Observatio & demonstratio cometae aetherei, qui anno 1577 et 1578 constitutus in sphaera veneris, apparvit, cum admirandis eius passionibus, varietate scilicet motus, loco, orbe, distantia à terrae centro, &c. adhibitis demonstrationibus geometricis & calculo arithmetico, cuiusmodi de alio quoquam cometa nunquam visa est

The path of the Comet under the starry sky.

This Comet was seen by me initially on the 12th of November, not far from the arc of Sagittarius in the sphere of the fixed stars, near the northern limit of the Zodiac, toward the ecliptic (if indeed the Zodiacal latitude ought to be set at 16 degrees), in the first degrees of Sagittarius, in the Milky Way, more western and more northern than Saturn, which then stood by the stars in the head of Sagittarius. It was also joined almost to the Moon according to the longitude of the circle of signs, except that she had receded some degrees to the south. At that hour the Sun was at the beginning of Sagittarius, Venus at 18 degrees 30 minutes of Sagittarius, Jupiter at 6 degrees 30 minutes of Aries, Mars at 28 degrees of Taurus, Saturn from the observation of the sky at 9 degrees 30 minutes of Sagittarius, having a latitude of 0 degrees 52 minutes north. The Moon, however, by apparent motion through parallax, was at 5 degrees of Sagittarius, with a latitude of 5 degrees 40 minutes south. But the Comet was at nearly 4 degrees of Aries, having a latitude of 7 degrees north, as will be said below. From this place, day by day it began to approach closer and closer to the zenith of our head, initially moving most swiftly, but afterward gradually restraining its course until it became very slow at the end of its appearance. It chose a great circle as the way of its journey under the ultimate sky from the arc of Aries straight to the breast of Pegasus; for it progressed from the place of its first appearance through the altar of Antinous (as some call it) and his knees and the hands of the one praying, leaving the star in his knees slightly to the north, and casting away that which is in his hands to the south, touching both. Then it directed its course to the little horse, through whose two small stars in its nostrils it passed. On the last day on which it was contemplated and observed by me, it was closest to the two small stars in the breast of Pegasus. It vanished entirely before it reached the brighter stars of Pegasus, which the common people are accustomed to call the Cross of the Savior because they are placed in a quadrangular figure.

Position, shape, and magnitude of the tail.

When the hair was first observed by me, it extended from the first degrees of Capricorn, established in the north, through the stars in the veil of Sagittarius, a little beyond those which are in the forehead and jaw of Capricorn, and it cast it a few degrees beyond the ecliptic into the south. In this trajectory, it involved the stars which are in the canonical description of the asterism of Sagittarius, the 13th, 14th, 15th, and 17th, but those which are the 12th and 16th it did not cover, but grazed the former with its southern edge and the latter with its northern. From there it descended to Capricorn, and walking under its horns, it occupied those which are located in the forehead and jaw; the star in the neck of Capricorn was also barely seen free from the rays of the hair. For from that point, the width of the hair inclined toward the south by several degrees beyond the ecliptic. Moreover, that width was very unequal to itself, namely, narrower at the beginning, wider near the end. For emanating from the head, which was not so large, the hair spread out, dilating more and more on both sides, until at the forehead of Capricorn, it obtained a width of five degrees and somewhat more.