De Celestia wiki

Celestia est sous licence GNU General Public License, ce qui signifie, je cite :

This program is free software; you can redistribute it and/or modify it under the terms of the GNU 
General Public License as published by the Free Software Foundation; either version 2, or (at your
option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without 
even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  
General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, 
write to the Free Software Foundation, Inc., 59 Temple Place, Boston, MA 02111.

Le programme est donc gratuit, mais sans aucune garantie. Il faut aller jeter un coup d'oeil sur la version de base GNU General Public License :

the GNU General Public License is intended to guarantee your freedom to share and change free 
software--to make sure the software is free for all its users.

Gratuité, et les modifications sont autorisées.

For example, if you distribute copies of such a program, whether gratis or for a fee, you must give 
the recipients all the rights that you have.  You must make sure that they, too, receive or can get 
the source code.  And you must show them these terms so they know their rights.

Distribution des copies autorisées uniquements avec la sources les accompagnant.

Théoriquement, il faudrait ajouter ça :

a) You must cause the modified files to carry prominent notices stating that you changed the files 
   and the date of any change.
b ) You must cause any work that you distribute or publish, that in whole or in part contains or is 
    derived from the Program or any part thereof, to be licensed as a whole at no charge to all third
    parties under the terms of this License.
c) If the modified program normally reads commands interactively when run, you must cause it, when 
   started running for such interactive use in the most ordinary way, to print or display an 
   announcement including an appropriate copyright notice and a  notice that there is no warranty (or
   else, saying that you provide a warranty) and that users may redistribute the program under these 
   conditions, and telling the user how to view a copy of this License.  (Exception: if the Program 
   itself is interactive but does not normally print such an announcement, your work based on the 
   Program is not required to print an announcement.)

Toutes les créations découlant de Celestia seront soumises aux mêmes gauche d'auteur.