Tuesday, 18 October 2011

planet - Calculating RA/dec from JPL ephemeris data

I'm using JPL planetary ephemerides to calculate the position of planets. Using DE405 ephemeris (using a parser provided by Project Pluto with all tests passing). I am fetching the position of Mars with the observer set as the Earth (geocentric).



My first assumption is that the result is a set of geocentric equatorial coordinates in rectangular form (x, y, z). This may be incorrect.



My goal is to calculate RA/Dec. My second assumption is that I should use a formula converting from Cartesian to Spherical:



r=sqrt(x2+y2+z2)r=sqrt(x2+y2+z2)
ra=arctan(y/x)ra=arctan(y/x)
dec=pi/2arccos(z/r)dec=pi/2arccos(z/r)



Using Julian day 2457134 (21st Apr 2015, noon UTC), and cross-checking the result with NASA's Horizons service which apparently uses the same ephemeris (DE405) gets me almost there but my results are a little off:



x 1.721427968227801 y 1.5802812301744067 z 0.6897426775298559



RA 02h50m12.51s Dec +16°26'41.37"



Horizons:



RA 02h50m11.52s Dec +16°26'36.5"



I'm not sure what I'm missing. Do I have to factor distance of the planet due to the time it takes for light to reach us? Am I not using the correct time? Are my assumptions wrong?

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