A nocturnal is a star clock. They first appeared in the 13th century, and remained in use into the 19th century. Nocturnals came in three parts: a pointer, and separate date and hour discs rotating round a common centre. Using one involved sighting Polaris through a hole in the rivet or bolt holding the components together. A competent navigator would expect to find the time to within 15 minutes.
A simple nocturnal can be made from two discs. One is a clock face with the months round its edge and the other is a very simple star chart which fits inside the 24 hour clock face with midnight at the top and noon at the bottom (see chart and template shown in Figure 10.6 and Figure 10.7).
To use the nocturnal turn the clock face until the month is vertical. Next, hold the nocturnal up to the sky and turn the star chart until the stars on it match up with those in the sky. Finally, read the time.
Was this article helpful?