Astronomical Images : Astrolabe rete and plate for latitude 42 degrees

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. The traditional astrolabe is based on a planispheric projection of the heavens on to the equatorial plane, the point of projection being the south celestial pole. A projection of coordinate lines for altitude and azimuth was fixed ' since the Earth was understood to be stationary ' while that for the celestial sphere could move in imitation of the diurnal rotation. This meant that a separate coordinate projection was required for each latitude. The most notable development in the sixteenth century was the dissemination of universal astrolabes, whose projection could be used at any latitude. The figure at the top shows a projection for 42 degrees north, and the lower figure shows the rete of the astrolabe.</p>