Why this? Why here? Why now?
I’ve always wanted a sundial. Not a cute little round garden dial that’s hard to read and rarely made for its location. I wanted a large, honest-to-Sol vertical dial, accurate for both time and date. After decades of playing with ideas and nearly a year of sporadic effort my New Mexico dial is up and running. The only technology I used was an accurate time signal; otherwise it was constructed with just straight edges, string and a bubble level. The South face of my garage is now fully synchronized with my favorite star of all.
Every accurate sundial is unique, made for its specific location and orientation on Earth. This site gives the story of how I created this one and some information on how to make your own. There are also sections on the history of sundials with links and pictures of other interesting dials. It really isn’t hard to create an accurate sundial. The fun part is in the artistic presentation of that information.
Actually completing this medieval science project has taught me more than all the years I spent studying them. Hopefully you’ll get enough inspiration to start a project of your own. In fact, the goal of this website is not to show off my dial, but to encourage others to make their own. May a thousand sundials bloom!
Connecting the dots, a couple months into the project
First - How to read the time?
Find the starburst shadow
The spot of light in the center of the shadow is the mark. This photo was taken at a solar time of 2:00 pm.
Read the time
There are small lines for each quarter hour, and the color changes at each half hour. Interpolate between the lines, reading from Left to Right. This photo reads 11:15 am.
Convert Solar time to Clock time
This is where the magic happens - more details on the ‘why’ later, this is the ‘how’ part of the story. Briefly, sundials are only the same as clock time four days each year. On all other days they are just a few minutes slow or fast relative to clock time. To correct for this difference (NOT an error, just a difference), you need to use the black Figure-8 along the centerline of the dial. This is the Analemma.
First, estimate the gap between the red center line and the black Analemma for your date. This photo shows about a 4 - 5 minute difference between the two (about 1/3 of the 15 minute section). Because the analemma is left of the red line, the sundial was SLOW to clock time - at 1 o'clock on a clock, the mark was on the black line. It took about 5 more minutes for the mark to reach the red hour line. That difference needs to be added to the initial reading. For this picture, the initial reading would be about 1:01 pm. Adding the difference adjusts it to 1:05 - 1:06 pm.
When the analemma is right of the line it means the sundial is FAST to clock time. You must subtract those minutes from the initial reading.
Next - How to read the date?
It’s all about the shadow length
The length of the shadow changes every day, getting longer from late December until late June. It then shrinks back for the next six months. At its very shortest, on December 21st, the mark moves along the top of the Yellow & White field. On the longest day of the year, usually June 21st, the mark moves along the bottom of the field.
The top photo was taken in early February.
This photo was taken on the Autumnal Equinox, September 22nd.
This photo was taken on the Summer Solstice, June 21st.