[dl, db] = ERFA.eqec06(date1, date2, dr, dd)Transformation from ICRS equatorial coordinates to ecliptic coordinates (mean equinox and ecliptic of date) using IAU 2006 precession model.
date1,date2 double TT as a 2-part Julian date (Note 1)
dr,dd double ICRS right ascension and declination (radians)
dl,db double ecliptic longitude and latitude (radians)
- The TT date date1+date2 is a Julian Date, apportioned in any convenient way between the two arguments. For example, JD(TT)=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.
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No assumptions are made about whether the coordinates represent starlight and embody astrometric effects such as parallax or aberration.
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The transformation is approximately that from mean J2000.0 right ascension and declination to ecliptic longitude and latitude (mean equinox and ecliptic of date), with only frame bias (always less than 25 mas) to disturb this classical picture.
eraS2c spherical coordinates to unit vector
eraEcm06 J2000.0 to ecliptic rotation matrix, IAU 2006
eraRxp product of r-matrix and p-vector
eraC2s unit vector to spherical coordinates
eraAnp normalize angle into range 0 to 2pi
eraAnpm normalize angle into range +/- pi
This revision: 2021 May 11
Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.