On 10 October the autumn equinox has passed and the cursor reading point has moved out from the inner circle of the dial towards winter solstice at the circumference. The sighting arm carries a short vertical gnomon which casts its shadow in line with the sun.
The azimuth sundial is an intriguing instrument. It finds the time from the direction of the sun. This idea sounds simple, the sun rises in the East, at Noon it is due South, and it sets again in the West, but it is complicated by the time of year. This note is about a new design for a precision azimuth sundial to go in a garden.
Azimuth sundials take many forms. Often the position of the gnomon is adjustable for the time of year, and some are pocket dials. The human sundial laid out on the ground is another type, often found in public parks. Here, the person casting a shadow steps further forward to find the time in the summer and further back in winter.
The inspiration for the new design comes from the 17th century. A magnetic azimuth dial by Henry Sutton of London in the Oxford History of Science Museum is particularly interesting. It has a calendar time chart, and the museum has a separate azimuth dial card which helps to clarify the mathematics. The card or chart is a plot of date, sun direction and time for a whole year. Knowing the date, and lining up with the sun, it will show the time. Our new design has features for precision, including a date scale with divisions of two days, a sighting arm and gnomon for accurate sun direction or azimuth, and time divisions of ten minutes.