Good points. Thanks Anselmo. Shadow astronomy works better that horizon astronomy. An east west line is easily established using the "Indian Circle" method. The meridian can be found, to limited accuracy, as the direction of the shortest shadow.
Don't give up on the equinox straight shadow line. Yes, the declination changes rapidly, 0.4 degrees per day, so any line will be slightly curved. But if the line is curved enough to detect the curvature, you should be able to detect the change in curvature from concave to convex, and set the day of the equinox. The best documented early determination of the equinox is outlined in Ptolemy's Almagest, when Hipparchus perceived the equinoctial day from observations made on the equatorial bronze ring in the Square Hall in Alexandria". Almagest is truly a great book! Enjoy the Solstice, Tuesday, 21 Dec at 12:42 UT Roger Bailey Walking Shadow Designs N 48.6 W 123.4 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of anselmo Sent: December 20, 2004 3:20 AM To: [email protected] Cc: Roger Bailey Subject: RE: equinox in the stone age..... 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 - -
