Ephemeris trail
Build VSOP87
Truncating VSOP87
Testing VSOP87
Class organization
Reference frames
Handling frames
Handling precision
Handling time
Putting all together
Swiss Ephemeris
Handling frames
Here, we see the classes used to represent frames and deal with coordinate transformations.

A simple model

To be flexible, the program should be able to respond to a request like "I want the coordinate of this body in a frame materializing this reference system, having this center, this reference plane and this reference axis".
It should also possible to specify if we want geometrical or apparent coordinates ; if apparent coordinates are wanted, it should be possible to specify from where they are apparent.

For the first version, I simplified and developed JEphem to be just able to respond to "I want the coordinates in this frame", for frames currently used in astronomy. In JEphem API, to identify a frame, constants of interface jephem.astro.spacetime.SpaceConstants are used.
In fact, the notion of frame, as it is used in the API, corresponds to "a way to express the coordinates" : it includes (sometimes implicitely) the information about apparent / geometrical coordinates.

The frames known by JEphem

The API names are the constant names of jephem.astro.spacetime.SpaceConstants ; these names are use throughout the program to designate the different frames.

API nameDescriptionNames of the spherical coordinates
(order : r, q, j)
FRAME_THEORY The coordinates are expressed as given by the theory used to compute them, without any transformation.
Center : depends on the theory
Ref. plane : depends on the theory
Ref. axis : depends on the theory
Reference system : depends on the theory
Geometric coordinates
Depends on the theory
Ref. plane : Mean ecliptic J2000
Ref. axis : Mean equinox J2000
Reference system : FK5
Geometric coordinates
(D, b, l)
Ref. plane : Mean ecliptic of date
Ref. axis : True equinox J2000
Reference system : FK5
Apparent coordinates
(r, l, b)
Ref. plane : True Equator of date
Ref. axis : True equinox J2000
Reference system : FK5
Apparent coordinates
(r, a, d)

A method, jephem.astro.spacetime.Space.getFrameLabel(int whichFrame) permits to get the English name of a given frame.

Coordinate labels

I developped constants and methods to identify the coordinates, and retrieve their names. This is implemented in classe jephem.astro.spacetime.Space and jephem.astro.spacetime.SpaceConstants.
  • Six constants (SpaceConstants.COORD_xx) permit to identyify a coordinate.
  • The notion of coord group was introduced for convenience (the groups are identified by SpaceConstants.COORDGROUP_XXX). Groups are accessed with Space.getCoordGroup(int frame, int sphereCart) This is useful to get the labels.