system |
symbols |
longitude |
latitude |
x-y plane |
long. zero |
RH/LH |
---|---|---|---|---|---|---|

horizon | - | azimuth | elevation | horizontal | north | L |

equatorial | R.A. | Dec. | equator | equinox | R | |

local equ. | H.A. | Dec. | equator | meridian | L | |

ecliptic | ecl. long. | ecl. lat. | ecliptic | equinox | R | |

galactic | gal. long. | gal. lat. | gal. equator | gal. centre | R | |

supergalactic | SGL,SGB | SG long. | SG lat. | SG equator | node w. gal. equ. | R |

The routines sla_EQECL and sla_ECLEQ transform between ecliptic coordinates and ; there is also a routine for generating the equatorial to ecliptic rotation matrix for a given date: sla_ECMAT.

For conversion between Galactic coordinates and there are
two sets of routines, depending on whether the is
old-style, B1950, or new-style, J2000;
sla_EG50
and
sla_GE50
are to and *vice versa* for the B1950 case, while
sla_EQGAL
and
sla_GALEQ
are the J2000 equivalents.

Finally, the routines
sla_GALSUP
and
sla_SUPGAL
transform to de Vaucouleurs supergalactic longitude and latitude
and *vice versa.*

It should be appreciated that the table, above, constitutes
a gross oversimplification. Apparently
simple concepts such as equator, equinox *etc.* are apt to be very hard to
pin down precisely (polar motion, orbital perturbations ...) and
some have several interpretations, all subtly different. The various
frames move in complicated ways with respect to one another or to
the stars (themselves in motion). And in some instances the
coordinate system is slightly distorted, so that the
ordinary rules of spherical trigonometry no longer strictly apply.

These *caveats*
apply particularly to the bewildering variety of different
systems that are in use. Figure 1 shows how
some of these systems are related, to one another and
to the direction in which a celestial source actually
appears in the sky. At the top of the diagram are
the various sorts of *mean place*
found in star catalogues and papers;^{} at the bottom is the
*observed* , where a perfect theodolite would
be pointed to see the source; and in the body of
the diagram are
the intermediate processing steps and coordinate
systems. To help
understand this diagram, and the SLALIB routines that can
be used to carry out the various calculations, we will look at the coordinate
systems involved, and the astronomical phenomena that
affect them.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk