Dear Frank, As ever, you prompt several interesting trains of thought...
> In fact any run-of-the-mill astro-navigator > with a merely passable sextant could at > least manage an accuracy of a minute of > arc... I have never used a sextant but I have heard this arc-minute figure so many times I am willing to believe it! > ...or four seconds of time. Careful here! Most of the year the sun goes round in a small circle so an arc-minute change in position takes longer than four seconds. Also, I am not sure what angle you are measuring? You can't mean the solar altitude. That doesn't change anything like an arc-minute in 4s where I live! > For a really accurate dial could the > east or west limb of the sun be > projected and utilised? Well you can project the whole solar disc using a continental camera obscura sundial and thereby see both east and west limbs of the sun. I have observed transit at the Basilica di S. Maria degli Angeli in Rome many times, sometimes with enthusiasic Italian diallists adding to the fun! Each diallist shouts "Ora" [Now] at what he perceives to be the critical moment and the chorus lasts two or three seconds, so that's an observational error for a start. The very first time I did this, I noted the time of the middle "Ora" on my radio-controlled watch and did the reductions. This includes allowing for the difference between UTC and UT1. It was pretty good, the error was about 3.5 seconds. Alas, I have done this many times since and noted errors of up to 10 seconds. There are too many things to go wrong... Mario Catamo says that you get a build-up of muck on one side of the hole which shifts the apparent centre. There would need to be a lot of muck to account for 10 seconds though! I believe the line isn't quite north-south but since I always go at much the same time of year the error should be consistent! That said, I think 10 seconds is pretty good. Frank Frank King Cambridge, U.K. --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
