Article of the month July

Universal ring dial

Here are a drawing and photograph of a modern replica of a universal ring dial.

The sundial consists of a ring in the equatorial plane.
Engraved in the middle of the inside of the ring is a line, forming the hour scale, although the numerals are on top of the ring.
The distance between any two hour points is 15 degrees.

Square to the ring, and hence parallel to the polar axis, is a rotating date scale with an adjustable indicator.
The indicator has a small hole through which the sun may shine.

The equator ring and the date scale are fitted inside a vertical ring, suspended on a string..
The point of suspension is adjustable for the desired latitude.
There are two latitude scales: one for northern, the other for southern latitudes.

In use, the sundial is set to the correct latitude and date.
One should also know whether it is before or after noon.
The sundial is suspended from its string, and turned such that the sunbeam from the indicator casts a spot of light on the hour scale inside the equator ring. This spot indicates the time.

Notice that the vertical ring is now automatically aimed north-south; this sundial is self-aligning.

In former days, sundials such as these were in use as pocket dials. The equator ring was collapsible, so that the dial, when not in use, was quite flat.
Larger and more precise table models were also in use.