Spring that year (1683) started showing off a most unusual sight in the sky. Cassini described it as follows: 

a light, similar to that which whitens the Milky Way, but brighter and more brilliant at its centre and fainter at its ending tips, spread through the signs the Sun crosses during this part of the season. 

I started seeing it from the Royal Observatory on the evening of 18 March, two days before the equinox. Shortly, after observing the changes that arose on the planet Saturn, I recognized the first star of the Ram. With the help of lenses it can be seen as a twin star: each star is distant from the other the length of both their diameters put together. 

I saw this constellation and that of Taurus much more luminous than ordinary around seven and three quarters, half an hour after dusk ended. 

On the Western side, only the rays at the horizon retained such a luminescence up to a height of two, three degrees and the brighter part extended for about eight, nine degrees. 

It extended, along the Zodiac, almost obliquely, reaching the Northern edge of the two most luminous stars of the Ram’s head. It crossed the whole body of the Ram, extended towards Pleiades and vanished quietly and gently in Taurus’s head. 
 
 
 
 
 
 

                                       Resemblance with the tail of a comet ... 

This part of the sky was extremely clear; by naked eye it was possible to see the stars with magnitude six and seven. This luminescence, though similar to a mist floodlit by the Sun rays, did not prevent the vision  of these small stars, even in its middle, which was thicker, in a similar way as they could be seen through the tails of the comets. Its width, however, was too extended to mistake it for a comet’s tail, as it was 3 or 4 times the width of the biggest (tails) I ever saw up to now. 

Except for its width, all other particulars were very much similar to a tail, not only for its transparency, but also for its colour and its position with respect to the Sun, towards which it, more or less, pointed lengthways. 

It was soon understood it followed the motion of the sky towards West. Following this motion, it always stationed within the same constellations and, together with them, plunged into the fog at the horizon. I doubted whether it slightly moved towards North, as  the two most brilliant stars of the Ram became a part of this luminescence, which, at first, just touched the Northern side of this constellation. This event was also confirmed by the observations carried out during the following days. 

There and then I was not completely sure, nor was I days later. Everywhere the extreme end of this luminescence was uncertain. It became more and more feeble. It was extremely difficult to exactly determine its border lines through the different shades of brightness the air showed at different distances. At twilight, during the following days, it appeared more or less diffused. Therefore, when the evening just started, an hour after sunset, that part of luminescence, that was mostly visible, extended in breadth up to the most brilliant stars of the Ram and lengthways up to Pleiades. Shortly after it modelled both. Its central part, however, as could be ascertained by naked eye, seemed always motionless in the same point and pointed towards the centre of the Ram. 

Once this constellation and that of Taurus waned, I tried to see if some trace of this luminescence was still present where it had showed at first, at a certain height and position, but nothing extraordinary could be seen there anymore. This made me understand it followed the daily revolution of these two constellations around the Earth: it waned with them during the following days and appeared together with them in the very same points where it had been seen during the previous days. For the Copernicans this is alike being motionless in the sky, always in the same point, during the daily revolution of the elementary sphere about the Earth’s axis from West to East. 
 
 
 
 
 
 

                                             Observation peculiarities ... 

Samely, I observed it from 18 to 26 March, whenever, in the evening, the sky was clear  at West, without noticing any further evident changes, except for those noticed during my last observation, which I carried out on 26 March: this light did not seem to extend towards the horns of Taurus, but more towards North, while the most brilliant star of the Ram, that in the beginning just passed closed to its side plunged  more than one degree into this light. 

During this last observation I could not discover the first star of this constellation, because it was quite low and deep amidst fogs, that at West diminished the extension of the light much more than in previous observations. 

Most probably without this obstacle and without the twilight, it could have been seen more extended towards West and quite near to the Sun that, in the beginning, was in the penultimate of Pisces. It wandered away 30 degrees from the first of Ram and during the last observation, carried out on 26 March, it moved away just slightly more than 22 degrees. If this luminescence could have been seen conjoined with the Sun, it would have shown up as a coma. 

After this period, as the sky appeared cloudy at West in the evening, I was not able to verify if such luminescence had dissolved, except on 14, 22, 24 and 28 April. During these days, even if after dawn, the constellation of the Ram was hidden, the same luminescence could still be seen in Taurus, extended up to its Northern horn. Northerly, it neared the head of Medusa and the Southern knee of Perseus, while its Southern foot was immersed into this light. 

