This is the master page for the SOFA Libraries Issue 2021-05-12 which was released on 2021-05-12 at 14:00 UTC. From here you can find your way to all the resources comprising this Issue, including notes and source download pages. A summary of changes for the new release is available.
PLEASE NOTE: The Fortran and ANSI C versions of the SOFA Libraries have been unified and will be referred to as the seventeenth release of SOFA.
This SOFA release contains support for the IAU 2006 precession model, used in conjunction with the existing IAU 2000A nutation model.
The IAU 2006 precession and IAU 2000A nutation are completely separate standards, and associated with the IAU 2006 precession is a transformation to be applied to the IAU 2000A nutation in order to make them match. The mismatch that this transformation eliminates stems partly from the purely geometric effect of the revised obliquity and partly from phenomena taken into account in the IAU 2006 precession that are absent from the IAU 2000A nutation. In the present era the adjustments are very small (microarcseconds) and are of formal rather than practical significance.
Certain SOFA subroutines apply this transformation, and have names that include the string "06A" as shorthand for "IAU 2006+2000A precession-nutation". It is important to understand that this is done simply to keep names concise and does not imply the existence of "IAU 2006A nutation". Care has been taken in the program comments for each affected SOFA routine, and in the other documentation, to keep IAU 2006 precession, IAU 2000A nutation, and the matching transformation all separate.