Accuracy

There are two ways of evaluating the accuracy of an ephemeris, the first by comparison with observation, the second by comparison with other ephemerides. Since the first is impractical, and requires observations reaching an accuracy of a very few seconds, which means only of the last three centuries while Alcyone Ephemeris extends from -2999 to +3000, we have evaluated AE by comparison with JPL's Horizons ephemeris generator. This ephemeris is based on Jet Propulsion Laboratory's DE406 ephemeris,  what is now considered the most accurate available long-period ephemeris.

The comparisons with DE406 are for geocentric longitude, latitude, distance (from the earth) for the planets and Moon, geocentric longitude of Sun and have been computed at intervals of 2130 days forward and backward from AD 0 Jan 1 0h at Greenwich in Ephemeris Time (ET). The result is 1025 comparisons extending from -2998 Jan 1 (JD 626038.5) to +2973 Aug 21 (JD 2807158.5). The period of 2130 days was chosen to give the maximum range of dates with the maximum number of calculations forward and back, 1025 (= 512 + 1 + 512), possible in AE. Each calculation is of the difference: Difference (AE - DE406) = AE - DE406.
The differences in the graphs are scaled in seconds; the distance is compared in astronomical units to seven places (10^-7 AU) and the graphs are scaled in 10^-6 AU = 0.000001 AU.

Moshier's ephemeris, upon which AE is based, was calibrated to DE404 from -3000 to +3000 for the outer planets, Jupiter, Saturn, Uranus, Neptune, Pluto, and from -1349 to +3000 for the inner planets, Mercury, Venus, Earth, Mars. In order to bring AE into better agreement with DE406 for the entire period -2999 to +3000, we have applied further corrections to heliocentric coordinates, which in turn correct geocentric coordinates. The graphs show only the corrected geocentric coordinates except for the moon, Jupiter, Uranus, and Neptune, for which uncorrected and corrected longitude is shown for reasons that will be explained below. The most notable corrections are for Mars for which the corrections extend from -2999 to +300, and Pluto for which the corrections extend the entire range from -3000 to 3000. The corrections for the earth, although much smaller, affect the geocentric coordinates of all the planets, the closer planets more, the distant planets less.