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Re: Agnostic gnomon and horary error
From: Geoff Hitchcox
Date: 2024 May 31, 18:15 -0700

> Does anyone "gnow"??

"I think" I might 'gnow' Frank ;-)

I'm interested in this question because I was given an Armillory Sundial that is a few degrees out to my Latitude, so I am curious how one might evaluate the error.

Here is my method - which indeed may be incorrect - is it similar to what you did Frank in the past?

I assumed these coordinates:

--------------------------

Treworgy Sundial Coordinates = N 41.36235 W 71.96382

Lake Saint-Jean (QC, Canada) = N 48.63765 W 71.96382 (co-latitude of Treworgy Sundial)

--------------------------

I assume that if the Sundial was (very carefully) mounted on the surface of Lake Saint-Jean, it would read the correct sundial time.

So for the 20 June 2024, I tabulated the Solar Azimuth for both locations for every minute of the day - the Sun was above the horizon. I then subtracted the Solar Azimuth at the same UTC minute, (Lake - Treworgy) and plotted this difference (as per attached graph). If this is how one would find the answer Frank - I find the graph quite interesting. The maximum time error of about +40 minutes in the morning, and -40 minutes in the afternoon. Like a broken watch (always accurate once a day), the Treworgy Sundial will be useful each day at Solar Meridian Transit!

Frank, if you think the above method is correct, I could do two more graphs for the Equinox and Winter Solstice to see how they compare to the attached Summer Solstice graph.

Regards, Geoff Hitchcox, Christchurch, New Zealand.

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