rm = ERFA.ecm06(date1, date2)ICRS equatorial to ecliptic rotation matrix, IAU 2006.
date1,date2 double TT as a 2-part Julian date (Note 1)
rm double[3][3] ICRS to ecliptic rotation matrix
- 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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The matrix is in the sense
E_ep = rm x P_ICRS,
where P_ICRS is a vector with respect to ICRS right ascension and declination axes and E_ep is the same vector with respect to the (inertial) ecliptic and equinox of date.
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P_ICRS is a free vector, merely a direction, typically of unit magnitude, and not bound to any particular spatial origin, such as the Earth, Sun or SSB. No assumptions are made about whether it represents starlight and embodies astrometric effects such as parallax or aberration. The transformation is approximately that between mean J2000.0 right ascension and declination and ecliptic longitude and latitude, with only frame bias (always less than 25 mas) to disturb this classical picture.
eraObl06 mean obliquity, IAU 2006
eraPmat06 PB matrix, IAU 2006
eraIr initialize r-matrix to identity
eraRx rotate around X-axis
eraRxr product of two r-matrices
This revision: 2021 May 11
Copyright (C) 2013-2021, NumFOCUS Foundation. Derived, with permission, from the SOFA library.