Giovanni Battista Riccioli · 1665
Read the translation (PDF) · Editorial footnotes (PDF)
A mature reckoning with the heavens as the telescope was revealing them — a sequel to the vast Almagestum Novum (1651), built on further years of observation at Bologna with the collaborator Grimaldi, with fresh measurements and corrected parameters at center stage.
The work runs to ten books across two tomes, opening with prolegomena that argue the ground before any observation is reported: a series of articles on the fixed stars, on the Sun, and — in an epilogue on the system of the world — on the arrangement of the cosmos itself.
The books then take the heavens in turn. The first establishes the observations of the Sun and the foundations of the solar hypotheses; the rest treat eclipses, the Moon, the fixed stars, and each planet in its own right, before reducing the whole to working tables a practising astronomer could compute from.
Three strands make it a distinctive record of its moment. It carries some of the earliest sustained telescopic accounts of Saturn’s changing shape, written before anyone understood that the changes were a ring seen at varying angles. It describes the bands and spots crossing Jupiter. And it measures the apparent sizes of the fixed stars — then turns that measurement into an argument, since stars of such apparent size, placed as far away as a moving Earth requires, would have to be implausibly vast.
Riccioli is the most careful of the astronomers who concluded against the Earth’s motion, and this book shows why that position was defensible on the evidence available. The star-size argument was a real difficulty for Copernicanism, not an evasion of it — it dissolved only once telescopic optics were understood well enough to show the measured discs were artifacts. Read now, Astronomia Reformata is a vivid window onto astronomy at the dawn of the telescopic age, and onto what it was like to reason well from measurements that were quietly misleading.
The translated source is the 1665 printing in two tomes, which opens with a dedicatory title page and epistle and supplies two separate indexes — one of the books, and a fuller one of books and chapters following the prolegomena.