There is a small two-storey house on the calle Italia in central Rosario, Argentina, that until about 1971 carried, on its flat roof, the cast-iron base of a small equatorial mount and the rusted remains of a hand-driven shutter mechanism for a wooden observatory dome.
The dome itself was removed in 1953, after a roof leak. The mount was sold for scrap when the house changed hands. The shutter mechanism remained for another eighteen years, because no one knew what to do with it.
The observatory had belonged to Petronila Astengo, who was born in Rosario in 1872 and died there in 1949, and who, between 1898 and about 1924, made and recorded an estimated twelve thousand variable-star observations from the roof of her own house.
Her observing notebooks, eleven of them, are held in a corner cabinet of the library of the Museo Histórico Provincial Dr. Julio Marc, in the city's Parque Independencia. They were acquired in 1974 from a great-niece who had been clearing the attic of the calle Italia house and who had not, until then, known what they were.
Astengo's biography is briefly told. Her father, Vicente Astengo, was a Genoese-born merchant who had emigrated to Rosario in 1858. Her mother, Teresa Maggiolo, was the daughter of an Italian-Argentine bookseller. The family was prosperous, Catholic, and unusually willing to send its daughters to school.
Petronila was the third of five children and the only one who never married. She studied at the Escuela Normal de Maestras de Rosario, qualified as a primary-school teacher in 1893, and taught for six years at a state school in the Pichincha district before resigning her post, at twenty-seven, on the death of her father.
The inheritance was small but sufficient. She used the first portion of it to commission the construction of the rooftop dome, in 1898, from a local boilermaker who had previously built grain elevators along the Paraná river. The mount and the four-inch equatorial refractor were purchased, second-hand, from a dealer in Genoa, brought across the Atlantic in 1899 on a freighter, and delivered to the Rosario port by a young friend of her brother's.
The friend was Wilhelm Knobel, a German-born clerk at a Rosario shipping firm and an amateur astronomer himself. He had agreed to oversee the installation. The two became correspondents and, for about fifteen years, occasional joint observers. Knobel's letters survive in the Astengo notebooks, folded into the back covers.
Astengo's choice of programme was practical. The southern variable stars were, in 1899, badly under-observed. The Argentine national observatory at Córdoba, which had been founded by Benjamin Apthorp Gould in 1871, had focussed its energies on the great Uranometría Argentina and on the southern photographic durchmusterung. Variable-star monitoring required patient, repeated visual estimation, which was labour the Córdoba staff could not spare.
Astengo offered to do it. Beginning in 1900, she began sending monthly digests of her observations, by post, to Charles Perrine at Córdoba and to Edward Pickering at the Harvard College Observatory.
Perrine acknowledged her letters with courtesy and filed them. Pickering, who had been collecting amateur variable-star data from any source he could find for nearly two decades, replied at length, sent her the latest copy of the Harvard Annals, and asked for more.
Over the next twenty years, Astengo supplied Pickering, and after his death in 1919 his successor Solon Bailey, with periodic estimates of roughly four hundred southern variables. The estimates were made by the standard method of the day, comparing each variable to nearby stars of known magnitude and interpolating by eye.
Her notebooks suggest that on a clear night she could make between thirty and fifty estimates. She observed roughly two hundred nights a year. The arithmetic is plausible.
The Harvard end of the correspondence is preserved in the observatory's incoming-letters file, where Astengo's contributions are catalogued by the name of the local intermediary, a Boston-born missionary's wife in Rosario who hand-carried letters to the post.
When the resulting data were eventually published, in the Harvard Annals and in supplements to the Henry Draper Catalogue, the attribution was, with two exceptions, generic. The phrase used was communicated from Rosario, Argentina.
Astengo herself appears, by name, in print only three times. Once in a footnote to a 1908 paper by Pickering on long-period variables in Carina. Once in a 1916 note by Annie Jump Cannon thanking her for a series of comparisons of the Mira-type variable R Centauri. And once in her obituary, which ran in the Rosario daily La Capital on 14 March 1949 and which described her, accurately and unhelpfully, as una dama soltera de antigua familia, aficionada a la astronomía.
The recovery of her career, such as it has been, began in the late 1980s, when a Córdoba historian of astronomy named Ricardo Platzeck noticed her name in a marginal note in a Pickering letter and began to look for the underlying observations. He found the notebooks in the Marc museum in 1991.
Platzeck published a short biographical article in the Boletín de la Asociación Argentina de Astronomía in 1994. The article was the basis for the entry on Astengo in the third edition of the Biographical Encyclopedia of Astronomers, in 2014.
Even at that date, her contribution to the Harvard variable-star programme had not been fully reconstructed. The Harvard plates from the period are still being scanned and re-reduced under the DASCH project at the Harvard-Smithsonian Center for Astrophysics. As that work has progressed, the Astengo estimates have been cross-matched with the photometric record, and her observational accuracy has been assessed, decade by decade, against the modern values.
The assessment is favourable. Her magnitudes, on the standard scale of the period, are within about two tenths of a magnitude of the modern values for the bulk of her sample. For an observer working alone, with a four-inch refractor, from the roof of a house in a river port at thirty-three degrees south latitude, the agreement is creditable.
There is no plaque on the calle Italia house. There is no monument. The city of Rosario has no street named for her, although a small lecture room at the Planetario Municipal Luis C. Carballo carries her name on a printed card pinned to the door.
The card was put there in 2016 by a high-school astronomy club. It is laminated. It does not say very much. It says that the room is named for an astronomer who lived in this city and who looked at the sky from her own roof for a long time.
The summary is, on its own terms, sufficient.
