Description
Nocturnal

Nocturnals have been known since at least the 10th Century, and may date from even earlier times. They were generally designed for sailors’ use, to find the time at night by the stars. Most of the ones used on board ship would have been made of wood. However, many metal nocturnals with elaborate decoration also survive.

Various different stars can be used to find the time at night, but the most common ones that nocturnal-makers chose were the Pole Star and the Great Bear. The inner disc of the nocturnal was adjusted so that the short pointer was positioned over the correct date. Then the instrument was lifted up until the user could see the Pole Star through the central hole. With the nocturnal still in this position, the long pointer was turned until it lined up with the two ‘pointer’ stars at the end of the Great Bear. Where the edge of the pointer crossed the inner disc it would show the time.

Many nocturnals had teeth carved into the inner disc to mark the hours so that the time could be read in the dark, by counting round the number of teeth between the big pointer and the small one.

Horizontal Dial

The horizontal dial is the most common form of sundial. The portable version proved very popular with the upper classes during the 19th Century. During this period, it could best be described as the wristwatch of its day.

The hour lines are engraved onto a horizontal surface, with a gnomon (pointer) in the centre. Seasonal variations (caused by the earth’s orbit being elliptical, not circular) have to be compensated for, as they can affect the raw reading by up to 18 minutes.
18/10/2002
Created by: Saffron Clackson and Boris Jardine on 18/10/2002