The altitude sundial in sunshine on 21st July 2026. The dial reads 3:50 pm, which is only 4 minutes difference from clock time, with corrections made for BST, Longitude, and EOT. The reading point is the tip of the shadow, which is the shadow of the gnomon nodus.
The altitude sundial is an intriguing instrument. It relies on measuring the height of the sun to tell the time. It also requires the user to adjust for the time of year before making a reading. There are many altitude sundials and there is one particular one with a circular dial, from around the year 1600, which we find very interesting. We have now designed and built one like it at full-scale for the garden.
We started with a working prototype before going full-scale. This was a table-top version constructed in plastic. It proved the principle of the dial calculations, but it was not stable enough to show accurate readings. The plastic tended to warp and deflect so the gnomon shadow was unreliable. However, the plastic mock-up was excellent for illustrating how turning the sundial on the table changed the length and direction of the shadow. In this specific type of altitude dial, rotating about a vertical axis and finding a correct shadow point is the actual method for setting the date before reading the time.
Our full-scale garden sundial is hand-forged in steel, galvanized and painted black. To provide rotation, a swivel bearing fits inside the box at the base of the column. The user turns the sundial by hand to take a reading. There are two dials back to back mounted inside the circular frame, each spanning half a year. These are in patinated bronze. Detailed notes on the dial calculations and the design are to follow.