Planets in their courses, the year set by rising stars—Wittenberg astronomy in elegant dress, a collector’s pleasure and a scholar’s tool. THE BOOK Bartholomaeus Schönborn. Computus Astronomicus. Wittebergae: Haeredes Ioannis Cratonis, 1579. Small 8vo. Collation: ⁸ A–S⁸ t⁴ T–Y⁸. Numerous woodcuts. Very rare edition. Bound with Erasmus Sarcerius, Rhetorica (Wittenberg, 1583). Both complete. MOUSEION CURATOR NOTE “Wittenberg poises the heavens into calm utility: Schönborn’s computus turns seasonal light into ordered hours, lets the planets keep courteous procession, and tunes the year by rising stars. It is Reformation astronomy with manners—astrology trimmed to what reason allows—equally persuasive to scholar and collector. With early use and the Owen Gingerich ex-libris, this copy links a sixteenth-century “science of time” to the modern study of the skies.” DESCRIPTION Elegant, usable, and scarce—a choice addition to science and Reformation shelves. A quintessential Reformation-era “science of time,” uniting astronomy, calendrics, and chastened astrology. Educated in the Melanchthonian milieu and later a professor at Wittenberg, Bartholomaeus Schönborn (1530–1585) wrote this advanced guide to the structure of time. Beginning from the motions of Sun and Moon, he derives equinoctial and temporary (unequal) hours, explains their conversion, and provides hour-wheels that let the reader pass from seasonal day-lengths to the variable “planetary hours” used in medical, devotional, and—cautiously—astrological practice. Schönborn lays out the Chaldean order and the modular rule by which the ruler of the day and the succession of hourly lords are obtained: dividing a day into 24 parts and cycling the seven rulers produces the well-known weekday sequence. A substantial section treats planetary hours and their computation. The tables enable one to: determine the day-ruler from sunrise, compute the lengths of diurnal and nocturnal hours from the Sun’s declination (giving longer daytime hours in summer, longer night hours in winter); And map each hour to a planetary lord for electional purposes (choosing auspicious times) while signaling the limits of such practice. Schönborn then turns to lunar theory and the calendar. He analyzes the Metonic 19-year cycle, quantifying its residual drift (roughly 1½ hours per 19 years, cumulating to c. one day in ~304 years) and explains the practical consequences for the age of the Moon, new and full moons, and festal reckoning. This sober arithmetic—very much in the Wittenberg spirit—anticipates the technical discussions that, within a few years, would be taken up in the Gregorian reform debates (Wittenberg itself remaining officially Julian). The calendar at the end fuses astronomy and liturgy: dominical letters; phases and ages of the Moon; heliacal risings and settings of prominent fixed stars (e.g., Orion’s head, Sirius, the Pleiades, Regulus), expressed in degrees and minutes; and concise verse distichs—useful for agriculture, navigation, and prognosis. It is the Protestant computus at work: astronomical data aligned to church time, and astrological material trimmed to what can be defended mathematically. PROVENANCE Early contemporary binding (dated 1585) with initials MWV—very likely the first owner. Owen Gingerich (1930–2023), astronomer and historian of astronomy (Harvard/Smithsonian): printed armorial bookplate “Coeli enarrant gloriam Dei” on the front pastedown; a resonant association for early modern calendar science. Early reader’s notes in the calendar (a neat “Index” at December; occasional marks to saints’ days and stellar columns), indicating active computational use. CONDITION REPORT Small 8vo in contemporary parchment over boards, sound and honest: some uniform toning and minor use consistent with a working manual; scattered early marginalia; binding rubbed in places yet entirely serviceable. Red-stained, gauffered edges present a handsome survival. A well-preserved copy in contemporary parchment —an appealing survival. ADDITIONAL INFORMATION The Wittenberg interpretation of astronomy—accepting precise computation and instruments while bracketing cosmological commitments—Schönborn’s manual shows how a mixed astronomical–astrological toolkit could be made rigorous: planetary hours built from observed day-lengths; lunar months fitted to solar years; fixed stars used as seasonal markers; and residuals of cycles openly quantified rather than hidden. It is the mathematization of time at pocket scale. Bound after the Computus is Erasmus Sarcerius’s Rhetorica, plena ac referta exemplis (Wittenberg: Matthaeus Welack, 1583), a compact Lutheran school manual that furnishes exempla “which can serve as brief declamations,” sharpening invention and arrangement for preaching, teaching, and civic oratory; attributed also to the Wittenberg circle through Reinhard Lorichius’s involvement, it embodies the Melanchthonian program of rhetoric as practical pedagogy and makes an apt humanist counterweight to the astronomical handbook it accompanies.