Subj: Shadow Velocity Calculation?
Hello List:
Would this be a true statement?
The average velocity of the shadow (m/hr) on the sundial face as it moves
along a line connecting the time point markers is equal to the total length
of these lines (m) divided by the longest possible daylength (in hours).
In this case that would be 118 meters /14.17 hours = 8.327 m/hr (or
27ft.hr).
John,
The above method will give you the average velocity over the whole path. Near the style (near noon) the velocity will be less, and in the early morning and late afternoon it will be substantially higher. I assume you want to know the speed of movement of the shadow in order to know how much time you have to mark it or to make corrections if you do not mark it on time (a very dubious process). I also assume you have a scale drawing of the dial from which you can take measurements along the projected path of the shadow between hour marks. I that case, for a better approximation, simply take the distance, along the projected path of the sun, between two hour marks in the area you are planning to work, and divide the distance by one hour, or by 3600 seconds to get the speed in m/sec. The latter will result in a very small number, and it might be more convenient top divide 3600 seconds by the distance in centi! meters to get a figure for the distance traveled in one second.
Note, this will still be an approximation to how fast the shadow is moving. It will be close to the speed at the center of the time interval you use. You can do better by measuring the distance between lines 10 minutes on either side of the line at which you wish the speed. and dividing by 20 min or 1200 sec.
Hope this is clear enough and that it provides the information you need.
Bill Walton
Plymouth, MA, USA
42 N 71 W
