[tz] Dealing with Pre-1970 Data

David Patte ₯ dpatte at relativedata.com
Sun Sep 1 13:03:48 UTC 2013

On 2013-09-01 6:35, Zefram wrote:
>> However the one I was really thinking about was the 'equation of
>> >time' calculation in a format we can use as a 'standard'.

Equation of time calculations are non-trivial requiring the knowledge of 
the variations of the orbit of the earth (to calculate sun position), as 
well as variations in earth revolution speed (to calculate DeltaT). For 
accuracy, astronomical software often uses large tables and series of 
thousands of polynomials to estimate local apparent time in the past. 
This is clearly outside the scope of a tz database.

There are simplifications to these formulas, but they are only accurate 
for short timeframes.


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