From a13aa0b9e3f59078e5e049c0f70974e7a465f78b Mon Sep 17 00:00:00 2001
From: Jürgen Knödlseder <jurgen.knodlseder@irap.omp.eu>
Date: Fri, 26 Feb 2016 09:45:06 +0100
Subject: [PATCH] Add HowTo for sky coordinate transformation.

---
 doc/source/howto/sky.rst | 33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/doc/source/howto/sky.rst b/doc/source/howto/sky.rst
index 90cc081..1c85a56 100644
--- a/doc/source/howto/sky.rst
+++ b/doc/source/howto/sky.rst
@@ -1,6 +1,37 @@
-Sky maps, sky coordinates and sky regions
+Sky coordinates, sky maps and sky regions
 -----------------------------------------
 
+How to convert a sky coordinate from celestial to Galactic?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To convert from celestial to Galactic coordinates you can set a sky 
+direction in celestial coordinates and read it back in Galactic
+coordinates.
+
+Python:
+
+.. code-block:: python
+
+    >>> import gammalib
+    >>> dir=gammalib.GSkyDir()
+    >>> dir.radec_deg(83.63,22.01)
+    >>> l=dir.l_deg()
+    >>> b=dir.b_deg()
+    >>> print(l,b)
+    (184.55973405309402, -5.7891829467816827)
+
+C++:
+
+.. code-block:: cpp
+
+    #include "GammaLib.hpp"
+    GSkyDir dir;
+    dir.radec_deg(83.63,22.01);
+    double l = dir.l_deg();
+    double b = dir.b_deg();
+    std::cout << l << ", " << b << std::endl;
+
+
 How to convert sky map projections?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
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