### SLA_NUT

Nutation Matrix

ACTION:
Form the matrix of nutation (SF2001 theory) for a given date.
CALL:
CALL sla_NUT (DATE, RMATN)
##### GIVEN:
 DATE D TDB (formerly ET) as Modified Julian Date (JD$-$2400000.5)

##### RETURNED:
 RMATN D(3,3) nutation matrix

NOTES:
(1)
The matrix is in the sense:

${}_{}$vtrue = M ×vmean

where ${}_{}$vtrue is the star vector relative to the true equator and equinox of date, M is the $3×3$ matrix rmatn and ${}_{}$vmean is the star vector relative to the mean equator and equinox of date.
(2)
The matrix represents forced nutation (but not free core nutation) plus corrections to the IAU 1976 precession model.
(3)
Earth attitude predictions made by combining the present nutation matrix with IAU 1976 precession are accurate to 1 mas (with respect to the ICRS) for a few decades around 2000.
(4)
The distinction between the required TDB and TT is always negligible. Moreover, for all but the most critical applications UTC is adequate.
REFERENCES:
(1)
Kaplan, G.H., 1981. USNO circular No. 163, pA3-6.
(2)
Shirai, T. & Fukushima, T., 2001, Astron.J., 121, 3270-3283.