# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Question on geocentric to heliocentric conversion
From: Marcel Tschudin
Date: 2011 Jun 10, 15:43 +0300
George,
The path of the sun (or rather its shadow) can only be predicted when knowing the shape and position of the surface and the position of the gnomon relative to the surface. If the surface isn't exactly level and the gnomon not exactly vertical to it you will have difficulties to find the shadow's path by extrapolation. The shadow's path is then a projection on a surface of unknown obliquity.
Marcel

On Fri, Jun 10, 2011 at 2:51 PM, George Becker PLS wrote:
Marcel, thank you for responding.

Basically, I am trying to define the path of the sun across the ground. I was hoping to "nail down" the sun's path with field measurement, time & date, then extrapolate the paths for the rest of the year. I have this gnomon chart ( http://solardat.uoregon.edu/SunDialProgram.html ) but something is not aligning correctly when I overlay it in AutoCAD.

-----Original Message-----
From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com]On Behalf Of Marcel Tschudin
Sent: Friday, June 10, 2011 5:40 AM
To: NavList@fer3.com
Subject: [NavList] Re: Question on geocentric to heliocentric conversion

After a second thougth: Yes, for your measurements the difference between topocentric and geocentric are likely to be negligible. You can verify the level of your gnomon set-up and the heliocentric coordinates by using the data from http://www.calsky.com/

Define under the "Intro"-tab your location and set the experience to "Astronomer". Select then the "Sun"-tab and next the "Apparent view/Data". Enter date and time of your gnomon readings and press "go". You should now have all data you need.

Marcel

On Fri, Jun 10, 2011 at 12:04 PM, Marcel Tschudin wrote:
George, I'm not really sure what you aim at. Could there be some misunderstandings? With a gnomon you take *topocentric* measurements. Geocentric refers to the center of the earth and heliocentric to the center of the sun. The accuracy of measurements taken with a gnomon depend on how much the surface is leveled and the gnomon at right angle to it. I question whether your ground surface was really leveled. An artificial horizon might have been a better choice.

Marcel

On Fri, Jun 10, 2011 at 7:33 AM, G Becker wrote:

Hello, I mostly lurk here.. but I have a question.

Today, I stuck a 1/2" rebar into the ground as a gnomon and recorded the bearing and distance of the shadows over a few hours time. I used a 3" robotic total station, so the bearings and distances are very accurate. What I am looking for is the heliocentric ease/west. I have the geocentric state plane coordinates for the gnomon, the height of the gnomon, the beginning mid and end times of measurements, and the geocentric bearings and distances of the shadows. Am I on a wild goose chase doing it this way, or can the heliocentric east/west direction be found with the information I provided. Please look at the attached 2 page pdf file for a the graphic information. Any comments would be appreciated.

Thanks,
George
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