Hi Roger (& others) That was your clearest and best description yet of your seasonal markers. I think you should also point out to the group the other great feature of your program, the shadow length and path of the gnomon. Your spreadsheet lets one see the path and length of the gnomon's shadow on different dates with different sized gnomons. This is very useful for sundial design, as you know. It helps you to determine the size of an analemmatic dial based on the size of the user, the latitude and the dates of use. (By the way, I'm going to use this feature in the future if I ever need to see how shadows behave of things like buildings or trees. This feature will let you spot in advance shadow problems with "horizon pollution" from nearby objects and can be used for any sundial on the ground.)
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> ----- Original Message ----- From: "Roger Bailey" <[EMAIL PROTECTED]> To: "Sundial Mail List" <sundial@rrz.uni-koeln.de> Sent: Saturday, May 11, 2002 7:19 PM Subject: Seasonal Marker Position > In previous notes on the this topic, I have proposed seasonal sunrise and > sunset markers for analemmatic sundials. These points on the East/West axis > are used with the date marks on the North South axis to determine when and > where the sun rises and sets through the year. There are two markers: the > one on the west side is used for sunrise, the one on the east side for > sunset. Stand on the marker point and sight past the date mark to find the > point on the horizon where the sun rises or sets. Stand on the date mark on > the on the N/S axis and sight past the sunrise marker on the E/W axis to the > hour ellipse to tell the time of sunset. > > The marker is not a single point but varies a bit with date and latitude. > This is shown on the attached sketch "SM-Lat.pdf" showing for a 5 meter > major diameter dial, the extremes of the marker position. The red line > shows the marker position for the solstices at different latitudes. The blue > line shows the equinox position. The difference between these points is the > diameter of the epicycle shown on the expanded scale on the right axis. For > comparison purposes, the focus point for the hour ellipse is shown in green. > > As outlined before, the "Epicycle" can be used to correct for the periodic > error. It is a simple circle with the date marked around the circumference. > A perpendicular from the circumference to the axis determines the exact > marker point for that date. These exact positions allow you to use string, > plumb bobs etc to use the dial as an analogue computer to accurately > determine the time and azimuth of sunrise and sunset. Make two rotations > around the "Epicycle" per year, from solstice to equinox, and back to > solstice for each rotation. > > As shown in the sketch "SM-Lat.pdf", the epicycle diameter increases with > latitude. Even at common latitudes for Europe, Britain and Canada, the > concept works well. For common use, diameter of the marker point is > reasonable and in proportion to typical hour markers. For greater accuracy, > use the "Epicycle" to determine the precise marker point for that date. > > Helmut and I are working to include the calculations for this "Epicycle" on > the spreadsheet. This will be posted soon. For the full story you will have > to wait for the presentation at the NASS conference in Tucson. > > Roger Bailey > Walking Shadow Designs > N 51 W 115 > -
