The Vatican Observatory: Refoundation in 1891 and Early Photographic Surveys

On 14 March 1891, Pope Leo XIII issued Ut mysticam, formally confirming the refoundation and expansion of the Vatican Observatory. The document approved the statutes, governing committee and division of staff responsibilities already prepared, and allocated operating funds, giving the undertaking continuing institutional and financial support. The pope also explicitly connected support for astronomical research with answering accusations that the Church opposed science.

Practical preparations had already taken place. An exhibition marking the pope’s priestly jubilee in 1888 assembled meteorological, geophysical and astronomical instruments, which were subsequently gathered in the Vatican. Barnabite priest Francesco Denza became the director, with Giuseppe Lais and others taking part. Richard Gregory’s contemporary report of 1894 traced the new institution’s work to 1889; the 1891 document supplied formal confirmation.

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The first instruments were installed in the Tower of the Winds, dating from the reign of Gregory XIII. The location continued an earlier observing tradition, but surrounding buildings and the dome of St Peter’s Basilica obstructed its view. The refoundation document therefore also assigned the observatory a solid tower on the nearby Leonine wall, high on the Vatican hill, with a large equatorial telescope for stellar photography.

In 1891, the photographic telescope and an eight-metre rotating dome were installed on the Leonine tower. Thick walls, vaulting and an open view provided the stability needed for long exposures. The instrument followed the same basic specifications as the international photographic survey coordinated in Paris, allowing plates from different sites to be processed and compared consistently.

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The early Vatican Observatory worked in astronomy, meteorology, geomagnetism and geodynamics. Denza and his colleagues used automatic recorders to preserve continuous meteorological changes and also made manual observations. Magnetic instruments measured the absolute values of geomagnetic elements and their variation. The first publications appeared from 1891, enabling outside researchers to examine the work.

Lais gradually made photographic observation part of daily practice and visited Paris to study stellar photography and plate-position measurement. Photography for the star catalogue began in 1894, and photography for the sky charts in 1900. In the first years he worked on alternate nights in the Leonine tower with the engineer Mannucci. After Mannucci was reassigned, Lais took on most of the continuing photographic work.

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Denza secured the observatory’s participation in the Carte du Ciel collaboration organised in Paris, taking responsibility for the band of sky from +55° to +64° declination. The programme had two distinct products. Shorter exposures provided a photographic catalogue of stellar positions and brightness, targeting approximately eleventh magnitude. Longer exposures produced charts reaching approximately fourteenth magnitude. Larger magnitude numbers signify fainter stars, so the latter required longer exposures.

To distinguish actual stars from plate defects, chart plates could be given three exposures of forty minutes each, with slight movements of the telescope producing three images of each star in a small triangle. A fine grid was also photographed onto the plate as a positional reference. Taking the photograph was therefore the first step; subsequent measurement and reduction converted the images into publishable observational data.

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Lais’s sustained photographic work supplied plates for catalogue measurements. Johann Georg Hagen, who became director in 1906, prioritised measurement and cataloguing. He investigated practices at other observatories, acquired a measuring machine from Repsold in Hamburg, and organised nuns to measure plates. Plates had to be measured again in the opposite orientation to check stellar positions and image diameters.

This division of labour linked nighttime observing, precision measurement, numerical calculation and proof correction. The ten-volume catalogue recording the brightness and rectangular coordinates of 481,215 stars began appearing in 1914 and was completed in 1928. Work on the longer-exposure sky charts continued afterwards, producing a separate series of results.

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After Hagen died in 1930, Johann Wilhelm Stein became director. Rome’s urban lighting and the space needed for additional instruments prompted the observatory to consider another site. At the end of 1931, staff made test photographs of star trails and double stars at Castel Gandolfo to examine atmospheric and imaging conditions. Construction of the new station began in 1932.

Pius XI presided over the inauguration of the new observatory at Castel Gandolfo on 29 September 1935. That year’s bulletin of the Jesuit scientists’ association recorded the occasion and speeches. The new site had Zeiss telescopes and photographic measuring equipment, expanding work on variable stars and astrophysics. Relocation gave the existing photographic observing programme better conditions and provided equipment and space for new astrophysical research.

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