Article of the month April

Self-aligning sundial

The three previous instalments showed examples of azimuth sundials.
Here is a combination of such a sundial with an ordinary horizontal sundial.

In this combination, the azimuth sundial is an Oughtred dial. The ordinary sundial is drawn around it.
The time may therefore be determined by means of two systems:
- from the azimuth of the sun, using a vertical gnomon,
- from the hour angle of the sun, using a polar gnomon.
Two green dots show where to place these gnomons.

The advantage of the use of two systems in a combined sundial is that it makes the instrument self-aligning.
The sundial is turned until both dials show the same time. The sundial is then north south aligned.
Thus, even portable instruments need no compass to find the meridian.
For fixed sundials, such a combination is in fact redundant, but beautiful specimens exist.

Note that the self-aligning property does not work well around true noon.

Shown here is a 2006 replica of such a self-aligning combination.
The original by Henry Wynne, London, 1685(?) was at Staunton Harold, Leicestershire, but was moved in 1951 to Ditchingham, Norfolk, which is also where the replica was placed.
The round dial is approximately 75 cm (30 in) in diameter.
This sundial’s furniture is much more elaborate than here described, and one is referred to the references below.

John Davis and Michael Lowne of the UK did the research and designed the replica. Tony Moss, UK, executed the design in bronze.
The detail below states this, reading:
Hoc horologii antqui simulacrum delineavit Iohannes Davis Ipswichianus, fecit Antonius Moss Bedlingtoniensis, numeris investigavit Carolus Lowne Hailshamicus, ....