Antique brass ring sundial & compass

To read the correct time, move the aperture on the sun position scale until it is precisely aligned with the current date. The aperture moves along a track, with positions marked for the different months and seasons (e.g., June at one end, December at the other). A small point of light shines through the aperture onto the inner edge of the hour ring, where the true local time is read.

The integrated compass serves to precisely align the ring sundial with north, ensuring that the shadow measurement corresponds to the actual direction. To correctly adjust the instrument for a specific location, portable sundials typically have an adjustable latitude setting, as the angle of the sun's position varies depending on location. For accurate orientation, the longitude of the observation point and the current value of the equation of time must also be taken into account to display true local time.

The portable ring sundial differs from a conventional, stationary sundial primarily in its function as an altitude sundial. While stationary sundials usually measure the sun's hour angle directly using a gnomon, the ring sundial utilizes an adjustable nodus (a sliding pinhole). This is set according to the current date, and the sundial is rotated so that sunlight passes through the hole onto the scale inside. Furthermore, it must be manually aligned with the north before each use, which is achieved by suspending it freely and rotating it along its vertical axis.

The engraved latitudes and city names are essential for calibrating the instrument to the specific location, as the sundial's tilt angle must correspond to the geographical latitude. By moving the suspension point to the correct latitude, the Earth's axis is accurately simulated at the observation site, enabling a precise determination of local time. The city names serve as a practical aid for quickly finding the correct latitude for familiar places.