Equinox

[fer j. de vries]

Hello All,   Some members of this list have drawn an east-west line at the september equinox. But what accuracy this line will have?   Assuming a perfect horizontal plane, a perfect (pin) gnomon and a perfect sunny day the straight line still will not be perfect east-west.   Around the september equinox the change in the sun's declination is about 24 arcminutes or 0.4 degrees a day. For a period of 8 hours this is about 0.133 degrees.   Let's assume the equinox is exact at noon. Then at 8 in the morning the sun's declination is about 0.067 degrees and -0.067 degrees at 4 in the afternoon. At a latitude of 45 degrees the x,y coordinates of the shadowpoint of a gnomon of unit length 1 at those 2 times are : x1 = -0.7071 y1 = 0.4992 x2 = 0.7071  y2 = 0.5008 A line between these 2 points has an angle of 0.067 degrees to the east-west line. This value isn't constant for any latitude. lat 0   : value 0.094 degrees lat 60 : value 0.047 degrees   Is this accurate enough to you?   Fer J. de Vries [EMAIL PROTECTED] http://www.iae.nl/users/ferdv/ Eindhoven, Netherlands lat.  51:30 N      long.  5:30 E