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I find some variation in the values reported for ephemeris by the various sources I have access to. For example for 2012-11-27T03:31:55 UTC I get solar declination values of -21.1828°, -21.18296°, -21.1814°, -21.1834°, -21.1906° and (again) -21.1814°. Are the differences among these values meaningful? Is there an "authoritative" source (ideally online) for ephemeris data?

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Out of curiosity, have you gotten similar inconsistencies with point sources? The Sun might be special. Also, a quick glance tells me several of these sites give $\pm 1$ arcsecond (0.0003 degrees) as the precision. – Chris White Nov 28 '12 at 2:57
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The most authoritative solar system ephemeris system is the JPL Horizons system. There is a web interface that will let you perform computations and extract solar system data from the JPL development ephemerides, the international standard.

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Which, of course, gives yet another value: 21.1523°. – raxacoricofallapatorius Nov 27 '12 at 23:50
Wolfram|Alpha is a black box that I wouldn't trust compared to Horizons. As I said, it is THE definitive standard and assumes the user knows what is needed. What is your intended purpose? See the book Fundamental Ephemeris Computations (1999, Willmann-Bell) for details on the internals of JPL ephemerides. – user11266 Nov 28 '12 at 1:28
Agreed. That's what started me wondering. I was using Wolfram (Alpha and Mathematica's AstronomicalData) and was getting weird results. Do you know if there is a web-based API that uses their algorithms? Something I can call from code as a substitute for AstronomicalData. (I can check the docs, but if you know off hand that would be great.) – raxacoricofallapatorius Nov 28 '12 at 1:38
My purpose is just basic study, so I don't need especially precise ephemeris (e.g., I'm not aiming a research telescope). One thing I'm trying to do "rediscover" some basic astronomical patterns and properties (e.g. the equation of time) using the datasets such as Wolfram's AstronomicalData, but I keep running up against small discrepancies that I can't explain, and wonder of they're from inaccuracies in the data. (That matters because the alternative may be instructive, e.g. revealing something like precession or $\Delta T$.) – raxacoricofallapatorius Nov 28 '12 at 1:44
Are there any Python libraries that provide programmatic access to the JPL ephemeris data? – raxacoricofallapatorius Dec 2 '12 at 23:55

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