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https://www.physicsforums.com/insights/measuring-how-many-days-are-in-a-year/

Now often astronomers work in Julian days to have a consistent time scale without skips and bound.s. Various parameters are often presented as polynomials using JD , J year of even J millennia.

The days are human noon to noon days, so presumably these equate to mean solar days of 86400 s ( by definition ) but there again there may be qualifications due to tidal drag etc.

http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html

Code:

```
J E ratio
Sidereal orbit period (days) 4,332.589 365.256 11.862
Tropical orbit period (days) 4,330.595 365.242 11.857
```

(E=365.256 thus must be msd.)

pJ=4332.589 msd.

What I would like to know is how to compare orbital period of Jupiter , polynomial functions of say lunar precession period in JD and eclipse series Saros periods.

Supplementary question: what is the best average value for Saros centred on J2000 ? I need a value compatible with the previous, not a derivative approximation in year,months,days and hours, or N synodic months etc.

I looked at data from NASA and averaged over series of similar eclipses but always came up with results a little shorter than values found stated elsewhere for Saros.

https://eclipse.gsfc.nasa.gov/SEsaros/SEsaros135.html

thanks to anyone who can clarify this infernal mix of different "days".