From d54fe7c1f704a63824c5bfa0ece65245572e9b27 Mon Sep 17 00:00:00 2001 From: Joseph Hunkeler Date: Wed, 4 Mar 2015 21:21:30 -0500 Subject: Initial commit --- src/slalib/sun67.htx/node202.html | 254 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 254 insertions(+) create mode 100644 src/slalib/sun67.htx/node202.html (limited to 'src/slalib/sun67.htx/node202.html') diff --git a/src/slalib/sun67.htx/node202.html b/src/slalib/sun67.htx/node202.html new file mode 100644 index 0000000..a161c65 --- /dev/null +++ b/src/slalib/sun67.htx/node202.html @@ -0,0 +1,254 @@ + + + + +Celestial Coordinate Systems + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: Precession and Nutation +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Using vectors +

+

+

+Celestial Coordinate Systems +

+SLALIB has routines to perform transformations +of celestial positions between different spherical +coordinate systems, including those shown in the following table: +

+

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
systemsymbolslongitudelatitudex-y planelong. zeroRH/LH
horizon-azimuthelevationhorizontalnorthL
equatorial$\alpha,\delta$R.A.Dec.equatorequinoxR
local equ.$h,\delta$H.A.Dec.equatormeridianL
ecliptic$\lambda,\beta$ecl. long.ecl. lat.eclipticequinoxR
galactic$l^{I\!I},b^{I\!I}$gal. long.gal. lat.gal. equatorgal. centreR
supergalacticSGL,SGBSG long.SG lat.SG equatornode w. gal. equ.R
+Transformations between $[\,h,\delta\,]$ and $[\,Az,El~]$ can be performed by +calling +sla_E2H +and +sla_H2E, +or, in double precision, +sla_DE2H +and +sla_DH2E. +There is also a routine for obtaining +zenith distance alone for a given $[\,h,\delta\,]$,sla_ZD, +and one for determining the parallactic angle, +sla_PA. +Three routines are included which relate to altazimuth telescope +mountings. For a given $[\,h,\delta\,]$ and latitude, +sla_ALTAZ +returns the azimuth, elevation and parallactic angle, plus +velocities and accelerations for sidereal tracking. +The routines +sla_PDA2H +and +sla_PDQ2H +predict at what hour angle a given azimuth or +parallactic angle will be reached. +

+The routines +sla_EQECL +and +sla_ECLEQ +transform between ecliptic +coordinates and $[\,\alpha,\delta\,]$; 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 $[\,\alpha,\delta\,]$ there are +two sets of routines, depending on whether the $[\,\alpha,\delta\,]$ is +old-style, B1950, or new-style, J2000; +sla_EG50 +and +sla_GE50 +are $[\,\alpha,\delta\,]$ to $[\,l^{I\!I},b^{I\!I}\,]$ 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 $[\,l^{I\!I},b^{I\!I}\,]$ 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 +$[\,\alpha,\delta\,]$ 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 $[\,Az,El~]$, 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. +

+
+

  + + + +
Figure 1: +Relationship Between Celestial Coordinates
\begin{figure} +\begin{center} +\begin{tabular} +{\vert cccccc\vert} \hline +& & & &... + ...2000, all of the precession and E-terms corrections +are superfluous.\end{figure}
+
+
+
+ +next + +up + +previous +
+ Next: Precession and Nutation +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Using vectors +

+

+

+SLALIB --- Positional Astronomy Library
Starlink User Note 67
P. T. Wallace
12 October 1999
E-mail:ptw@star.rl.ac.uk +
+ + -- cgit