To cite PROJ in publications use:

  Stellarium contributors (2023). Stellarium v23.4 Astronomy Software. 
  URL https://stellarium.org/.
  DOI: 10.5281/zenodo.10427779

A BibTeX entry for LaTeX users is

.. code-block:: latex

  @Stellarium{,
    title = {{Stellarium} v23.4 Astronomy Software},
    author = {{Stellarium contributors}},
    year = {2023},
    url = {https://stellarium.org/},
    doi = {10.5281/zenodo.10427779},
  }