Ecliptic and Equatorial Coordinate Transformations

 

Coordinate Transform Between Equatorial and Ecliptic Systems
Notes: The equations below will transform coordinates between the equatorial (RA, DEC) and ecliptic
(LAT, LONG) systems of locating objects on the celestial sphere. While the equatorial system is standard for locating objects in the sky, the ecliptic system is more convenient in calculating the position of an object orbiting the sun. Calculations can be done for input coordinates that are of mean or true equinox.
 
Resources: [AA: pp 88,89]
 
Function Returns Mean or True Obliquity of the Ecliptic
Input: JD and IsMean value: 0 = True equinox, anything else for Mean.
Output: Obliquity in degrees.
 
Equatorial RA and DEC to Ecliptic Longitude
Input: RA, DEC, JD, and IsMean (defined above). The setting for IsMean must match the RA and DEC coordinates as being mean equinox or true equinox.
Output: Ecliptic longitude in degrees.

 
Equatorial RA and DEC to Ecliptic Latitude
Input: same as above.
Output: Ecliptic latitude in degrees

.

 
Ecliptic LAT and LONG to Equatorial Right Ascension
Input: LONG, LAT, JD, IsMean (explained above).
Output: RA in decimal degrees.

 
Ecliptic LAT and LONG to Equatorial Declination
Input: LONG, LAT, JD, IsMean (explained above).
Output: DEC in decimal degrees.

 
Example: Transform coordinates of the star from equatorial to ecliptic system and back.
 
Enter RA/DEC Equatorial positions:
Indicate Mean or True Equinox and date as Julian day number:
 
Resulting ecliptic longitude and latitude:
 
Use ecliptic result to determine equatorial coordinates:
Check: The final results should match the initial input coordinates.

 

Astro Utilities Electronic Book Copyright 1999 Pietro Carboni. All rights reserved.