[rv, ra2, dec2, pmr2, pmd2, px2, rv2] = ERFA.starpm(ra1, dec1, pmr1, pmd1, px1, rv1, ep1a, ep1b, ep2a, ep2b)Star proper motion: update star catalog data for space motion.
ra1 double right ascension (radians), before
dec1 double declination (radians), before
pmr1 double RA proper motion (radians/year), before
pmd1 double Dec proper motion (radians/year), before
px1 double parallax (arcseconds), before
rv1 double radial velocity (km/s, +ve = receding), before
ep1a double "before" epoch, part A (Note 1)
ep1b double "before" epoch, part B (Note 1)
ep2a double "after" epoch, part A (Note 1)
ep2b double "after" epoch, part B (Note 1)
ra2 double right ascension (radians), after
dec2 double declination (radians), after
pmr2 double RA proper motion (radians/year), after
pmd2 double Dec proper motion (radians/year), after
px2 double parallax (arcseconds), after
rv2 double radial velocity (km/s, +ve = receding), after
int status:
-1 = system error (should not occur)
0 = no warnings or errors
1 = distance overridden (Note 6)
2 = excessive velocity (Note 7)
4 = solution didn't converge (Note 8)
else = binary logical OR of the above warnings
- The starting and ending TDB dates ep1a+ep1b and ep2a+ep2b are Julian Dates, apportioned in any convenient way between the two parts (A and B). For example, JD(TDB)=2450123.7 could be expressed in any of these ways, among others:
epna epnb
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.
-
In accordance with normal star-catalog conventions, the object's right ascension and declination are freed from the effects of secular aberration. The frame, which is aligned to the catalog equator and equinox, is Lorentzian and centered on the SSB.
The proper motions are the rate of change of the right ascension and declination at the catalog epoch and are in radians per TDB Julian year.
The parallax and radial velocity are in the same frame.
-
Care is needed with units. The star coordinates are in radians and the proper motions in radians per Julian year, but the parallax is in arcseconds.
-
The RA proper motion is in terms of coordinate angle, not true angle. If the catalog uses arcseconds for both RA and Dec proper motions, the RA proper motion will need to be divided by cos(Dec) before use.
-
Straight-line motion at constant speed, in the inertial frame, is assumed.
-
An extremely small (or zero or negative) parallax is interpreted to mean that the object is on the "celestial sphere", the radius of which is an arbitrary (large) value (see the eraStarpv function for the value used). When the distance is overridden in this way, the status, initially zero, has 1 added to it.
-
If the space velocity is a significant fraction of c (see the constant VMAX in the function eraStarpv), it is arbitrarily set to zero. When this action occurs, 2 is added to the status.
-
The relativistic adjustment carried out in the eraStarpv function involves an iterative calculation. If the process fails to converge within a set number of iterations, 4 is added to the status.
eraStarpv star catalog data to space motion pv-vector
eraPvu update a pv-vector
eraPdp scalar product of two p-vectors
eraPvstar space motion pv-vector to star catalog data
This revision: 2021 May 11
Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.