Hi John, Some reality checks need to be applied to the equinox rising and setting directions. You mentioned the fact that the declination rapidly changing. This will affect the exact direction. Bigger effects are the semi diameter of the sun, refraction, dip and horizon pollution. The equation, when Alt = 0, Cos Z = Sin Dec / Cos Lat, solves for the centre of the sun while sunrise is generally taken as the first crack of dawn. Refraction varies with atmospheric conditions but is generally taken as ~34". Semi diameter is typically 16".
About 150 BC, Hipparchus determined the time of the equinox and the length of the solar year by noting when the shadow exactly covered the inner surface of the equatorial ring of an armillary sphere. This was accurate enough for him to determine the precession of the nodes. Horizon astronomy is different from the shadow astronomy that we are used to. The ancient "Medicine Wheels" on the western plains often show solar orientations. For example the Majorville site near Calgary has east west alignments for the "equalday" rather than the equinox. The equalday is that day when the crack of sunrise and sunset are observed to be truly east west and the time from sunrise to sunset is exactly 12 hours. These orientations were determined by Dr. Gordon Freeman of U of A and published about 10 years ago. See http://www.ucalgary.ca/UofC/faculties/SS/ARKY/Dept_Files/papers.html#4 for the abstract of proposed paper by Gordon Freeman on a related topic. Roger Bailey Walking Shadow Designs N 51 W 115 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Behalf Of John Carmichael Sent: September 22, 2001 7:47 AM To: Dave Bell Cc: [email protected] Subject: Re: equinox solar declination Dave: Thanks so much Dave for looking that up for me. I can't wait to test it today! Because the declination changes throughout the day, I guess it's not quite true that the sun rises exactly due east and sets due west on the equinox... Is that correct? John John L. Carmichael Jr. Sundial Sculptures 925 E. Foothills Dr. Tucson Arizona 85718 USA Tel: 520-696-1709 Email: [EMAIL PROTECTED] Website: <http://www.sundialsculptures.com>
