NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Level of observation accuracy
From: Frank Reed CT
Date: 2004 Jul 24, 16:29 EDT
From: Frank Reed CT
Date: 2004 Jul 24, 16:29 EDT
Dan wrote:
"Anyway, then I could measure with an accurate tape measure the height of the line, measure the length of the shadow, and take some arctangents and get an angle. If the line and length of shadow are long enough, serious accuracy could be achieved, no? Anyone done this or see any obvious deficiencies in this inexpensive method?"
The penumbra of the shadow will be a problem. Since the shadow edge is "fuzzy", it would be difficult to be more accurate than a quarter of a degree in the angle estimate. This is the same reason that sundials can't give local apparent time to better than about one minute accuracy.
Speaking of the penumbra, here's a fun trick (which I have never seen in print): Measure the width of the penumbra of a shadow cast by the Sun or the Moon. The distance to the object casting that shadow is approximately 108 times that shadow width. This can be very handy for estimating the distance to buildings in a city (heights of buildings when the Sun is at high altitude). If you walk along the edge of a shadow from a tall building and find that it's six feet from complete shadow to complete sunlight, then the edge of the building that's casting the shadow is about 650 feet away. Note that the shadow has to be measured on a surface that is perpendicular to the light in at least one direction but that's not hard to find in practice.
-FER
Frank R
[ ] Mystic, Connecticut
[ ] Chicago, Illinois
[X] in transit
"Anyway, then I could measure with an accurate tape measure the height of the line, measure the length of the shadow, and take some arctangents and get an angle. If the line and length of shadow are long enough, serious accuracy could be achieved, no? Anyone done this or see any obvious deficiencies in this inexpensive method?"
The penumbra of the shadow will be a problem. Since the shadow edge is "fuzzy", it would be difficult to be more accurate than a quarter of a degree in the angle estimate. This is the same reason that sundials can't give local apparent time to better than about one minute accuracy.
Speaking of the penumbra, here's a fun trick (which I have never seen in print): Measure the width of the penumbra of a shadow cast by the Sun or the Moon. The distance to the object casting that shadow is approximately 108 times that shadow width. This can be very handy for estimating the distance to buildings in a city (heights of buildings when the Sun is at high altitude). If you walk along the edge of a shadow from a tall building and find that it's six feet from complete shadow to complete sunlight, then the edge of the building that's casting the shadow is about 650 feet away. Note that the shadow has to be measured on a surface that is perpendicular to the light in at least one direction but that's not hard to find in practice.
-FER
Frank R
[ ] Mystic, Connecticut
[ ] Chicago, Illinois
[X] in transit
