2025-05-09

Last modified: 2025-05-10 09:00:23

Sundial

Some inspiration:

I like the idea of showing landmark distances and directions.

I don't know what the month names are supposed to convey.

ChatGPT has told me that you can add "date lines". So 3 rings around the dial, and the tip of the gnomon touches one ring on the summer solstice, one ring on the winter solstice, and the other on the equinoxes.

Are the rings concentric with the dial?

Intuitively they would be concentric if the dial were angled perpendicular to the gnomon, and not concentric otherwise.

Apparently Earth's rotation axis is tilted 23.44 degrees from the orbital axis. So I think the sun is 23.44 degrees higher in summer solstice and 23.44 degrees lower in winter solstice, compared to a centre at the equinoxes.

And actually I think that would stay broadly constant throughout the day. I think I would want to write code to plot SVGs to work this stuff out.

You can tell where the lines should go by drawing a line through the centre of the sun and the tip of the gnomon and seeing where it hits the dial.

So the sundial wants to have:

• solstice/equinox date lines
• all possible sunlit hours marked
• arrows and distances to landmarks
• cardinal directions marked
• dial calibrated to solar time in Greenwich (i.e. rotate by ~2.5 deg.?)? or to local solar time and write on the dial that there is a 10 minute offset?
• show equation of time on the "unused" portion of the dial