Planetary Position Computing

Planet Position Display

C2A calculates planetary positions from files of polynomials developed by the Institut de Mécanique Céleste et de Calcul des Ephémérides (IMCCE) in Paris. C2A is delivered with two different files of polynomials:

The choice of theory to use (and therefore the date interval covered) is made through the SolSys tab in the Options dialogue box.

By default, C2A calculates geocentric equatorial coordinates at the J2000 mean epoch. This means that the J2000 equinox is used as a reference and that the speed of light is taken into account in calculating equatorial positions. However, C2A lets you calculate the positions of solar system objects in the topocentric rather than geocentric system. Similarly, you can express equatorial coordinates at the Equinox of the date rather than the J2000 equinox. These two options are also accessible through the SolSys tab in the Options dialogue box. It is strongly advised to only work with the J2000 Equinox when plotting solar system objects against the star background since most catalogues are given in the J2000 equinox. This will ensure a correct positioning of the planets, moon, sun, asteroids and comets in the maps with respect to the stars.

For example, here are some comparisons of the accuracy of positions obtained with C2A and other software.

Equatorial geocentric coordinates of the Sun on 19 January 1994 at 0 hours UT

Software

Details

Right Ascension

Declination

C2A

Equinox J2000, equatorial geocentric coordinates, theory SLP98

 20h 03m 34,33s

 -20° 24' 28,46"

IMCCE

(production of ephemerides on website)

Mean J2000, geocentric, DE406/LE406

 20h 03m 34,32983s

 -20° 24' 28,4747"

Other software available on the Internet

Equinox J2000

 20h 03m 10,91s

 -20° 25' 40,6"

 

Equatorial topocentric coordinates for the Sun on 19 January 1994 at 0 hours UT from Paris (2° 20' 13,56" E; 48° 50' 10,564" N; 45.77 m)

Software

Details

Right Ascension

 Declination

C2A

Equinox J2000, equatorial topocentric coordinates, theory SLP98

 20h 03m 34,33s

 -20° 24' 32,68"

IMCCE

(production of ephemerides on website)

Mean J2000, topocentric Paris, DE406/LE406, Relativistic perturbations, coordinate system 0

 20h 03m 34.32769s

 -20° 24' 32.6932"

(1) P. Bretagnon. Theory for the motion of all the planets - The VSOP82 solution. Astron. Astrophys., 114:278, 1982.

Note that a file of polynomials covering the period 1500 to 2100 is available on the C2A website (page Downloads then Miscellaneous).

Display of the Satellite Positions

C2A calculates the positions of the main satellites of Mars, Jupiter, Saturn and Uranus. The sattelites taken into account are as follows:

Mars: Phobos, Deimos
Jupiter: Io, Europa, Ganymede, Callisto
Saturn: Mimas, Encelade, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus
Uranus: Miranda, Ariel, Umbriel, Titania, Oberon

The positions of these satellites are calculated for the period 1987-2040 by using coefficients supplied by IMCCE. Outside this period the satellites are not displayed. Note that you must zoom sufficiently on the chosen planet for the satellites to appear (it is preferable to use a Field type view, so that the planet does quickly leave the map).

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