From his observations of the a.d. 1572 supernova and the a.d. 1577 comet, Tycho Brahe concluded that such transient celestial objects are outside the Earth’s atmosphere, and he quoted the ninth century a.d. Persian astrologer and astronomer Abū Macshar: Dixit Albumasar, Cometa supra Venerem visus fuit; i.e., the latter had reported much earlier that comets were seen beyond Venus. However, even from a more detailed Latin translation, the observations and logic behind Abū Macshar’s conclusion were not understandable. We present here the original Arabic text (manuscript Ankara, Saib 199) together with our translation and interpretation: Abū Macshar reported that he had observed Venus in (or projected onto) the tail of a comet and concluded that the comet was behind Venus because he had observed the extinction of Venus due to the cometary tail to be negligible (light of Venus was unimpaired). He then concluded that the comet would be located behind Venus. He also mentioned that others had observed Jupiter and Saturn in cometary tails, so that those comets would even be located beyond those two outer planets – in the sphere of the stars. The dates of the observed close conjunctions were not mentioned; using known orbital elements for a few comets, we found a few close conjunctions between comets and planets from a.d. 770 to 868, but we cannot be sure regarding which conjunctions were reported. While the argument of Abū Macshar is not correct (as cometary tails are optically thin), the conclusion – namely that comets are outside the Earth’s atmosphere and beyond the moon – is correct. This may have helped Tycho Brahe to come to his revolutionary conclusion.
