era.epv00.md

eraEpv00

[rv, pvh, pvb] = ERFA.epv00(date1, date2)

Earth position and velocity, heliocentric and barycentric, with respect to the Barycentric Celestial Reference System.

Given:

   date1,date2  double        TDB date (Note 1)

Returned:

   pvh          double[2][3]  heliocentric Earth position/velocity
   pvb          double[2][3]  barycentric Earth position/velocity

Returned (function value):

                int           status: 0 = OK
                                     +1 = warning: date outside
                                          the range 1900-2100 AD

Notes:

1. The TDB date date1+date2 is a Julian Date, apportioned in any convenient way between the two arguments. For example, JD(TDB)=2450123.7 could be expressed in any of these ways, among others:

          date1          date2

       2450123.7           0.0       (JD method)
       2451545.0       -1421.3       (J2000 method)
       2400000.5       50123.2       (MJD method)
       2450123.5           0.2       (date & time method)

The JD method is the most natural and convenient to use in cases where the loss of several decimal digits of resolution is acceptable. The J2000 method is best matched to the way the argument is handled internally and will deliver the optimum resolution. The MJD method and the date & time methods are both good compromises between resolution and convenience. However, the accuracy of the result is more likely to be limited by the algorithm itself than the way the date has been expressed.

n.b. TT can be used instead of TDB in most applications.

2. On return, the arrays pvh and pvb contain the following:

      pvh[0][0]  x       }
      pvh[0][1]  y       } heliocentric position, au
      pvh[0][2]  z       }

      pvh[1][0]  xdot    }
      pvh[1][1]  ydot    } heliocentric velocity, au/d
      pvh[1][2]  zdot    }

      pvb[0][0]  x       }
      pvb[0][1]  y       } barycentric position, au
      pvb[0][2]  z       }

      pvb[1][0]  xdot    }
      pvb[1][1]  ydot    } barycentric velocity, au/d
      pvb[1][2]  zdot    }

The vectors are with respect to the Barycentric Celestial Reference System. The time unit is one day in TDB.

3. The function is a SIMPLIFIED SOLUTION from the planetary theory VSOP2000 (X. Moisson, P. Bretagnon, 2001, Celes. Mechanics & Dyn. Astron., 80, 3/4, 205-213) and is an adaptation of original Fortran code supplied by P. Bretagnon (private comm., 2000).

4. Comparisons over the time span 1900-2100 with this simplified solution and the JPL DE405 ephemeris give the following results:

                              RMS    max
         Heliocentric:
            position error    3.7   11.2   km
            velocity error    1.4    5.0   mm/s

         Barycentric:
            position error    4.6   13.4   km
            velocity error    1.4    4.9   mm/s

Comparisons with the JPL DE406 ephemeris show that by 1800 and 2200 the position errors are approximately double their 1900-2100 size. By 1500 and 2500 the deterioration is a factor of 10 and by 1000 and 3000 a factor of 60. The velocity accuracy falls off at about half that rate.

5. It is permissible to use the same array for pvh and pvb, which will receive the barycentric values.

This revision: 2021 May 11

Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.