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      SUBROUTINE slERTH (IY, ID, FD, PV)

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      E R T H
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  Approximate heliocentric position and velocity of the Earth

  Given:
     IY       I       year
     ID       I       day in year (1 = Jan 1st)
     FD       R       fraction of day

  Returned:
     PV       R(6)    Earth position & velocity vector

  Notes:

  1  The date and time is TDB (loosely ET) in a Julian calendar
     which has been aligned to the ordinary Gregorian
     calendar for the interval 1900 March 1 to 2100 February 28.
     The year and day can be obtained by calling slCAYD or
     slCLYD.

  2  The Earth heliocentric 6-vector is mean equator and equinox
     of date.  Position part, PV(1-3), is in AU;  velocity part,
     PV(4-6), is in AU/sec.

  3  Max/RMS errors 1950-2050:
       13/5 E-5 AU = 19200/7600 km in position
       47/26 E-10 AU/s = 0.0070/0.0039 km/s in speed

  4  More precise results are obtainable with the routine slEVP.

  P.T.Wallace   Starlink   23 November 1994

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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