Accuracy Enumeration

• nutation calculations use the IAU 2000A model [iau2000a, nutation_angles];
• gravitational deflection is calculated using three bodies: Sun, Jupiter, and Saturn [grav_def];
• the equation of the equinoxes includes the entire series when computing the “complementary terms" [ee_ct];
• geocentric positions of solar system bodies are adjusted for light travel time using split, or two-part, Julian dates in calls to ephemeris and iterate with a convergence tolerance of 10-12 days [light_time, ephemeris];
• ephemeris calls the appropriate solar system ephemeris using split, or two-part, Julian dates primarily to support light-time calculations [ephemeris, solarsystem_hp, light_time].

In reduced-accuracy mode,

• nutation calculations use the 2000K model, which is the default for this mode;
• gravitational deflection is calculated using only one body, the Sun [grav_def];
• the equation of the equinoxes excludes terms smaller than 2 micro arc seconds when computing the "complementary terms" [ee_ct];
• geocentric positions of solar system bodies are adjusted for light travel time using single-value Julian dates in calls to ephemeris and iterate with a convergence tolerance of 10-9 days [light-time, ephemeris, solar system];
• ephemeris calls the appropriate solar system ephemeris using single-value Julian dates [ephemeris, solar system].

In full-accuracy mode, the IAU 2000A nutation series (1,365 terms) is used [iau2000a]. Evaluating the series for nutation is usually the main computational burden in NOVAS, so using reduced-accuracy mode improves execution time, often noticeably. In reduced-accuracy mode, the NOVAS 2000K nutation series (488 terms) is used by default [nu2000k]. This mode can be used when the accuracy requirements are not better than 0.1 milliarcsecond for stars or 3.5 milli arc-seconds for solar system bodies. Selecting this approach can reduce the time required for Earth-rotation computations by about two-thirds.

Reference