  From his observations of the A.D. 1572 super-nova and the A.D. 1577 comet,
Tycho Brahe concluded that such transient celestial objects are outside the
Earth's atmosphere, and he quoted the 9th century A.D. Persian astrologer and
astronomer Abu Mashar: Dixit Albumasar, Cometa supra Venerem visus fuit, i.e.
that he had reported much earlier that comets were seen beyond Venus. However,
even from a more detailed Latin translation, the observations and logic behind
Abu Mashar's conclusion were not understandable. We present here the original
Arabic text (MS Ankara, Saib 199) together with our translation and
interpretation: Abu Mashar 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 Abu Mashar is not correct (as
cometary tails are optically thin), parts of the conclusion - namely that
comets are outside the Earth atmosphere and beyond the moon - is correct. This
may have helped Tycho Brahe to come to his revolutionary conclusion.
