https://github.com/CelestiaProject/Celestia/blob/68c192a55abb5565a104ab60567e918a2d6d7da4/README.md; avsnitt, vers eller paragraf: License; verkets språk: engelsk; besøksdato: 18. februar 2018; sitat: 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 of the License, or (at your option) any later version..