I will list a few of my practical applications for a sundial with declination lines, in architectural and urban design.
1. Take an architectural model of a group of buildings. Place a small sundial next to the model with the north-point of the sundial parallel to the north-point of the model. (I use a sundial in a matchbox tray). Shine an electric lamp on the model. The shadow on the sundial will indicate the date and the time of day at which the sun will shine from that direction on the group of buildings. Note the times when each building will overshadow its neighbours, altering the layout to optimise solar penetration or protection. Ideally this is a job for three people. One to hold the lamp as far away from the model as possible to ensure near-parallel light. One to watch the sundial and tell the lamp-person to move up, down, left or right. One to photograph the sunlit model, taking care to note the time and date on the sundial in each photograph. 2. Alternatively, if the sun is shining, mount the model and sundial on a drawing board. Tilt and rotate the board so that successive shadows on the sundial indicate the times and dates you have chosen to study. 3. Instead of using an electric lamp indoors, view the architectural model by eye. Glance past the nodus to read on the sundial scale the time and date at which the sun would shine from the direction of your eye. Parts of the model you can see will then be sunlit. Parts concealed from your view will be overshadowed: a shadow is simply an area which the sun cannot see - the sun never sees a shadow. 4. Another application is to detect the periods at which a given piece of ground will be overshadowed, perhaps by trees or by surrounding buildings. Place a small mirror flat on the ground. Mark a cross on the centre of the mirror. Move your head to align the mirrored reflection of a possible obstruction to sunlight with the cross on the mirror. Without moving your head, place your small sundial over the cross, remembering to rotate the sundial so the its north-point faces south. Read past the nodus the time and date at which the direction of the sun will coincide with the direction of the potential obstruction. 5. If you turn a vertical sundial upside down and reverse it from left to right, the declination lines will form a perspective of the sun's apparent orbit across the sky. Indeed if you place your eye at the nodus of the rotated sundial, each point on the sun's path will appear in the same direction, in relation to your eye, as the real sun at the corresponding time of day. This feature is especially valuable when you are asked to investigate complaints of solar glare or distraction in a workplace. Superimpose your sunpath perspective on a photograph or perspective drawing of the environment and read off the time when the sun may be in view through a window. Obviously the orientation of the vertical sundial and the drawing or photograph must be the same, and the perspective distance of the drawing, or the effective focal length of the camera, must match the distance of the nodus from the plane of the sundial. So either the sundial or the photograph may need enlarging. 6. Finally if you need to design external shading to mitigate the effect of solar overheating, you can superimpose on the sunpath perspective a perspective of a horizontal canopy (a horizontal line seen from the window sill) or a vertical or sloping fin to show the percentage of window shielded from direct sunlight at different times of day or at different seasons. 7. ... and I'm told that sundials have also been used for telling the time ... ... John Lynes
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