Proper Motion

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/node207.html | 119 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 119 insertions(+) create mode 100644 src/slalib/sun67.htx/node207.html (limited to 'src/slalib/sun67.htx/node207.html') diff --git a/src/slalib/sun67.htx/node207.html b/src/slalib/sun67.htx/node207.html new file mode 100644 index 0000000..6c74c4e --- /dev/null +++ b/src/slalib/sun67.htx/node207.html @@ -0,0 +1,119 @@ + + + + +Proper Motion + + + + + + + + + + + + +

+ +

+
+ Next: Parallax and Radial Velocity +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Epoch +

+Proper Motion +

+Stars in catalogues usually have, in addition to the + $[\,\alpha,\delta\,]$ coordinates, a proper motion $[\mu_\alpha,\mu_\delta]$ .This is an intrinsic motion +of the star across the background. Very few stars have a +proper motion which exceeds $1\hspace{-0.05em}^{'\hspace{-0.1em}'}$ /year, and most are +far below this level. A star observed as part of normal +astronomy research will, as a rule, have a proper motion +which is unknown. +

+Mean $[\,\alpha,\delta\,]$ and rate of change are not sufficient to pin +down a star; the epoch at which the $[\,\alpha,\delta\,]$ was or will +be correct is also needed. Note the distinction +between the epoch which specifies the +coordinate system and the epoch at which the star passed +through the given $[\,\alpha,\delta\,]$ . The full specification for a star +is $[\,\alpha,\delta\,]$ , proper motions, equinox and epoch (plus something to +identify which set of models for the precession etc. is +being used - see the next section). +For convenience, coordinates given in star catalogues are almost +always adjusted to make the equinox and epoch the same - for +example B1950 in the case of the SAO catalogue. +

+SLALIB provides one routine to handle proper motion on its own, +sla_PM. +Proper motion is also allowed for in various other +routines as appropriate, for example +sla_MAP +and +sla_FK425. +Note that in all SLALIB routines which involve proper motion +the units are radians per year and the + $\alpha$ component is in the form $\dot{\alpha}$ (i.e. big +numbers near the poles). +Some star catalogues have proper motion per century, and +in some catalogues the $\alpha$ component is in the form + $\dot{\alpha}\cos\delta$ (i.e. angle on the sky). +

+ +

+
+ Next: Parallax and Radial Velocity +
+Up: EXPLANATION AND EXAMPLES +
+ Previous: Epoch +

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

+ + -- cgit