Tycho Brahe · 1923
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For twenty-one years on the island of Hven, and afterwards in Bohemia, Tycho Brahe and his assistants recorded what they saw. The Thesaurus Observationum is that record, gathered from the surviving logbooks by J. L. E. Dreyer for his edition of Tycho’s complete works.
This first volume opens with Dreyer’s Prolegomena, his account of the manuscripts, how they came down, and how he has edited them. Then come the observations, year by year. They begin in 1563 with the first observation Tycho is known to have recorded, made at sixteen. They run through his student years and his time in Augsburg, then through the Hven years up to 1585.
What the logs hold is measurement: positions of the Sun, Moon and planets, the distances and meridian altitudes of the fixed stars, and right ascensions worked out by triangulation from the planets. The instruments used are often named alongside the readings.
The published books of Tycho Brahe give his conclusions. The logbooks give the evidence. This is the observational record from which Kepler, working from Tycho’s data, derived the laws of planetary motion. Reading it shows what astronomical observation was in the last generation before the telescope: how often it was done, how carefully it was checked, and how much labour lay behind each position in a table.
The source is volume X of Dreyer’s Tychonis Brahe Dani Opera Omnia (Copenhagen, 1923), the first of its volumes of observations. The next volume, covering 1586 to 1589, is Thesaurus Observationum, Tomus Secundus.