[time-nuts] Crude Survey Technique

[Jim Lux]

On 11/21/13 3:28 PM, Chris Albertson wrote:
> On Thu, Nov 21, 2013 at 11:11 AM, Don Latham <djl at montana.com> wrote:
>> Lord no, John. No red wagon is needed. Use a pole and the equation of
>> time, and a good watch or clock. At local noon, a shadow will be a n-s
>> line.
>
> How accurate do you need to be?   The above requires a very tall pole
> to case a 300 foot long shadow.  Then you have to be quick to measure
> because the Earth turns at  .25 degrees per minute.

The original request was for "accurate to a degree or two", so he has 
4-8 minutes to make the measurement.

The sun is 1/2 degree wide, which is actually the practical challenge in 
measuring using shadows, because the shadow is not sharp edged.

I don't know why you need a 300 foot long shadow.

Let's assume I have a 1 meter tall rod that is 1cm in diameter.
The sun isn't that high in the sky at noon these days (in the Northern 
Hemisphere).. let's say it's about 45 degrees, so the 1 meter long stick 
casts a shadow that is 1 meter long.  If the sun were a point source, 
the shadow would be 1 cm wide, or 1 part in 1/100 which is about half a 
degree.  That's comparable to the width of the sun, so you might want to 
choose a "bigger stick".  maybe a 2" piece of pipe?  estimate the center 
of the shadow, which could easily be done within 1/2".

You now have your north direction.

Solar noon is trivial to find out.  The USNO Astronomical Applications 
page will give you a solar ephemeris for a specified lat/lon.  or 
knowing your longitude and applying the equation of time, you can 
calculate when solar noon is.

Or, whip out your current copy of the Nautical Almanac




>
> If you need a very tall pole that is 100% vertical then hang a
> weighted rope from a tall support.  Then go to the other end and watch
> the seconds tick down.
>
> A GOOD magnetic compass can do this job too.  Easier then finding s
> 1,000 food tall pole.   The better compasses have some kind of optical
> aid for sighting a line.
>


Getting 1 degree accuracy with a magnetic compass is challenging. 
Finding out the *current* magnetic variation is only part of the 
challenge, because it varies (about 1 degree in 10 years in Southern 
California, last I checked).