These handy travel sundials were particularly popular in the 17th and 18th centuries. Travel sundials because they could be used for all geographic latitudes by moving the suspension. The hour ring, which lies in the plane of the celestial equator when the geographic latitude is set correctly and the instruments are hanging vertically, gives the sundial its special name: Equatorial ring sundial.
The principle of time measurement is based on the change in altitude of the sun during the course of the day. This makes determining the cardinal direction (azimuth) and a compass unnecessary. On the contrary: if the time reading is correct, the meridian ring assumes an exact north-south position, which means that the device can also be used as a solar compass. The prerequisite is knowledge of the date and the geographic latitude of the observation location.
The complete description of the instrument as PDF for download
