Dear Friends
I have spend many happy hours during this wet, wet winter investigating and
learning how to calculate all the solar parameters that a gnomonist might
possibly need - Equation of Time, Declination, RA, Altitude, Azimuth, Time of
Sunset/Rise, etc, etc.
I have been surprised to find that - with traditional calculation methods and
an absolute minimum of astronomical information - it is possible to calculate
everything from first principles to a surprising degree of accuracy.
Other than location and local time, only six pieces of astronomical information
are required - obliquity, eccentricity, Sun’s GHA at 1/1/2000, longitude of
perihelion, a single precessional constant and the length of the tropical year.
Accuracies for the EOT are +/- 2 seconds of time For altitudes/azimuths, less
than 1 minute of arc - much better than needed by most gnomonic problems.
If any of you are interested in such calculations, I have loaded a document
with all the astronomical theory and background plus the code onto my website
www.precisedirections.co.uk/sundials
The code is written in Python, a language available on every type of computer,
which is very easily understood, quite easily learnt and very easily translated
into any other coding language you might like.
If you own an iPad or iPhone, and are prepared to buy a cheap little app called
Pythonista, the code will extract locational & time information from your phone
- so you do not even have to input this to get your calculations done
You might also like to see a graphic of a civil mean time horizontal dial,
which I think is called a hectomoros dial, that is destined for my garden.
This is also on the website.
Enjoy
Kevin
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