The Carte du Ciel photographic survey from this observatory mapped southern sky zones unreachable from Europe. Its 1900s plates help calculate proper motions of stars over a century.
Claudio Elias, Public domain, via Wikimedia Commons
A Grand Map of the Southern Sky
In 1887, astronomers from around the world met in Paris to begin one of the most ambitious collaborative projects in the history of science: the Carte du Ciel, or "Map of the Sky." The goal was to create a photographic atlas of the entire celestial sphere, a task so immense it required dividing the sky into zones assigned to 18 different observatories. The La Plata Astronomical Observatory, founded just a few years earlier in 1883, was an important participant in the Southern Hemisphere.
To accomplish this task, a specialized instrument called an astrograph was installed in a dedicated dome in 1891. This photographic refracting telescope had an objective lens with a 34.2-centimeter aperture and a focal length of 3.42 meters. The telescope was designed to hold large glass photographic plates, each capturing a 2-by-2 degree section of the sky. The original plan was for La Plata to photograph a specific zone of the southern sky, but a broken objective lens caused delays. The work was eventually taken over by the Córdoba Observatory. Despite this early setback, the La Plata astrograph went on to a long career of photographic observation, capturing thousands of plates of asteroids, comets, and star fields throughout the 20th century.
A Time Machine Made of Glass
The true scientific value of the La Plata plate collection is more than a simple static map. These glass plates, numbering over 15,000, are a precise record of the sky from the early 1900s to the 1980s. They act as a time machine, providing a baseline of stellar positions from over a century ago. By comparing the positions of stars on these old plates with their positions in modern digital sky surveys, astronomers can measure their "proper motion"—their apparent movement across the celestial sphere.
This long time-baseline is important for the collection. The tiny shifts, measured in arcseconds, become clear over decades, allowing for precise calculations of stellar velocities and trajectories. This data helps identify stars that are gravitationally bound in binary systems, find stars that are near neighbors to our own solar system, and understand the dynamics of our galaxy. The information stored in the delicate photographic emulsion is so important that a major project, known as ReTrOH (Recuperation of Historical Observational Work), began in 2019 to digitize the entire collection. Using specialized open-source software called PlateUNLP, researchers are converting the analog data on the glass plates into digital FITS files, the standard format for modern astronomical images. This effort preserves the fragile originals and makes their unique historical data accessible to scientists worldwide.
💡Fun Facts
The observatory was founded shortly after the city of La Plata itself, which was established in 1882 as a new provincial capital.
The original core instruments for the observatory were acquired in 1882 from a French astronomical mission that came to Argentina to observe a rare transit of Venus.
The first asteroid discovered at the observatory was (1029) La Plata, found by astronomer Johannes Hartmann on April 28, 1924.
The observatory is part of the National University of La Plata and is housed within a large 7-hectare park containing about 20 scientific buildings and domes.
The observatory and its museum offer guided tours and activities, but schedules vary. It is best to check the official website for current times, especially for evening observation events.
Admission
Visits are generally free, but specific events or planetarium shows may have a fee.
Accessibility
The observatory is located in a park with paved paths. However, the historic dome buildings may have stairs and limited accessibility.
