Richard Mallett <[EMAIL PROTECTED]> writes: > >> As for determining the "length of the tropical year ... with a > gnomon between successive solar solstices", I don't believe this is a good > method. One can determine the exact date/time of an equinox much more > accurately than that of a solstice (although the solstice is conceptually a > bit easier to deal with). << > > Can you elucidate please ? I would have thought that the > solstices, representing the extremes of solar altitude (measured when the > Sun crosses the meridian) would be easier to determine.
Suppose you can measure the declination give or take one tenth of a solar diameter, i.e., to +/- 3'. Around the equinox, the declination changes by about 1' per hour, so your measurement would allow you to pin down the time of the equinox to +/- 3 hrs. At the solstice, the declination varies quadratically from its extreme value by about 0.22'/dy^2. In the worst case, you measure a value 3' below the maximum, so you might actually be right on the solstice, but you could also be at a date, either before or after the solstice, where the declination is 6' smaller than the extreme value. So the uncertainty in your measurement of the soltice can be as large as +/- sqrt( (6') / (0.22'/dy^2) ) = +/- 5 days. --Art Carlson
