USNO Circular 179 - U.S. Naval Observatory

30 EPHEMERIDES
DE200 nor DE405 have been the subject of any IAU resolution, although they have been frequently
reported on at various IAU-sponsored meetings, and DE405 is a recommended standard of the IERS
(IERS Conventions 2003). A comparison of DE405 with DE200, with an estimate of their errors,
has been given by Standish (2004).
The JPL ephemerides are computed by an N-body numerical integration, carried out in a
barycentric reference system which is consistent, except for the time scale used, with the Barycentric
Celestial Reference System (BCRS) described in Chapter 1. The equations of motion, the method of
integration, and the techniques used to adjust the starting conditions of the integration so that the
results are an optimal fit to observations are described in Chapter 5 of the Explanatory Supplement
(1992). That chapter specifically describes DE200, but the basic procedures are largely the same
for all of the JPL ephemerides.
The position and velocity data provided by the JPL ephemerides represent the centers of mass
of each planet-satellite system (although data for the Earth and Moon can be extracted separately).
Therefore, the positions, when converted to geocentric apparent places — angular coordinates as
seen from Earth — do not precisely indicate the center of the apparent planetary disk. Displacements
can amount to a few tens of milliarcseconds for Jupiter and Saturn, a few milliarcseconds
for Uranus and Neptune, and about 0.1 arcsecond for Pluto.
4.2 DE405
The JPL DE405/LE405 ephemeris provides the coordinates and velocities of the major planets,
the Sun, and the Earth’s Moon for the period 1600 to 2200 (Standish 1998b). The position and
velocity 3-vectors are in equatorial rectangular coordinates referred to the solar system barycenter.
The reference frame for the DE405 is the ICRF; the alignment onto this frame, and therefore onto
the ICRS, has an estimated accuracy of a few milliarcseconds, at least for the inner-planet data.
Optical, radar, laser, and spacecraft observations were analyzed to determine starting conditions
for the numerical integration and values of fundamental constants such as the Earth/Moon mass
ratio and the length of the astronomical unit in meters. In addition to the planetary and lunar
coordinates and velocities, the ephemerides, as distributed, include the nutation angles of the Earth
and the rotation (libration) angles of the Moon. (Note, however, that the nutation angles are not
derived from the IAU 2000A theory described in Chapter 5.)
As described in Chapter 2, DE405 was developed in a barycentric reference system using Teph,
a barycentric coordinate time (Standish 1998a). Teph is rigorously equivalent to TCB in a mathematical
sense, differing only in rate: the rate of Teph matches the average rate of TT, while the
rate of TCB is defined by the SI system. The IAU time scale TDB, often (but erroneously) considered
to be the same as Teph, is a quantity that cannot be physically realized, due to its flawed
definition. So, in fact, the use of the name TDB actually refers to quantities based on or created
with Teph. 1 Astronomical constants obtained from ephemerides based on Teph (or TDB) are not
in the SI system of units and must therefore be scaled for use with TCB or other SI-based time
scales.
The ephemerides are distributed by JPL as plain-text (ASCII) computer files of Chebyshev
series coefficients and Fortran source code. Third-party C versions of the code are also available
and, for Unix users, the data files can be downloaded in binary form. Once the system is installed
on a given computer, a Fortran subroutine named PLEPH can be called to provide the position and
1 Because of this, the IAU Working Group on Nomenclature for Fundamental Astronomy has recommended chang-
ing the definition of TDB to be consistent with that of Teph.