I wanted to learn how sundials work and figured the best way to do that is to try to make one myself. After extensive research on https://en.wikipedia.org/wiki/Sundial, among other sites, I felt I knew most of what I needed. However, I kept making adjustments as I discovered new factors I didn't account for.
The first thing I learned was that sundials are specific to a location because the sun's relation to time is different everywhere. The sundial I made was specific to MIT and would only work around there, so I put a small icon of the great dome on it to signify this.
I used some data online to layout the hour angles based on my location and thought I was pretty much set. Then I learned that you sometimes need to account for the width of the gnomon (the part of the sundial that casts the shadow). Because as the sun passes solar noon, the shadow goes from being cast from the left edge of the gnomon to the right edge and that will cause a jump in time if not accounted for. For a thin gnomon on a large base this effect is small and sometimes neglected, but I was making it out of half inch plywood on a one foot diameter base so I thought I should account for it.
The next interesting thing I learned was that sundials are designed for the average solar path, but the solar path changes throughout the year. The result is that a sundial can be off by up to around 20 minutes depending on your location. That was no good for me so I thought I should account for it. The difference between mean solar time and apparent solar time (measured by the sundail) is known as the equation of time. Data for this relation, which is also location specific, can be easily found online. I took the data, reversed it, and graphed it directly on the sundial to give an easy way to lookup what offset you have to apply to the sundial to arrive at actual time based on the month of the year. For example, if it's around the end of October, you look at the end of the 10th bar on the X axis and the graph reads +14, so you add 14 minutes to the time you read on the sundial.
Once the pattern was ready, I brought it over to a laser cutter and engraved it into some wood. Engraving went well but the laser wasn't strong enough to cut all the way through the wood so I brought it over to a waterjet to cut out the circular shape as well as the rectangular notches in the center that I used to insert the gnomon. The gnomon was also cut on the waterjet.
I assembled it then took it out in the morning to try it out. Looks like it was a little passed 8 at first glance but given the offset of -10 minutes in the middle of July, I would say it was 7:55.
