Dear all, I can't understand as well how people did in the stone age to mark equinoxes. Even solstice days are quite difficult because the sun 'stops' in a couple of weeks or so, so it is difficult to tell which of these days is the real solstice.
As regards to equinoxes... > harder to mark using "horizon astronomy". Okay, switch to "shadow > astronomy". The equinox is special because the path of the shadow of > a point traces a straight line on any plane on the equinox, and only > on the equinox. This straight line equinox shadow path is an east > west line on a horizontal plane. I speculate that this was observed > and marked during the stone age. Nope again! During the equinoxes solar declination has its greatest rate of change so the shadow of a point does *not* describe a true straight line... Once again, the approximation we commonly use in sundialing is very crude and would only give the day of the equinoxe in a week or so. I guess that they simply used perpendicularity: when the sun rises at the East, that is, the direction perpendicular to meridian line, it is the equinoxe, but then we've got more or less the same problem with declination change. Any clue about this? Anselmo Perez Serrada -
