Implementation of "Keplerian Elements for Approximate Positions of the Major Planets" by E M Standish.
Approximate errors, from http://ssd.jpl.nasa.gov/?planet_pos
1800 -- 2050 3000 BCE to 3000 CE
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RA Dec. r RA Dec. r
(arcsec) (arcsec) (1000 km) (arcsec) (arcsec) (1000 km)
Mercury 15 1 1 20 15 1
Venus 20 1 4 40 30 8
Earth-Moon
Barycenter 20 8 6 40 15 15
Mars 40 2 25 100 40 30
Jupiter 400 10 600 600 100 1000
Saturn 600 25 1500 1000 100 4000
Uranus 50 2 1000 2000 30 8000
Neptune 10 1 200 400 15 4000
Pluto 5 2 300 400 100 2500