3.5" Brass Dollond Portable Sundial

Our wooden-boxed brass sundial is a completely self-contained portable instrument, inspired by a design from the 18th century instrument maker Peter Dollond, founder of a great optical empire. The original is in the London Science Museum. Instruments like this were once used by explorers and navigators to tell the time anywhere in the world. For this to be possible, the gnomon, which casts the shadow, can be set for latitude on a sliding scale and the whole instrument oriented for direction by means of a magnetic compass in the base. Height adjustable legs ensure that the instrument is used upright. Box measures 4" square, 2.5" tall.

Instructions

1. Raise the gnomon (the angled bit that will cast the shadow).
2. Raise the latitude arc (0-60) to vertical
3. Raise the circular dial plate to local latitude (e.g. 51.5)
4. Release the compass needle by means of the lever on the side.
5. Visually level the base by means of the screw legs (this is not critical).
6. Rotate the sundial so that the compass needle N points to 0 on the scale underneath.
7. Read the time from the shadow of the gnomon on the dial plate, adjust if necessary for daylight saving.

This procedure has set the gnomon's edge so the the Sun rotates around it (in our frame, not reality!) - picture a stick stuck in the ground and then pushed over a bit to be perpendicular to the plane of the Sun's orbit (again, ignoring reality). This is the principle of the 'equinoctial' sundial, of which I believe Dollond is a variant (I'm not an expert) in having the gnomon angled a bit on the dial plate, to cast a better shadow.

A sundial is a device that measures time by the position of the Sun. In common designs such as the horizontal sundial, the sun casts a shadow from its style (a thin rod or a sharp, straight edge) onto a flat surface marked with lines indicating the hours of the day. As the sun moves across the sky, the shadow-edge progressively aligns with different hour-lines on the plate. Such designs rely on the style being aligned with the axis of the Earth's rotation. Hence, if such a sundial is to tell the correct time, the style must point towards true North (not the north or south magnetic pole) and the style's angle with horizontal must equal the sundial's geographical latitude.