In an earlier post "How horizontal and equatorial sundials work" we can read about the working principles of the two main types of sundials. As I mentioned in that post, I was planning to make one such sundial myself. I wanted to build the sundial from stainless steel because it is very corrosion resistant and has a beautiful appearance. Because stainless steel is quite expensive and all the details are laser cut, everything had to be thought through first. A little bit of experimenting and calculating, and a few months later, I finished building my first stainless steel equatorial sundial. You can see what they look like in the gallery on my webpage www.ajavari.ee. But some time ago, an order came for a horizontal sundial. At this time, the budget was also smaller, and I decided to make the sundial from carbon steel.
Construction and positioning of horizontal sundial
The image below shows the main parts of the constructed horizontal sundial. The dial plate can be of any geometric shape, but since the customer of the sundial is engaged in the production of honey, I decided to make it a hexagonal honeycomb shape. The circumference of the hexagon is 570 mm in diameter, and the height of the gnomon is 300 mm. Gnomon and dial plate are welded together from below. Because of this, the gnomon has a wedge, which goes through the opening of the dial plate. As all the details are made of carbon steel, and the sundial stays outdoors, it is powder-coated.
The horizontal sundial is mounted on a flat surface with the tip of the gnome facing true north in the northern hemisphere, and in the southern hemisphere, it must point to the true south. It should be noted that the direction of the Earth's geographical and magnetic north and south may not coincide. The difference depends on your longitude. In Estonia, where I live, the difference is about +10 degrees, which means that the magnetic north direction is 10 degrees east of the true north.
At what angle should the gnomon be?
The angle formed by the gnomon with the dial plate must be equal to the sundial's geographical latitude. Your local latitude is the distance from the Earth's equator, measured in degrees. This particular sundial will be installed in a small Estonian village near Tallinn, with a geographical latitude of 59,3 degrees north. Using Google Maps, you can find geographical coordinates of any place on earth by right-clicking the place or area on the map.
When the angle is set correctly, and the tip of the gnomon is pointing true north (in the northern hemisphere), then the longer side of the gnomon becomes parallel to the Earth's axis of rotation.
How high should the gnomon be?
When making a sundial, it's good to know how high the gnomon should be, because this determines how long the shadow will be. When the gnomon height is small, it is even possible to indicate the date. The image below shows a horizontal sundial template created with Shadows software. The geographical latitude of the sundial is 59,3 degrees north. Selecting the desired dial plate dimensions, we find that the height of the gnomon will be 30 mm. In this case, we can draw on the dial plate declination lines.
Around June 21st when the Sun rises higher in the sky, the shadow is shorter, and its tip moves along the summer solstice line. Around December 21st, when the Sun is the lowest in the sky, a shadow will be longer, and its tip moves along the winter solstice line. Twice a year, in the autumn and spring, when the day and night on the Eart are the same lengths, the tip of the shadow moves along the equinox line.
Since the customer wanted the sundial to be with the larger gnomon, I didn't mark the declination lines on the dial plate. Although it is possible to cut a small notch to the longer side of the gnomon and use declination lines anyway, they would have needed engraving.
I planned the height of the gnome to be 300 mm, but is that enough? To be sure that the shadow reaches the edge of the dial plate with the desired dimensions at any time of the year, we need to know how long the shadow is when the sun is highest in the sky.
At noon on June 21, the Sun is highest in the sky, at an altitude of h = 90 - L + 23.5 = 54.2 degrees from the horizon. L in this formula is my sundial's geographical latitude, which is at 59,3 degrees. At this time, the length of the shadow is 216 mm, which means that it extends far enough over the edge of the dial plate. At other times the sun is already lower in the sky, and thus the shadow will be even longer.
How to mark the hour lines?
On the equatorial sundials, regardless of where it's located, the hour lines are always spaced 15 degrees apart. Unlike the hour lines on the equatorial sundial, the hour lines on the horizontal sundial are not uniformly spaced. Their spacing depends on the sundial's geographical latitude (L). Using a shadow angle calculator, or a formula, we can calculate how the hour lines of a horizontal sundial are spaced.
L in the upper formula is the sundial's geographical latitude, and t is the hour number before or afternoon. For example, 10 a.m. is 2 hours from noon, which means t = 2, and the angle between the noon line and 10 a.m. line is 26,4 degrees. If you take a closer look, you will see that the hour lines on the dial plate are not located radially from the center of the hexagon but from the edge of the gnomon. Only the noon line can be drawn from the center of the dial plate.
I used 5 mm thick steel for the construction of the sundial, and therefore the hour lines are radially outward from the center 1 or center 2. Center 1 and center 2 are 5 mm apart.
I also cut the sundial's location coordinates on the dial plate and the name of that location. Knowing the sundial's longitude coordinate, we can convert local solar time to standard time or vice versa. In this way, it is even possible to calibrate the sundial reading. Finally, when everything seemed in place, I sent my CAD files for laser cutting.
The picture below shows the parts of a horizontal sundial, cut with the sheet metal laser cutting machine. As soon as the assembly is welded and powder-coated, I will add the picture of the finished sundial.
And in this picture, the parts are welded together and powder coated to imitate the color of honey.
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