The Nocturnal

The nocturnal was a common instrument used by seamen of the sixteenth and seventeenth centuries to determine the time by observing the apparent rotation of a or b Ursae majoris or of b Ursae minoris around the pole star, Polaris. These instruments generally have four main parts: 1) the main plate with handle having graduated circles for months and hours, 2) the inner plate with pointers for a or b Ursae majoris and b Ursae minoris, having graduated circles for hours and days of the month (this disk is sometimes saw-toothed to aid in determining its relative position in the dark), 3) an index arm to line up with the indicator stars, and 4) a pierced bolt which also serves as a site to line up the Pole star.

Råmon Llull described a simple nocturnal around the end of the thirteenth century for telling time by the position of b Ursae minoris. Llull described a single disk, perforated in the center to site the Pole star, and engraved with two concentric graduated circles for the months and hours based on the positions of b Ursae minoris. Variations on this instrument in the fifteenth century and the development of the nocturnal to its height in the seventeenth century are discussed by Maddison1.

This project is based on Llull's description: a single disk perforated in the center with concentric graduated circles. One circle is graduated into 12 major divisions and marked with the months of the year, counter-clockwise from January to December. This circle may also be subdivided into days, though divisions to two or even five day intervals are more than adequate for the expected accuracy of this device. The second circle is divided into 24 major divisions corresponding to hours. It is not necessary or desirable to number them.

To use this disk with b Ursae minoris as the indicator star it is held with the highest midnight position (May 5) of this star at the top, and Polaris is sighted through the center hole. The disk is then moved out from the eye, carefully maintaining its vertical orientation until b Ursae minoris is visible near the edge. The star's position is noted (perhaps by griping the disk edge at this position), and then the number of hours from its position to its predicted midnight position (the current date) is counted and added to or subtracted from midnight to give the time.

Making a Simple Nocturnal

The blank for the nocturnal consists of a computer CD-ROM disk spray-painted on both sides with black lacquer. All graduations etc are made on the read (non-label) side of the disk, since the paint is easily removed to reveal the highly reflective CD surface. Graduating the disk into 24 hours requires a division every 15°, with major divisions at 30° intervals for the 12 months. For this workshop a series of templates were constructed in advance, graduated at 15° intervals with a protractor, and marked with the months at 30° intervals as seen in the figure.