Models and Accuracy

Mercury through Neptune

Model: The planetary model is VSOP87 (Variations Seculaires des Orbites Planetaires) by Bretagnon and Francou. The original model documentation, data files, and sample Fortran routines are available (http://vizier.cfa.harvard.edu/viz-bin/ftp-index?/ftp/cats/VI/81) NOTE: It is ‘’’not’’’ neccessary to download any data or sources from the original site. Astronomia provides everything required. The original data files require about 4.1 MB; combined and reduced in Astronomia to about 1.4 MB. The full accuracy of the model is retained.

Version: The VSOP model is available in several variants. Astronomia uses version “D” which produces heliocentric spherical coordinates for the equinox of date. This is the same as used in Meeus.

Accuracy: The model produces positions of better than 1 arc-second for Mercury through Mars for the years -2000 to +4000, for Jupiter and Saturn for the years 0 to +4000, and for Uranus and Neptune for the years -4000 to +8000. Within those ranges, the actual accuracies are:

Accuracy of planetary positions

Planet	Distance to Sun (km)	Latitude and Longitude (arc seconds)
Mercury	0.3	0.001
Venus	2.7	0.005
Earth	3.7	0.005
Mars	22.8	0.021
Jupiter	272	0.072
Saturn	1000	0.144
Uranus	230	0.017
Neptune	1891	0.087

A note on the Meeus version. Meeus prints a truncated version of the model in his book. The first edition had an optional software supplement which included the truncated data. This supplement is no longer available, but the complete data presented here can be used instead of the version on the diskette. Astronomia does use a different format.

Moon

The lunar model is Meeus’s simplified version of ELP2000-82 by Chapront. The complete model can be found at http://cdsarc.u-strasbg.fr/viz-bin/Cat?VI/79. Accuracy of the simplified version is about 10 arc-seconds in longitude and 4 arc-seconds in latitude.