Bessel, Friedrich (1839). „On the parallax of the star 61 Cygni”. London and Edinburgh Philosophical Magazine and Journal of Science. 14: 68—72. Bessel's statement that light employs 10.3 years to traverse the distance.
Ule, Otto (1851). „Was wir in den Sternen lesen”. Deutsches Museum: Zeitschrift für Literatur, Kunst und Öffentliches Leben. 1: 721—738.
Junor, W.; Biretta, J. A. (1994), „The Inner Light-Month of the M87 Jet”, Ур.: Zensus, J. Anton; Kellermann; Kenneth I., Compact Extragalactic Radio Sources, Proceedings of the NRAO workshop held at Socorro, New Mexico, February 11–12, 1994, Green Bank, WV: National Radio Astronomy Observatory (NRAO), стр. 97, Bibcode:1994cers.conf...97J