Function epochProp( tYr, astrom6 )

Description:
Propagates the astrometry parameters, supplied as a 6-element array, to a different epoch.

The input and output astrometry parameters are each represented by a 6-element array, with the following elements:

 index  gaia_source name  unit    description
 -----  ----------------  ----    -----------
   0:   ra                deg     right ascension
   1:   dec               deg     declination
   2:   parallax          mas     parallax
   3:   pmra              mas/yr  proper motion in ra * cos(dec)
   4:   pmdec             mas/yr  proper motion in dec
   5:   radial_velocity   km/s    barycentric radial velocity
 
The units used by this function are the units used in the gaia_source table.
Parameters:
tYr (floating point)
epoch difference in years
astrom6 (array of floating point)
astrometry at time t0, represented by a 6-element array as above (a 5-element array is also permitted where radial velocity is zero or unknown)
Return Value (array of floating point):
astrometry at time t0+tYr, represented by a 6-element array as above
Example:
epochProp(-15.5, array(ra,dec,parallax,pmra,pmdec,radial_velocity)) - calculates the astrometry at 2000.0 of gaia_source values that were observed at 2015.5
Signature:
double[] epochProp(double, double[])