Astronomical Images : Exemplum for finding the beginnings of the twelve houses with an astrolabe for a given time

Jacob Koebel

Astronomical Images

<p style='text-align: justify;'>Jacob Koebel was a surveyor and practical mathematician in Oppenheim, near Mainz. He was also a prolific printer and publisher of his own works and those of others. After publishing <i>Elucidatio fabricae ususque astrolabii</i> by his friend, Johann Stoeffler, in 1524, Koebel went on to produce his own treatise on the astrolabe ' a much shorter and simpler version than Stoeffler's, but following the same basic structure. Thus, Koebel's tract also begins with a practical section on the construction and uses of the astrolabe, its application to astrology and the use of the quadrant in surveying. The book proved popular, as Stoeffler's had done. This is the second edition, which presented more elaborate illustrations than the original work. This figure shows a horoscope, presented in the form of a square chart which was commonly used by sixteenth-century authors. The chart gave the details of the time of birth in the middle square, with the twelve astrological houses represented by the triangles surrounding it. This made it easy to distinguish visually between the highly influential angular houses, contained in the square tilted diagonally and the less powerful 'succeedent' and 'cadent' houses, contained between the tilted square and the outermost square. In the text, the author described the method for using an astrolabe to find the beginning of each of the houses for a particular nativity. An ephemeris was then to be used to find the position of the planets and the brightest fixed stars, which were to be marked on the figure. Presenting the results of this method in the traditional square arrangement may have been intended to demystify the use of the astrolabe in astrology.</p>