the Moncayo brought together some of the most prestigious scientists in the world and helped to unveil some of the secrets of the Sun.
