Buenaventura Suárez arrived at San Cosme in 1703 and built the first observatory in colonial South America, grinding lenses and having Guaraní craftsmen make the tubes, the quadrant and the clocks. His Lunario de un siglo predicted a century of eclipses accurately enough that European astronomers used his timings, which given the equipment is close to absurd. The stone sundial with its metal gnomon still stands in the courtyard.
The modern centre beside the mission is the tribute. There is a projection room on Guaraní astronomy, an armillary sphere, the solar system laid out to scale, a galaxy slide with a black hole at the bottom of it for children, and a planetarium that shows the sky as the Guaraní read it rather than as the Greeks did — the ñandu, the tapir's path, the Milky Way as a river.
The reason to stay the night, though, is the sky itself. San Cosme has almost no light pollution and the observatory stays open every evening from 19:30 to 21:00 with night telescopes; by day you look at the sun through a filtered one. Both are inside the same Gs 40.000 ticket that covers Trinidad and Jesús. Two mobiles answer, +595 982 743 438 and +595 985 732 956, and neither is a WhatsApp.