Arthur Carlson wrote: >"fer j. de vries" <[EMAIL PROTECTED]> writes:
>> Some members of this list have drawn an east-west line at the september = >> equinox. >> But what accuracy this line will have? > > > >Assuming a perfect horizontal plane, ... > >... > >A line between these 2 points has an angle of 0.067 degrees to the = > >east-west line. >It is interesting to note that this source of error isn't limited to >the equinox. The usual mark-two-shadow-crossings-on-a-circle method >is also affected, except near the solstices. Art, It appears to me that although there is not exception to the change in declination source of error, there is a period of several days near the equinoxes, when the small curvature of the hyperbola makes for higher precision in the practical application of the 'mark-two- shadow-crossings-on-a-circle' method. Assuming that an attempt is made to cut the hyperbola at equal distances, (i.e., times) west and east of a gnomon, the more nearly the process approximates cutting a straight line, the smaller the effect of asymmetry (not truly equal radii from the actual nodal shadow-caster.) There is a nice optimization to be done here, in balancing a fairly short time-interval (for smaller rate-of-change of south-north component of shadow motion) versus greater separation in the west-east position of the points that determine the desired direction line. Also, it is of course, possible to adjust the results for the predictable change in declination during the observation. All this is a bit moot, relative to other dialing uncertainties, but interesting to explore. Bill
