[John Flamsteed's horoscope for laying the groundwork of Greenwich astronomy and astrology].

The paper deals with the astronomical and astrological contents of a horoscope cast by John Flamsteed in 1675 for the foundation of Greenwich Observatory. So far no analysis of its astronomical contents has been made. It can be shown that the chart has been drawn correctly, as is to be expected from a competent astronomer. For calculating the planetary positions he most likely used tables issued by Johann Hecker, a pupil of Hevelius, based on Kepler's "Tabulae Rudolphinae" in 1627. The cusps of the twelve astrological houses Flamsteed calculated trigonometrically; so he used no table of houses. Flamsteed employed a method of house division (domification) which was commonly used in the 16th and 17th century and connected with the name of Johannes Regiomontanus. Positional circles joining in the north and south points of the observer's horizon are laid through distances of 30 degrees on the celestial equator, thus giving unequal sections of the ecliptic. By consulting contemporary sources for the interpretation of the chart (Ramesey's Astrologia Restaurata, 1653) it appears that the time for laying the foundation stone was well chosen from the astrological point of view. There were precursors in this practice, e.g. the Italian astrologer Luca Gaurico, who was commissioned to submit an astrological report for the foundation for the Franse Wing in the Vatican in 1543, and Tycho Brahe, who performed a solemn ceremony on the island of Hven in 1576 at the laying of the foundation stone of his observatory in an astrologically propitious moment. This leads to the question whether Flamsteed believed in astrology. Michael Hunter has already given evidence that Flamsteed was indeed well-versed with astrological techniques and supplied astrologers with data. But at the same time he expressed hostility towards astrological interpretations issued frequently by different parties during Civil War in England. In an unpublished preface for Hecker's Tables (edited by Hunter) Flamsteed tried to show the "Vanity of Astrology, & the Practice of Astrologers". Therefore he cannot be taken as an ardent astrologer, although he was well acquainted with the art.