Position Calculation Precision and Accuracy

Except for the topocentric horizon calculations, positions are displayed to the following precision:

• rectangular coordinates and distances: 10^-9 astronomical unit (AU) or, for the geocentric Moon, 10^-3 km
• equatorial spherical coordinates: 10^-3 second of time in right ascension, 0.01 arcsecond in declination
• ecliptic spherical coordinates: 10^-2 arcsecond in both ecliptic longitude and latitude
• the equation of time (see this note on the Apparent Geocentric Equator of Date position type) is given to a precision of 0.1 seconds of time

MICA's position calculations are based on standard algorithms and are carried out to a precision of better than one milliarcsecond, that is, to at least an order of magnitude better than the tabulation. However, do not confuse precision with accuracy. Note the following:

• In many cases the external accuracy of the calculated positions is limited by the quality of the basic reference data - the fundamental star catalogs and planetary ephemerides. Inadequacies in the current standard models for precession and nutation are also known which affect some position types. A full discussion of this subject is quite complex and cannot be given here, but, in practice, the uncertainties in MICA's tabulated angular coordinates will usually fall between 0.01 and 1 arcsecond, depending on object, position type, and date.
• The positions of solar system bodies are those of their centers of mass, which are not directly observable. However, except for the Moon, the angular coordinates of the center of mass can be assumed to be at the geometric center of the visible (but fully illuminated) disk to the accuracy of the tables.
• For all position types in which the selected time scale is UT1, the uncertainty in the ΔT extrapolation limits the accuracy of the coordinates of fast-moving solar system objects. For example, the Moon's geocentric angular coordinates, as a function of UT1, may be uncertain at a level of a few tenths of an arcsecond.
• Topocentric apparent horizon coordinates are given to a precision of 0.1 arcsecond in both zenith distance and azimuth. However, when the selected time scale is UT1, the accuracy is limited by the neglect of polar motion -- generally, the uncertainty will be of order 0.5 arcsecond. (In practice, the conversion of local clock time to UT1 could be more problematic.) In addition, atmospheric refraction is not taken into account (except for in rise/set calculations). Refraction can affect zenith distance by several arcminutes (more near the horizon) at optical wavelengths.
• The positions of components of double or multiple stars do not include the effect of orbital motion except that (linear) part included in the position and proper motion in the star catalog; in some cases, catalogs give data on the center of mass or the center of light.