Therefore, during these last observations I was able to see this luminescence advancing slightly towards North, in a better way than before. This, while the Sun neared Taurus, prevented it to show so brilliant, due to the evening twilight. 
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In past Memories it was very difficult to find something similar to this new light that, for many days, was stationary in the same celestial signs (zodiac signs) without showing any evident particular motion. Its extension, mainly in width, was great and no comet that could have been the cause of its origin was to be seen. 

What may be considered nearer to this circumstance, to its duration, consistency and direction towards the meeting with the Sun, was an event I saw in Bologna during year 1668, when I was invited over to France and where His Majesty, the King of France, requested me to join the Academie Royale des Sciences (Royal Academy of Sciences). It was a path of light similar to the tail of a comet: it took up a space 30 degrees in length and a little more than 1 degree in width. 

On 10 March, I observed it at the horizon, peeping out from the clouds that hid the constellation of Cetus or Whale, while on the Eastern side it pointed towards the foot of Orion and on the Western side towards the Sun. Its longitude was in relation with the signs of the Ram and of Taurus, but its latitude was manly Southerly. It changed position among the fixed stars, following a particular motion towards East and towards North, nearing Orion day after day. It remained visible up to 19 March and, during these nine days, it passed opposite different stars of Eridanus, never hiding any of them. 
 
 
 
 
 
 
 
 
 
 

Chardin, while describing the coronation of Suleiman, King of Persia, refers that in 1668 a similar event was observed in the capital of one of the Persian Provinces on 7 March, the second day after its first apparition. Later on it showed up on 10 March, at 7 in the evening, at Ispahal, capital of the Kingdom. 

It showed up in the Southern side, following the celestial vault. It was 30 degrees and 32 minutes long. Data meets with our observations. It was 6 degrees large almost everywhere, 4 times more than the one I observed in Bologna. Yet, some people saw it larger. Its width, however, was difficult to estimate, as at its extreme end it was feeble, dissolving little by little. Chardin added its upper part pointed towards the Orion’s Belt and the river Eridanus. 

From my point of view, longitude 72 degrees and latitude of the ecliptic 3 degrees do not comply with this position, as Eridanus and the Orion’s Belt are much more Northerly and Westerly. 

He also added its lower end was Cetus or the bend of Eridanus, which exactly coincides with my observation that focuses it where the hollow of Cetus touches the bend of Eridanus, without respect to the longitude and the latitude he gives to this ending part, where numbers seem to be incorrect. He says Persians called it Niazach, which means small spear because of its appearance. He also reported they had never seen or heard of a similar event. He said they thought it was a comet, its head hidden in the West, so that it could not be seen at the horizon. 
 
 
 
 
 
 
 
 
 
 

Cassini’s hypothesis ... 

In that occasion, I was able to prove that this event was quite similar to something else that had appeared two thousand years before. An event Carimandrus , as referred by Seneca in his work "Discussions on Nature (Naturales Quaestiones)" in 7 books, says had been observed by Anaxagoras: it was a great unusual light visible for many days  and it had the size of a big log . The same author states also Callisthenes observed this light before the two great towns of Achaea, Helix and Bure, disappeared into the sea due to an earth-quake. It had the shape of a fire, quite extended in length. According to Aristotle it was a comet that did not seem so at the beginning, because of its great blaze. When time passed by, it showed its true nature as soon as the Sun started to set. 

Seneca, in chapter 6^th of book 1^st "on Meteors", writing about this event that was observed in the sky during the time of the earth-quake and of the floods that destroyed Achaea, called it Great Comet, or Great Star. He said it became visible at the Western equinoctial point, in the same way as seen by Us. After many stories and notes on similar events, he added that the Great Star, of which before, became manifest when the weather was frosty and the evenings were very clear, in the year during which Aristaeus was Archon of Athens. The first day it did not show up at all, as the Sun set early. The next day it showed a little more, as the Sun delayed a little, and set after this light had extended up to the third part of the sky, taking the shape of a line. For this reason it was called "Pathway": it extended upward to the Orion’s Belt and there dissolved. Something very similar happened to the "Pathway" of light of 1668. 

Seneca, misunderstanding this event and considering it as a comet, treated Ephorus as a liar and a swindler. Ephorus stated this great light had divided into two stars. Nobody else made such assertion, even if everybody witnessed it everywhere on earth and considered it as an omen of the submersion of the two towns. Although the apparition of this great light was certain and witnessed by all the people that observed it, it was not possible to agree on its nature, similarly as it was not possible for the present event of our days. 

