Perihelion: 1680 December 18.49, q = 0.006 AU
This particular comet was undoubtedly one of the brightest comets of the 17th Century, but it is also one of the most important comets in history from a scientific perspective, and perhaps even from the perspective of overall human history. While there were certainly plenty of superstitions attached to the comet’s appearance, the scientific investigations made of it were among the beginnings of the era in European history we now call The Enlightenment, and indeed, in a sense the Great Comet of 1680 can perhaps be considered as one of the sparks of that era.
The significance began with the comet’s discovery, which was made on the morning of November 14, 1680, by a German astronomer residing in Coburg, Gottfried Kirch – the first comet ever to be discovered by means of a telescope. It was already around 4th magnitude at that time, and located near the star Regulus in the constellation Leo; from that point it traveled eastward and brightened rapidly, being closest to Earth (0.42 AU) on November 30. By that time it was a conspicuous naked-eye object with a tail 20 to 30 degrees long, and it remained visible for another week before disappearing into morning twilight.
Around the time of perihelion passage on December 18 the comet was reported as being visible during the daytime by several observers around the world. Within a few days it began to emerge into the evening sky, and during the last week of December was reported as being a brilliant object with a tail up to 70 degrees long. After being closest to Earth again (0.49 AU) on January 4, 1681 it faded as it receded from the inner solar system, remaining visible to the unaided eye until early February. The final telescopic observation was made on March 19 – by which time its heliocentric distance had increased to 2.2 AU – and was made by none other than the eminent British physicist Isaac Newton.
It was right around this time that Newton was developing his theory of Universal Gravitation, and while he was not the only person to attempt calculating an orbit for the Great Comet of 1680, he did so within the context of his ideas concerning gravity. A significant sticking point was that he initially believed that the pre-perihelion appearance of the comet in the morning sky, and the post-perihelion appearance in the evening sky, were in fact two separate comets, and it wasn’t until observations by his contemporary John Flamsteed and by his friend Edmond Halley conclusively demonstrated this that he finally, and somewhat begrudgingly, accepted that these were in fact one and the same comet. Once he did so he was able to apply his law of Universal Gravitation to the comet and determine a reasonable orbit for it. (Among other things, his work produced a mathematical foundation for Kepler’s Three Laws of Planetary Motion, which Kepler had arrived at empirically.) Newton’s work on the Great Comet of 1680 featured somewhat prominently in his Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), usually known as the Principia, wherein he laid out his ideas on gravitation.
The publishing of the Principia is often considered as being one of the first significant events of The Enlightenment. Halley played a major role in funding the publishing of the Principia, and encouraged his friend Newton to apply his methods to calculating the orbits of additional comets. It would be Halley himself, though, who would end up taking on that task, and in the process he would “discover” the comet that now bears his name. This story is told in that object’s “Special Topics” presentation.
The Great Comet of 1680’s unusually small perihelion distance is reminiscent of that of the Kreutz sungrazers, and while it can accordingly be considered as a sungrazing comet, it is not, however, a member of that group. Its orbital inclination (61 degrees) is dramatically different from that of the Kreutz sungrazers (in general, 142 to 144 degrees), and while it was well visible from the northern hemisphere, as will be demonstrated by next week’s “Comet of the Week” the visibility of a Kreutz sungrazer passing through perihelion at this time of the year is pretty much restricted to the southern hemisphere.