Apparent Place to Observed Place

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/node213.html | 181 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 181 insertions(+) create mode 100644 src/slalib/sun67.htx/node213.html (limited to 'src/slalib/sun67.htx/node213.html') diff --git a/src/slalib/sun67.htx/node213.html b/src/slalib/sun67.htx/node213.html new file mode 100644 index 0000000..4d2a0e1 --- /dev/null +++ b/src/slalib/sun67.htx/node213.html @@ -0,0 +1,181 @@ + + + + +Apparent Place to Observed Place + + + + + + + + + + + + +

+ +

+
+ Next: Refraction +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Mean Place to Apparent Place +

+Apparent Place to Observed Place +

+The observed place of a source is its position as +seen by a perfect theodolite at the location of the +observer. Transformation of an apparent $[\,\alpha,\delta\,]$ to observed +place involves the following effects: +

$[\,\alpha,\delta\,]$ to $[\,h,\delta\,]$ .
Diurnal aberration. +
$[\,h,\delta\,]$ to $[\,Az,El~]$ .
Refraction. +

+The transformation from apparent $[\,\alpha,\delta\,]$ to +apparent $[\,h,\delta\,]$ is made by allowing for +Earth rotation through the sidereal time, $\theta$ : +

$\begin{displaymath} +h = \theta - \alpha \end{displaymath}$

+For this equation to work, $\alpha$ must be the apparent right +ascension for the time of observation, and $\theta$ must be +the local apparent sidereal time. The latter is obtained +as follows: +

1. +: from civil time obtain the coordinated universal time, UTC +(more later on this); +
2. +: add the UT1-UTC (typically a few tenths of a second) to + give the UT; +
3. +: from the UT compute the Greenwich mean sidereal time (using +sla_GMST); +
4. +: add the observer's (east) longitude, giving the local mean + sidereal time; +
5. +: add the equation of the equinoxes (using +sla_EQEQX). +

+The equation of the equinoxes ( $=\Delta\psi\cos\epsilon$ plus +small terms) +is the effect of nutation on the sidereal time. +Its value is typically a second or less. It is +interesting to note that if the object of the exercise is to +transform a mean place all the way into an observed place (very +often the case), +then the equation of the +equinoxes and the longitude component of nutation can both be +omitted, removing a great deal of computation. However, SLALIB +follows the normal convention and works via the apparent place. +

+Note that for very precise work the observer's longitude should +be corrected for polar motion. This can be done with +sla_POLMO. +The corrections are always less than about + $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.3$ , and +are futile unless the position of the observer's telescope is known +to better than a few metres. +

+Tables of observed and +predicted UT1-UTC corrections and polar motion data +are published every few weeks by the International Earth Rotation Service. +

+The transformation from apparent $[\,h,\delta\,]$ to topocentric + $[\,h,\delta\,]$ consists of allowing for +diurnal aberration. This effect, maximum amplitude + $0\hspace{-0.05em}^{'\hspace{-0.1em}'}\hspace{-0.4em}.2$ , +was described earlier. There is no specific SLALIB routine +for computing the diurnal aberration, +though the routines +sla_AOP etc. include it, and the required velocity vector can be +determined by calling +sla_GEOC. +

+The next stage is the major coordinate rotation from local equatorial +coordinates $[\,h,\delta\,]$ into horizon coordinates. The SLALIB routines +sla_E2H +etc. can be used for this. For high-precision +applications the mean geodetic latitude should be corrected for polar +motion. +

+ + +

+Refraction +
+Efficiency considerations +

+ +

+
+ Next: Refraction +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Mean Place to Apparent Place +

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

+ + -- cgit