Dials

Dials are instruments used for recording time by means of shadows derived from the sun or moon. They were of much importance before the invention of clocks and watches, but are now of little more than historic interest. The principle upon which dials are made is readily explicable, though in the case of dials constructed for various positions of pointer and plate, certain difficulties of detail manifest themselves. The earth rotates uniformly about its axis, thereby causing an appearance of uniform rotation of external bodies, such as the sun or other stars, about the earth. It is true that the earth is travelling in an elliptic orbit round the sun, with a speed not strictly uniform; but its motion once round its own axis is so much more rapidly effected than its motion once round the sun that we may disregard this latter in discussing the daily position of the sun as seen from the earth. Assuming, then, that the sun appears to swing round the axis of the earth at the constant rate of one revolution per day, it follows that any rod lying in this axis or parallel to it will throw a shadow that also swings uniformly round once per day. We cannot adjust a rod in the actual axis of the earth where it may cast a shadow, but we can readily fix one parallel to the axis at any convenient spot on the earth's surface. We may also adjust a plate near the rod which shall exhibit the shadow and its motions. The plate is marked with lines showing the position of the shadow at the different hours of the day, the way in which the lines are drawn depending on the position of the plate. The marking is simplest when the rod or gnomon, pointing to the pole-star, is perpendicular to the plate or dial-plane; then the hour lines are at equal angular distances of 15° all round the dial. For the marking when the plate is vertical or horizontal, a little spherical geometry is required.