USNO Circular 181
Nutation Series Evaluation in NOVAS 3.0
George H. Kaplan
U.S. Naval Observatory
December 2009
Introduction:
This circular describes the computation of nutation in the Naval Observatory
Vector Astrometry Software (NOVAS), versions 3.0 and 3.1. The user's guides to the
Fortran and C
editions of NOVAS 3.1 provide a detailed description of current software, which supersedes NOVAS 3.0
described by USNO Circular 180.
Nutation is a small periodic oscillation of the Earth's axis with respect to a space-fixed reference
system ("inertial space") caused by the torque of the Sun and Moon — and to a lesser extent, the
other planets — on the Earth's equatorial bulge. Nutation is usually modeled by lengthy
trigonometric series that, when evaluated for a specific date and time, yield the values of two ecliptic
angles: the nutation in longitude, Δ ψ, and the nutation in obliquity, Δ ε. The
evaluation of the nutation series is generally the most computationally intensive part of calculating
the positions of celestial objects with respect to the traditional celestial coordinate system defined
by the true equator and equinox of date. NOVAS provides several nutation series with different numbers
of terms, which the user can select depending on the accuracy requirements.
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