Technical data

Chapter 5 Starmap 25
comets or planets, the Medium mode will give positions that are very close to the actual positions, but will take
slightly longer to calculate. Finally, if you desire precision comparable to the Astronomical Almanac, choose the High
mode. This mode calculates values using the most rigorous forms of all the equations and corrections needed to
accurately determine the objects position to sub-arcsecond accuracy.
For the most accurate Geocentric (Earth centered) values, select the Geocentric option then choose the High
precision mode. By selecting the Topocentric precision mode, you will use the geocentric values that are also
corrected for the current location and elevation. This will give the values that pertain precisely to your selected
location and time.
The precision setting will be reflected in the displayed values in the Ephemeris tables, and in the resulting Right
Ascension and Declination values shown in the Object Selection dialog box. Make sure you have selected the correct
precision and reference before comparing the positions with published values!.
The three precision settings (Low, Medium and High) correspond to the three basic position references, Geometric,
Astrometric and Apparent.
The Geometric position (Low precision) is calculated without any of the corrections for the effects of light time,
aberration, etc.. Additionally, the Geometric parameters are calculated at a lower precision to increase the speed of the
calculations.
The Astrometric position (Medium precision) is first calculated to the full precision (J2000.0) Geometric values, then
the correction for light time is added. The Astrometric mode is used for comparing the positions of Pluto and the
asteroids and comets with the position of the stars. The Astronomical Almanac shows the positions of Pluto and the
minor planets in the Astrometric Geocentric mode.
The Apparent position (High precision) mode starts with the high precision Geometric values, then corrects for the
light time effect, aberration, precession, nutation in obliquity and nutation in longitude. The positions are referenced to
the Equinox and Equator of Date, not the standard reference of J2000.0. The Astronomical Almanac show the
positions of the planets in the Apparent, Geocentric mode. For the current 'TRUE' positions of the objects use the
Apparent, Topocentric mode.
Reset Parameters
The Reset Parameters function sets all of the Starmap options, viewpoint, mirroring, and magnitude limits, to the
initial default settings. This is useful when a large number of specialized options have been chosen and you want to
start a new display session from a known starting point.