AbstractAt the very beginning of the last millennium Ibn al-Haytham greatly contributed to the investigation of the Milky Way. Here, the only three currently known versions of his work on the location of the Milky Way are compared to each other and discussed. A comparison of the texts and an early translation into German by E. Wiedemann in 1906 reveals several differences that triggered a new critical translation of the passed down text. We give detailed comments on the work and check the validity of Ibn al-Haytham's arguments. We also discuss his work in the framework of the ‘Great Debate’ on the Milky Way that took place around 1920, more than a decade after Wiedemann's translation. We find that Ibn al-Haytham's work is certainly at the peak of the unaided-eye era of the Milky Way's discovery. Through his own argumentation and in comparison to Ptolemy's observations Ibn al-Haytham clearly identifies the Galaxy as an extraterrestrial body that is not part of the atmosphere but much further away than the Moon. With some of his statements on the stellar positions passed down by Ptolemy, Ibn al-Haytham also anticipates the concept of stellar proper motions.
Stellar Proper Motions in the Orion Nebula Cluster
