Re: Equatorial monument in Brazil

[DrArthurCarlson]

That's a horse of a different color.  Then you have a geometrical image of the hole rather than a pinhole image of the sun.  The lack of "perfection" remains, but is no longer so apparent.  Now you might want to ask for how many days around the equinox the image stays "pretty much" on the line.  The deviation will be largest near dawn and dusk.  Let's use 7 AM and 5 PM as reference times and 65' as the height, so the horizontal projection distance is 65'/tan(15 deg) = 240'.  An offset of a quarter diameter -- 2' -- should be quite noticeable.  That corresponds to an angle of 2/240 = 8.3 mrad = 29 arc-min, nearly equal to the change of declination in one day.  It is also nearly equal to the angular diameter of the sun, so the image will be rather washed out at that distance.

All in all, you should be able to determine the exact day of the equinox.  That's close enough to perfection to leave me impressed.  I'd love to see it in action.

--Art