A Hunt for Southern Stars
The story of the Leoncito Astronomical Complex begins with a scientific need. In the mid-20th century, astronomers had thoroughly mapped the stars of the Northern Hemisphere, but the southern sky remained less charted. Yale University astronomer Frank Schlesinger had pioneered programs to measure stellar parallax and proper motion, but this work needed to be extended south of the equator. After a successful program in Johannesburg, South Africa, was compromised by increasing light pollution in the 1950s, a new site was needed.
In 1960, with a $750,000 grant from the Ford Foundation, the Yale-Columbia Southern Observatory (YCSO) corporation was formed to find a new home for southern sky observation. The primary goal was astrometry: precisely measuring the positions and movements of stars to understand the structure of the Milky Way. A survey of potential sites led them to the eastern foothills of the Andes in Argentina. At an elevation of 2,552 meters, the dry, stable air above El Leoncito in San Juan Province offered exceptional conditions. The site averages 250 clear, windless nights per year, an essential requirement for long-exposure photographic surveys. The aridity of the region significantly reduces atmospheric water vapor, which normally absorbs infrared radiation, making the site perfect for this type of observation.
The National University of Cuyo provided the land, and on March 31, 1965, the new observatory was dedicated. The first major project, the Southern Proper Motion (SPM) program, began that year, using a Double Astrograph telescope to photograph the sky. The U.S. Naval Observatory also relocated a Meridian Circle telescope to the site to extend its catalog of stellar positions to the Southern Hemisphere.
A Sanctuary for the Sky
The exceptional quality of the sky at El Leoncito was so important that it led to the creation of a national park. To protect the site from future light pollution and preserve the pristine atmospheric conditions, the 89,706-hectare El Leoncito National Park was established. This act formally recognized the sky as a natural resource.
Today, the site hosts two major institutions. The original observatory is the Dr. Carlos U. Cesco Observatory (CESCO), operated by the National University of San Juan. The larger, newer facility is the Complejo Astronómico El Leoncito (CASLEO), established in 1983. CASLEO’s flagship instrument is the 2.15-meter Jorge Sahade Telescope, a reflector that began regular operations in 1987. The complex also includes other instruments like the Solar Submillimeter Telescope, installed in 1999 to observe solar flares. The creation of the park ensures that the atmospheric clarity that first drew astronomers here in the 1960s will be maintained for future scientific work.