There are some further Memories of ambiguous comets, which only showed a great light, same as that observed after 10 till 29 November 1618, in the Southern side of the sky, towards Hydra, before the apparition of the great comet that showed up in the Northern side towards the end of that same month and that lasted till the end of January 1619.

About the nature of this light 

This extraordinary light able to shed rays upon the Earth seemed to possess some particular material: luminous itself or able to reflect or decompose the rays of the Sun or the rays of some other luminous body, directly or with the help of a foreign body. However the direction of its length from the Sun allowed to presume it originated from the Sun itself. 

In my Compendium of Observations no. 12 carried out on the comet of year 1681, I said: in the Upper Sky there may be a diffused substance able to reflect the light, similar to what can be found in the air (atmosphere?) that surrounds the Earth. This substance meeting the comets, where the Sky is more or less clear, may favour the apparition of their tails and become the cause of the changes that arise from them. 

As this light is similar to that of the comets, both in colour and brightness, in faintness and in the relation it has with the rays of the Sun, it is possible to believe that the substance that reflects this light has a similar nature, whether it originates from a comet hidden among the rays of the Sun (an argument I should not wish to foster, since its width is so dissimilar from that of all the other comet tails observed up to now) or whether it directly receives the rays of the Sun itself, in the same way as  we can see in the air (atmosphere?) events caused by refractions and by the direct reflexes of the Sun or by similar reflexes due to the intrusion of the Moon, such as the Iris and the Haloes of this or that Star. 

No other inconvenience is there except for the appearance that, in this substance spread out in the Upper Sky, is similar whether originating from the Sun or self originating or originating from the tail end of some comet. It could even reflect the light of some Star. This happens when certain fixed Stars show a coma, as referred by Aristotle, in accordance with the observations of the Egyptians and with what he emphasized, himself having seen a coma around one of the Stars in the Leg of the great Dog, initially not brilliant, but still evident to those who carefully observed it. 

It must be remarked that our light seemed to originate from the same place where many comets passed by during this century, in years 1652, 1665, 1672, 1680 and where numerous other passed by in previous centuries and that, due to their frequency, could be seen in that belt, I called "Comet’s Zodiac" in my Treatise. 
 
 
 
 
 
 
 

Hypothesis on the distance of this luminous substance. 

Regarding the distance of such substance that originates this light or that is a medium through which this light is reflected or refracted back to Earth, it is not possible to determine this distance using the parallax, principally because of the ambiguity of its ending (part), that, at different hours during night-time and from different places around the Earth, does not allow to accurately compare such distance with the fixed Stars. Anyway it is possible to ascertain such distance is great from the circumference of its 24 hour daily motion, next to the Stars. 

Following a common thinking, could a raging wind carry this substance for a whole month through the air without dispersing it with its impetuosity, pushing it around the Earth in such uniform way and allowing it to always match the same constellations? On the contrary, if the Copernican hypothesis must be followed, with what kind of force could it try to put up a resistance to the daily motion of the elementary sphere from West to East, without being dispersed or diverted? It then must be admitted that this substance is above the elementary sphere and consequently is in the Upper Sky. Considering that its particular motion is imperceptible, it must be assumed that this substance is extremely noble and near to the region of the Stars. 

Our ancestors were right when they deemed nearer towards the fixed stars those stars that, among the planets, are nearer to their universal motion and less to their particular motion. Only for this precise reason they deemed Saturn most noble among all other planets and placed Jupiter above it. No astronomer in the last 20 or 30 centuries ever doubted  this convincement. They even confirmed (their assumptions) with new hypothesis used to represent the motion of such events, even though (such hypotheses) were diverse one from another and sometimes even conflicting, as the Copernican theory was towards the Ptolemaic theory and that of Tycho’s. Each theory explained the order of the superior planets as ascertained by our ancestors. Being impossible to explain such order without the aid of a hypothesis, each single theory made use of particular conjectures.  

These two planets have no definite parallax, because the diameter of their circumferences and that of the Earth’s is not proportioned. It is therefore a good rule to establish the universal position of any new object, taking into consideration its motion and that of other bodies known to us, which, through astronomical observations, are found to be lined up at different distances in accordance to their different apparent velocity.