Astronomical Images : Universal rectilinear dial of Regiomontanus

Christophorus Clavius

Astronomical Images

<p style='text-align: justify;'>Christopher Clavius (1538-1612) was a Jesuit mathematician and astronomer. Clavius wrote many pedagogical works, and his textbooks were used widely, often as part of the Jesuit educational programme, in Europe and beyond. In 1579, Clavius was asked to reform the calendar so as to halt the drift of the Church's holidays through the year. Clavius' reform was adopted in 1582 as the basis for the Gregorian calendar. This figure comes from the comprehensive account of dialling, written by Clavius. His work covers 654 folio pages, dealing mainly with fixed sundials in various orientations and using different hour systems, but also with several types of portable dials and horary quadrants. This figure shows a universal rectilinear sundial designed by Regiomontanus. Dials of this type consist of a plate which is generally smaller than hand-size. As shown at the top of the main rectangle in this figure, the plate bore sighting holes for aligning the upper edge of the dial with the Sun's rays. This ensured that the inclination of the sighted edge was equal to the Sun's altitude. The instrument also featured an articulated arm, or <i>brachiolum</i>, which could be positioned on the plate according to the latitude. The plumb bob hanging from this arm carried a bead which was moved along the length of the string according to the declination. Once the dial was calibrated for local conditions in this way, it could be directed towards the Sun and the time of day could be estimated according to the position of the hanging bead. Notably, this figure shows the construction lines that would have been sketched out in order to construct the dial, so it may have been intended as a guide to the making of the dial, rather than as a ready-inscribed paper instrument.</p>