The Danzig halo display on February 20, 1661, is probably the most famous of all halo displays and is vividly discussed even nowadays. It was observed by the excellent astronomer Johannes Hevelius, who describes the phenomenon in his book “Mercurius in Sole visus “ with a drawing. A reproduction of this drawing can be found in almost every book dealing with halos. The special thing in this display is the observation of fragments of a 90°-halo around the sun, with sundogs visible at its points of intersection with the parhelic circle. The existence of these halos could not have been proved by unmistakable photographs up to now. There is also no theoretical explanation. So it is often supposed that Hevelius´ observation was not exact or that his drawing is misinterpreted. Tape proposes that the 90°-halo in reality is a subhelic arc. The same is what Greenler thinks. Hastings means that a halo display that is observed only once in about 250 years cannot exist.
During a research for old halo drawings I found a script by a parson from Danzig
with the title:
“Sevenfold wonder of the sun or seven sundogs, observed in this year 1661, on February 20 on Sexagesima Sunday from 11 to 12 o´clock here in the sky.”
The original is in the Herzog August Bibliothek Wolfenbüttel, which was so kind to make a copy on microfilm on my request. The script was written by the then parson of the St. Marien Church at Danzig, Georg Fehlau, and reports in German on the same halo display Hevelius also saw in Danzig. The script quotes a sermon Fehlau held at the St. Marien Church on March 6, that is only tow weeks after the halo display occurred. This sermon considers carefully the details of the halo display, “the town has been talking about during these 14 days and still talks and that will be talked of a lot in future.” Fehlau writes the phenomenon was observed by more than a thousand eyes. My first thought was that it was a parallel observation of Hevelius´observation which might solve the riddle around Hevel´s halo.
Unfortunately this speculation could not be confirmed. The description of the phenomenon by Fehlau is almost identical with that by Hevelius. First I was puzzled by that, because Hevelius book was published only in 1662, about one year after Fehlau´s script. In an annotation Fehlau says that the description of the display goes back to Hevelius. According to himself, he visited Hevelius on March 3, 1661, in order to watch a comet together with him. It is very probable that Hevelius left over the records on the halo display to Fehlau at this occasion. So Hevelius must have written down his observation directly after the halo display had occurred.
Here I quote the description of the display by Fehlau:
"1. We now come to the description of the recently appeared sundogs or parhelia, which had been observed a fortnight ago and which we talk of now. They were like this: A fortnight ago it was February 20 at about 11 a.m. when the sun was in the southeast and the air was all bright and clear. There were seven suns clearly visible in the sky at the same time, three colourful and three white ones additionally to the real sun. Around the latter one appeared a rather big and almost closed circle with very beautiful colours like a rainbow, on which at both sides two colourful sundogs were visible which were at the same elevation as the real sun. Both of them had long clear and white tails, tapering like comets, one pointing to the east and the other one to the west.
2. Second, on just that circle, directly above the sun, under a vertical line, there was a part of an inverted circle or rainbow, also with very beautiful colours, with another, a bit less bright sundog in it.
3. Third there was a very much larger circle with also a lot of beautiful colours around the sun, surrounding the other one, a bit fainter and not totally closed as the horizon was too near and the diameter of the circle too big, on which near the uppermost point an inverted part of a rainbow was visible, very bright and with beautiful colours.
4. Fourth there was a very white and silvery circle emerging from the two sundogs beside the real sun, which surrounded the whole horizon and was in all parts at the same distance, which was about 20 degrees: On this circle there were three other silver suns, one in the northwest, opposite the real sun. The other one in the northeast and the third one in the southwest. Through these two latter ones as eastern and western one passed a white piece of an arc coming from above, also through the big circle they were standing upon. So that through these two white sundogs a white cross seemed to pass, which appeared very amazing for about one hour and a half before everything disappeared. It was an overwhelmingly beautiful picture, in which seven suns were visible at the same time, which never had been observed before. Yes, scholars believe that, if this picture had been observed a little earlier, there would have been visible 9 suns at the same time.”
From the text, the halos can be easily identified. In the first paragraph, Fehlau
describes the 22°-halo with both parhelia. In the second paragraph he describes
the upper tangent arc. In the third paragraph he describes the 46°-halo
with the circumzenithal arc and in the fourth one the parhelic circle with anthelion
and two parhelia in the 90°-area. Through these pass fragments of a great
annular halo (Hevel´s halo).
If you compare this description with that of Hevelius, there is hardly a difference between them. Even the words and the division into paragraphs are the same. But, contrary to Hevelius´report, the exact angles are missing. Fehlau also stresses more than Hevelius, that the 90°-halo formed a cross with the parhelic circle. This pleads against the assumption that Hevel´s halo was the subhelic arc, because the subhelic arc intersects the parhelic circle in an acute angle, which hardly gives the impression of a cross. But as Fehlau´s script is no independent observation to Hevelius, the existence of a 90°-halo cannot be clarified.
Of special interest is a drawing on the last page of Fehlau´s script. Fehlau does not mention that drawing with a single word. So it cannot be said if it really is a drawing of the Danzig halo display, even if this is very probable. Circumzenithal arc, 46°-halo and upper tangent arc are missing. But the anthelion and the two anthelic sundogs with arcs passing through them, are delineated. If you look at it carefully, you can see that the anthelic sundog in the northeast is drawn at a distance of about 110° from the sun, while the southwestern one has a distance of 90° from the sun. Were these really no sundogs in the 90°-area, but 120°-parhelia? Even a versed observer can easily estimate a wrong distane of the 120°-parhelia. And the arc passing through the northeastern anthelic sundog in the drawing, looks very similar to the subhelic arc which at this sun elevation (about 27°) exactly passes through the 120°-parhelion. Even if the other arc through the other anthelic sundog projects steeper upwards, this drawing supports the theory that Hevel´s halo really was the subhelic arc and that the sundogs visible at the points where it intersected the parhelic circle really were 120°-parhelia.