Tycho's Supernova

Brahe made observations of a supernova in 1572. This was a "star" that appeared suddenly where none had been seen before, and was visible for about eighteen months before fading from view. Since this clearly represented a change in the sky, prevailing opinion held that the supernova was not really a star but some local phenomenon in the atmosphere. (Remember: the heavens were supposed to be unchanging in the view of Aristotle and his followers.)

Brahe's careful observations showed that the supernova did not change positions with respect to the other stars (no parallax). Therefore, it was a real star, not a local object. Because he could see no parallax for the supernova, Tycho was able to show that the supernova was at least as far away as the orbit of the Moon, as illustrated in the adjacent diagram. Notice that the further away the star is, the smaller the parallax angle. Since he could see no parallax, this implied that it was at least as small as the smallest angle he could reliably measure, and that in turn implied that it was further than the Moon. The diagram shown above is only schematic; in reality the parallax angles are much smaller than illustrated here and stars are much further away than the Moon.