Accuracy

Accuracy of astronomical software is an important point that should be understood by users. Care must be taken not to overstate the accuracy of cataloged data or calculated data in an effort to imply precision greater than that which is reasonable. For this reason, the accuracy of cataloged and calculated data are described below, along with suggestions for proper use of the many formatting options available to the user. The author thanks Roger Sinnott of Sky Publishing for sharing his insight into this problem.

Plan data

Plan documents can contain position data that is referred to J2000.0, J2050.0, or the epoch of date. Plan reports can display these data as they appear in a plan document, or they can be displayed as apparent topocentric positions. The necessary arithmetic is applied to each type of object to arrive at a highly accurate position. Typically, apparent topocentric positions differ from apparent geocentric (planets) or astrometric positions (asteroids, comets, deep-sky objects & stars) by arcseconds at most.

Cataloged data: Deep-Sky and Stellar databases

For deep-sky and stellar objects, the accuracy of the original catalog data dictates the level of accuracy, and this varies from catalog to catalog. The accuracy of these data cannot be improved by calculations. Click here for a description of the catalogs.

Orbital elements for some visual double star systems are included, permitting a highly accurate ephemeris computation of separation and position angles for a given instant. High accuracy ephemeris information is displayed on reports to greater precision than cataloged values, and the grades of the orbital elements used to compute the data are included.

When single catalog reports are created, size and magnitude data are given to the same precision as the catalogs from which the data is taken, so no error is introduced by calculation. When reports are created for multiple catalogs, size and magnitude are given to the greatest precision among the selected catalogs.

Positions are precessed, when applicable, to the epoch and mean equinox of J2000.0. These results are accurate for contemporary times, not those in the distant past or future. Positions are precessed to the epoch and mean equinox of J2000.0 rather than the epoch and equinox of date in deep-sky and star documents to facilitate cross referencing with most popular star atlases in use by amateurs.

Note that the plan document allows you to choose whether to display positions precessed to J2000 or to a given epoch and equinox of date, thereby providing apparent positions for a given instant. The plan document does not contain the accuracy of the position information so Deep-Sky Planner displays positions to a conservative precision. Full precision is retained internally.

Rise, set, and transit time, and their associated azimuth and altitude angles are calculated. Rise and set times, and the associated angles are affected by local circumstances, namely, horizons, air temperature, and air pressure. The calculated values are in error by at most a couple minutes, but local circumstances may affect that error further. The error in the calculated angles is less than a degree.

Calculated data: Planets, Sun and Moon

For planets, sun and moon, all must be calculated. The best theories available can produce extremely accurate results, but this level of accuracy comes at the expense of execution time and program size. The "truncated solutions" offered by Jean Meeus in Astronomical Algorithms and used herein offer less accuracy for solar system objects than the full solutions, but the shorter computing time and smaller program size reflect a well informed compromise.

An abridged version of Planetary theories in rectangular and spherical variables. VSOP87 solutions (Bretagnon and Francou , 1988) is used to calculate the apparent positions of the Sun and the planets Mercury through Neptune. Positions of these bodies are in error by less than a few arc seconds.

An Accurate Representation of the Motion of Pluto (Goffin, Meeus, and Steyaert, 1986) is used to calculate the apparent position of Pluto. Position of Pluto is in error by less than 1 arc second for the years 1885 to 2099.

An abridged version of Lunar Solution ELP 2000-82B (Chapront-Touze and Chapront, 1988) is used to calculate the apparent position of the Moon. Position is in error by less than 11 arc seconds. The mean error in the time of lunar phases is < 4 seconds in the period 1980 to 2020.

The times of planet events are accurate for modern times, not those in the distant past or future. The error in the times calculated may be as large as:

Suggested proper use of data formatting options

You may choose from several different formats for right ascension and declination. This gives you maximum flexibility when working with equipment, charts, or atlases that use different units of measure, but they should be used together properly to avoid a mismatch in precision. The following table gives equivalent units of measure that should be used together.

Help file version 9.2.3.0 ▪ Copyright © 2024-2026 Knightware, LLC

Help file version: 9.2.3.0 ▪ Copyright © 2026 Knightware, LLC