next up previous contents
Next: Example: galactic2equatorial program Up: Galactic Modelling Previous: Example: caltech_lst program   Contents


Function: galactic_to_equatorial()

void galactic_to_equatorial(float l, float b, float *alpha, float *delta)

This routine converts the coordinates of an object from the Galactic system--Galactic longitude $\ell$ and latitude $b$--to the equatorial system--right ascension $\alpha$ and declination $\delta$. The arguments are:

l: Input. The Galactic longitude $\ell$ (radians).
b: Input. The Galactic latitude $b$ (radians).
alpha: Output. The right ascension $\alpha_{1950}$ (radians).
delta: Output. The declination $\delta_{1950}$ (radians).

The transformation is the following:

\begin{displaymath}
\delta = \arcsin[
\cos b\, \cos\delta_{\scriptstyle\mathrm...
...{ascend}}) + \sin b\,
\sin\delta_{\scriptstyle\mathrm{NGP}} ]
\end{displaymath} (12.3.280)

and
\begin{displaymath}
\alpha = \arctan\left[
\frac{ \cos b\, \cos(\ell - \ell_{\...
...athrm{ascend}}) } \right]
+ \alpha_{\scriptstyle\mathrm{NGP}}
\end{displaymath} (12.3.281)

where
$\alpha_{\scriptstyle\mathrm{NGP}}=192\makebox[\deglen]{\makebox[0pt]{$^\circ$}\makebox[0pt]{.}}25$ is the right ascension (1950) of the North Galactic Pole
$\delta_{\scriptstyle\mathrm{NGP}}=27\makebox[\deglen]{\makebox[0pt]{$^\circ$}\makebox[0pt]{.}}4$ is the declination (1950) of the North Galactic Pole
$\ell_{\scriptstyle\mathrm{ascend}}=33^\circ$ is the ascending node of the Galactic plane on the equator.

Author: Jolien Creighton, jolien@tapir.caltech.edu
Comments: This routine is adapted from the method given in: Peter Duffet-Smith Practical Astronomy with Your Calculator, 3rd edition, (Cambridge University Press, 1988). The values used in this routine should be updated to epoch 2000 coordinates.


next up previous contents
Next: Example: galactic2equatorial program Up: Galactic Modelling Previous: Example: caltech_lst program   Contents
Bruce Allen 2000-11-19