John Carmichael wrote: > We noticed that the Shadows sundial generator program has analemmas with the > following dates of each month: 1,6,11,16,21,26. Why would this sequence be > better than: 1,5,10,15,20,25?
I won't venture to guess the motivation of creator of the Shadows program, but I will offer a historical reason. On historical astronomical instruments and in astronomical texts, the zodiacal calendar took priority over the civil calendar. If the civil calendar appeared, it was adjacent to the zodiacal calendar, showing that the date that the sun entered each sign was around the 21st of the appropriate civil month (after the Gregorian reform of the calendar). For example, the vernal equinox--the date that the sun entered Aries--was on March 21st. On many sundials, whether one is marking the sun's path through the ecliptic, solar declination, the seasons, or the analemma, it makes more sense from the geometry of the earth-sun system (which gives the traditional starting dates of our seasons) to mark scales primarily in terms of the sun's apparent motion through the signs. So when the figure-eight analemma was introduced to instruments, it too was marked zodiacally. However, over time people became more "removed" from the zodiacal calendar than the civil calendar. The zodiacal scale was dropped leaving the civil. But the zodiacal date markers remain as a vestige of that heritage. Happy dialling, Sara -- Dr. Sara Schechner Curator, Collection of Historical Scientific Instruments Harvard University, Science Center B-6 Cambridge, MA 02138 tel: (617) 495-2779 fax: (617) 496-5932
