Measurements of altitude and geographic latitude in Latin astronomy, 1100–1300

Abstract

This article surveys measurements of celestial (chiefly solar) altitudes documented from twelfth- and thirteenth-century Latin Europe. It consists of four main parts providing (i) an overview of the instruments available for altitude measurements and described in contemporary sources, viz. astrolabes, quadrants, shadow sticks, and the torquetum; (ii) a survey of the role played by altitude measurements in the determination of geographic latitude, which takes into account more than 70 preserved estimates; (iii) case studies of four sets of measured solar altitudes in twelfth-century Latin sources; (iv) an in-depth discussion of the evidence relating to altitude measurements performed in Paris in the period 1281–1290. The findings from the last part indicate that by the end of the thirteenth century Parisian astronomer had developed rigorous standards of observational practice in which altitudes were typically measured to a precision of minutes of arc and with a level of accuracy higher than ± 0;5°, and sometimes exceeding ± 0;1°.

On some early Latin European measurements of the eccentricity of the solar orbit (1308–1314)

“…0;24°/d) to estimate the time until/since the equinox. 16 Rather than using this specific recipe, the astronomer responsible for the equinox observations of 1314 presumably converted the declination at the observed solar noon altitude into an ecliptic longitude. Subsequently, he divided the longitude difference relative to the equinox (at 0° or 180°) by a solar velocity adequate to the relevant time segment.…”