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/node223.html | 139 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 139 insertions(+) create mode 100644 src/slalib/sun67.htx/node223.html (limited to 'src/slalib/sun67.htx/node223.html') diff --git a/src/slalib/sun67.htx/node223.html b/src/slalib/sun67.htx/node223.html new file mode 100644 index 0000000..497a385 --- /dev/null +++ b/src/slalib/sun67.htx/node223.html @@ -0,0 +1,139 @@ + + + + +Geocentric Coordinates + + + + + + + + + + + + +
+ +next + +up + +previous +
+ Next: Ephemerides +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Calendars +

+

+

+Geocentric Coordinates +

+The location of the observer on the Earth is significant in a +number of ways. The most obvious, of course, is the effect of latitude +on the observed $[\,Az,El~]$ of a star. Less obvious is the need to +allow for geocentric parallax when finding the Moon with a +telescope (and when doing high-precision work involving the +Sun or planets), and the need to correct observed radial +velocities and apparent pulsar periods for the effects +of the Earth's rotation. +

+The SLALIB routine +sla_OBS +supplies details of groundbased observatories from an internal +list. This is useful when writing applications that apply to +more than one observatory; the user can enter a brief name, +or browse through a list, and be spared the trouble of typing +in the full latitude, longitude etc. The following +Fortran code returns the full name, longitude and latitude +of a specified observatory: +

+            CHARACTER IDENT*10,NAME*40
+            DOUBLE PRECISION W,P,H
+             :
+            CALL sla_OBS(0,IDENT,NAME,W,P,H)
+            IF (NAME.EQ.'?') ...             (not recognized)
+
+

(Beware of the longitude sign convention, which is west +ve +for historical reasons.) The following lists all +the supported observatories: +

+             :
+            INTEGER N
+             :
+            N=1
+            NAME=' '
+            DO WHILE (NAME.NE.'?')
+               CALL sla_OBS(N,IDENT,NAME,W,P,H)
+               IF (NAME.NE.'?') THEN
+                  WRITE (*,'(1X,I3,4X,A,4X,A)') N,IDENT,NAME
+                  N=N+1
+               END IF
+            END DO
+
+

+The routine +sla_GEOC +converts a geodetic latitude +(one referred to the local horizon) to a geocentric position, +taking into account the Earth's oblateness and also the height +above sea level of the observer. The results are expressed in +vector form, namely as the distance of the observer from +the spin axis and equator respectively. The geocentric +latitude can be found be evaluating ATAN2 of the +two numbers. A full 3-D vector description of the position +and velocity of the observer is available through the routine +sla_PVOBS. +For a specified geodetic latitude, height above +sea level, and local sidereal time, +sla_PVOBS +generates a 6-element vector containing the position and +velocity with respect to the true equator and equinox of +date (i.e. compatible with apparent $[\,\alpha,\delta\,]$). For +some applications it will be necessary to convert to a +mean $[\,\alpha,\delta\,]$ frame (notably FK5, J2000) by multiplying +elements 1-3 and 4-6 respectively with the appropriate +precession matrix. (In theory an additional correction to the +velocity vector is needed to allow for differential precession, +but this correction is always negligible.) +

+See also the discussion of the routine +sla_RVEROT, +later. +

+

+ +next + +up + +previous +
+ Next: Ephemerides +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Calendars +

+

+

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