Stars - Apparent Position

 

 

    The apparent position of an astro is his position observed in the Celestial Sphere without the correction of the local effects (diurnal aberration, geocentric parallax and refraction).

    To determine that position we need to introduce some concepts:

 

    · Mean Equinox is the intersection of the ecliptic of the date with the mean equator of the date;

    · True Equinox is the intersection of the ecliptic with the true (instantaneous) equator, i.e., the equator affected by the nutation effect;

    · there is not "Mean" Ecliptic since that this one have a regular motion.

 

Fig. 01: Relationship between the Ecliptic and the Mean and the True Equator.

 

    The problem of the reduction of a position of a star to her mean position in a certain date (for instance from an epoch and an equinox standards as J2000.0, that is the case of  FK5) to her apparent position in another date we must take into account the following corrections:

 

Proper Motion of the Star between the two epochs;
Precession effect;
Nutation effect;
Annual Aberration effect;
Annual Parallax effect;
Gravitational Light Deflection.

 

    To determinate the apparent position of a Star it will be important to determinate first of all her Mean Position, then her True Position and finally the Apparent Position.

 

• Mean Position

    The mean coordinates (a1, d1) of a star are her coordinates in the barycentre celestial sphere referred to the mean equator and equinox of date (i.e., the epoch of the equator and the equinox are the same as the date of observation).

    These coordinates are only affected by the Precession and by the Proper Motion of the star.

• True Position

    The true coordinates (a2, d2) of a star are the coordinates in the barometrical sphere referred to the true equator and equinox of date.

    These coordinates include the effect of  Nutation (besides the Precession effect and the Proper Motion in the corresponding fraction of the year), but they are rarely used. General, they are not more than a passage between the mean coordinates to the apparent coordinates.

• Apparent Position

    The apparent coordinates (a, d) of a star are her coordinates in the geocentric celestial sphere referred to the true equator and equinox of date.

    These coordinates involves the corrections for the Annual aberration and for the Parallax, differing from the observable ones by only the local effects – Refraction and Diurnal Aberration.