Re: Precession / EoT

[Pete Swanstrom]

Title: Analemmic Sundial Gnomon

Thanks to all for your comments regarding precession of the earth's axis, the movement of the perihelion, and the effect they cause on the EoT. It has certainly been an interesting discussion, and I have learned (that I have yet to learn) a lot! Luke, thanks for your graphs and references to J. Meeus' texts. I will borrow or purchase a copy and spend some considerable time with it. I do still have one big question that I wish to resolve, which pertains to my analemmic equatorial sundial. The question is, how long will it be before the changing analemma renders my current analemmic gnomon inaccurate? (Say until either the date positions on the gnomon have shifted by 1/4 day or more, or the EoT for some date has changed by over 15 seconds.) Thanks again! Pete Swanstrom. The Analemmic Sundial Gnomon       This analemmic gnomon is a flat plate with a Figure-8 pattern cut out of the center. The gnomon plate is suspended in the exact center of the Equatorial Dial Ring and can swivel about the North Celestial Pole axis of the sundial. A solar azimuth of 180° and altitude of 0° corresponds with the exact center of the gnomon plate.       The analemma shape and date marks were calculated by the following method. Last Updated March 12, 1998 Return to: | Sundials |