Transportability of a Sundial

Extract

It does not seem to be generally known that a sundial, which has been correctly constructed and graduated for a particular latitude, can be used in any other latitude if it is set up at the proper angle. This appears from the very simple observation that if a sundial is moved in any way without rotation (of course within a range small compared with the sun’s distance), the shadow of the style will continue to fall upon the same figure of the dial, and if it remains on the same half-meridian of the Earth, where the local time is the same, the dial time will be still the same; when transported to another meridian, without change of latitude, it will give the correct local solar time for that meridian. (In transporting to another meridian, the dial will of course be rotated, so that its inclination and orientation relative to the earth’s surface shall be unaltered.) Thus a horizontal sundial constructed for north latitude ø will be accurate when erected as a prime-vertical sundial, facing north, in south latitude 90° – ø (Fig. 1).