Slot-gnomon Equinoctial

Christmas 2003

For my parents

Garden plot, with sundial pillar

Garden plot, with sundial pillar

Some years ago, my Dad had a problem with his front lawn subsiding. After several attempts to keep it level, he gave up and replaced the sunken area with a wabe and a pillar for a sundial. After several years though, there still wasn't a sundial.

On a blacksmithing course earlier this year, I set about fixing this.

The course was held at Fire and Iron gallery in Leatherhead and was taught by Peter Parkinson. I'd initially been impressed by his book The Artist Blacksmith.

Sundial, as installed

Sundial, installed

The design is a fairly simple equinoctial (often also called an equatorial). This is one of the simplest designs of sundial, certainly the simplest to understand. The gnomon (the shadow-casting bar) is set at an angle to be parallel to the polar axis of the Earth. The hour-ring is parallel to the equator. Hour divisions are thus equally spaced (24 hours in 360°, or 15° each)

One unusual feature of this sundial is that it uses a slot gnomon, rather than a solid bar. Instead of a shadow, this indicates time with a bright line inside a broader shadow.

Close-up of the completed sundial

Close-up of the completed sundial

Construction is entirely of steel. The frame was made on my blacksmithing course, of three main parts with various drawn points and decorative twists. The slot gnomon was made by splitting the bar whilst hot. It's held to the frame with a mortice and tenon joint, the tenon hot-split and bent over to hold them together. The hour-ring was made later, by plasma cutting graduations and numerals into a curved steel strip.



A few useful boks on sundials