An Equatorial Heliochronometer
Posted: Sat Feb 23, 2013 12:03 pm
Aka: "Sundial". Growing up on the beach in Atlantic City, I used to make simple sundials by sticking popsicle sticks in the sand at the right angle. I've meant ever since to make a proper sundial, and the lower lawn at Coconuts provided a very fitting location. I did most of the brass machining, rolling, and CNC time strip engraving back home, but dug, hand-mixed, and cast the footing, base, column, and crown while at Coconuts, using light-weight plastic molds that I found online. My first footing had to be moved over by ten yards, as I realized the next day that the Flamboyant would shade it on winter mornings. Time is read, with one minute graduations, by the shadow cast by a brass rod (the gnomom), against the engraved time strip, and once the polar alignment is tweaked, it should be accurate to about a minute.
As it happens, the sun is not the most reliable of clocks; due to the tilt and the ellipticity of the Earth's orbit, even the best sundial will run fast or slow by up to 16 minutes during the course of a year. The corrections to address this constitute the "Equation of Time", which some of you may recall as that enigmatic lazy figure-eight off in the Pacific on globes of the Earth. The Equation of Time was first captured visually in a brilliant multiple exposure photograph (one year in the making) by Dennis di Cicco, back in 1978-79 (attached below). To implement the correction for the Equation of Time, the brass time strip slides left to right on teflon guides within a slot that I machined into the brass equatorial arc. A knob and a brass wire are attached to the right-hand end of the time strip, and guests simply align the wire with the current month on a small EOT graphic to calibrate the sundial. A laminated paper EOT is there for now, but this quickly bleaches under the tropical sun, so I'm CNC engraving a brass version to be installed during our next visit. I'm not sure if there are any math geeks on this forum (cocosmom?), but FWIW, I've attached a copy of the spreadsheet that I wrote to calculate the Equation of Time.
We're really pleased with how the sundial came out; this objet d'art et science adds an elegant touch to our lawn. A small brass plaque on the landing of the stairs to the lawn reads "Le Jardin du Temps".
All the best,
Kevin
As it happens, the sun is not the most reliable of clocks; due to the tilt and the ellipticity of the Earth's orbit, even the best sundial will run fast or slow by up to 16 minutes during the course of a year. The corrections to address this constitute the "Equation of Time", which some of you may recall as that enigmatic lazy figure-eight off in the Pacific on globes of the Earth. The Equation of Time was first captured visually in a brilliant multiple exposure photograph (one year in the making) by Dennis di Cicco, back in 1978-79 (attached below). To implement the correction for the Equation of Time, the brass time strip slides left to right on teflon guides within a slot that I machined into the brass equatorial arc. A knob and a brass wire are attached to the right-hand end of the time strip, and guests simply align the wire with the current month on a small EOT graphic to calibrate the sundial. A laminated paper EOT is there for now, but this quickly bleaches under the tropical sun, so I'm CNC engraving a brass version to be installed during our next visit. I'm not sure if there are any math geeks on this forum (cocosmom?), but FWIW, I've attached a copy of the spreadsheet that I wrote to calculate the Equation of Time.
We're really pleased with how the sundial came out; this objet d'art et science adds an elegant touch to our lawn. A small brass plaque on the landing of the stairs to the lawn reads "Le Jardin du Temps".
All the best,
Kevin