‘Why, sad sun, do you deny the rays?’ – The blue sun over Europe in September 1465 as an atmospherical and cultural-historical phenomenon
Since a few days the particle clouds of the Canadian wildfires, burning mainly in Quebec and Ontario since mid of May this year, have reached the coasts of Western Europe and will spread to East-Central Europe until Friday to dissipate (Image 1).
A diffuse high-altitude fog, extreme discoloration or reduced luminosity of the sun are the manageable consequences that we Europeans will experience from what is probably the earliest onset of the wildfire season in Canada’s large forests. And this is certainly not the first time that Europeans witnessed high-altitude smoke from North America. In mid-September 1465, Angelo de Tummulillis, a notary writing in the small town of Sant’ Elia near Montecassino in southern Italy, reported unusual phenomena in the sky:
But on the 14th of September around the 20th hour the sun’s color changed to blue. In this month of September […] several days and nights all the air in the upper sky was foggy and hazy. The sun seemed to be like the non-shining moon, and its glance at noon was not natural, but very limited. […] In this week on Friday 13th, it got even darker and the following day, on Saturday 14th , in the morning it seemed to be dyed with saffron. […] From noon until the evening the sun was of blue color and hardly shone within fog and haze. And this endured for ten days, accompanied by silence and calm of the air without any breeze. There were no clouds in the air or over the mountains, just this haze without any movement.
And the notary was by no means the only one to make this irritating observation. The Roman Senate clerk Stefano Infessura noted on the same day: In this year  on the 14th of September, day of the Holy Cross, the sun showed a lot of mutations. At times during the day it appeared green, at times blue and sometimes yellow. In the chronicle of Ser Guerriero di Gubbio from Umbria we find another confirmation: On the 13th of September, a Friday, around noon, the sun changed its color to a light blue. And it remained liked this until Saturday, 14 September, when it set this way. Also north of the Apennines, in Bologna, the chroniclers confirm the phenomenon: The month of September was hazy, and the sun showed little splendor. It was of a color like the moon by night, sometimes also blue, but most of the time the disk of the sun was white like milk. Even far north of the Alps we find almost identical observations. In Soest, Westphalia, the municipal books recorded: On the eve of the Holy Cross the sun was so blue in the sky and the air so yellowish, […] and there were rings around the sun and its glow was so bluish and sometimes red and white. A chronicle from Braunschweig writes: On the Friday before the day of the Holy Cross at noon, the sun was surrounded by blue circles like a blue cornflower, and it gave no glow.
So, from Braunschweig to Naples, six sources reported the same phenomenon: a discoloration of the sun into blue and other colors on September 13–14, 1465, accompanied by weak luminosity and yellowish hazy air. The observations are strikingly reminiscent of the consequences of the Canadian forest fires for the eastern parts of the USA and Canada, especially the apocalyptic mood of the discolored sun, the orange skies, admittedly minus the acrid smoke smell and the harmful effect on humans‘ respiratory tract, and supplemented by a remarkable optical phenomenon that we have not yet observed in 2023: the blue sun. Due to the multitude of independent sources, the factual background of the observation cannot be doubted. But what exactly was the physical cause of the blue sun in 1465?
Three possibilities arise, in descending probability:
1. a so-called dry fog of sulfate aerosols after a large volcanic eruption,
2. the smoke cloud of large forest fires, probably taking place in North America,
3. dust clouds from the Sahara carried at high altitude to Europe.
All three phenomena have in common that fine particles are carried into high air layers – either sulfuric acid aerosols, burnt biomass or mineral dust, and there change the composition of the atmosphere in such a way that the scattering allows blue or green light from the sun to penetrate to earth. Last but not least, the coloration of the surrounding sky determines whether the sun appears yellowish, blue or green to the observer. At the same time, a relatively high particle density in the atmosphere is necessary to produce the blue sun effect. However, blue sun sightings have so far only been documented and investigated in the context of volcanic eruptions such as Krakatau in 1883 or large forest fires in northern Canada, which led to blue sun sightings in northwestern and central Europe in 1950. For historical volcanic eruptions, such dry fogs have been extensively studied, and based on long-lasting changes in the atmosphere, detectable for instance in reduced plant growth, the volcanic scenario is the most likely cause for the blue sun of 1465. This is supported by reports that the change in luminosity lasted longer, as a vine grower from Nepi near Rome recorded in his diary: On the eigth day [of September] the sun started to lose its shining and it remained weakened in this way for about ten days. Also a chronicler from Constance in Southern Germany, Gebhard Dacher, had noted for 1465, less specific in time, that the sun did not have its shine as clear as it should have been […] It had been distorted by its color many times during the year.
However, this is opposed by an as yet unresolved contradiction of the findings of dendroclimatology, ice core research and climate history: The years identified in ice-cores don’t fit with drops in temperature recorded in tree-rings, and atmospherical and meteorological phenomena described in historical sources. Hence, no specific dates can yet be identified for which all findings could be brought equally into agreement. In addition, historical findings for other years can be verified, such as discolorations of the sun in 1463, as reported from Poland: On Thursday, the day of St Bartholomew (24 August 1463), in Proschowice and some other places, when the sun was about to set in the west, the picture of the Crucified appeared in the air, yielding a sword. At first it was reddish, then turned pale and then black. More than two hours many saw this great miracle that moved from west to south. The historiographer and astronomer of Count Palatine Frederick I, Matthias von Kemnat (ca. 1430–1476), can also be cited for various sightings of the blue sun. In retrospect he remembers with a vague reference to the several decades of the 15th c., i.e. 1440-1476: the time Emperor Frederick III ruled, […] the sun has been seen blue many times a day and a cross was spotted in the moon as well as many more miracles in the sky.
For the time being, therefore, it cannot be determined with certainty whether large North American forest fires were behind the phenomenon of the blue sun as they definitely were in 1950, or whether the contradictory dating concerning the large volcanic eruptions in the middle of the 15th century can still be unified and thus an explanation can be found. Against a Saharan dust storm speaks that the frequently associated, so-called blood rain is mentioned by no Europen source – the red coloring of precipitation and waters by the desert dust transported in the air is a key marker for such events.
The disturbed entry of the bride
But beyond the question of what turned the sun blue over Naples on September 13/14, 1465, it is worth noting the social and cultural effects connected to this event: Precisely on September 14, the daughter of the Duke of Milan, Ippolita Maria Sforza (1445–1488) had arrived in Naples. The 20-year-old, most educated and beautiful Milanese princess had married the son of the Neapolitan King Ferrante, thus consolidating the most important political alliance of the time for Italy. Her entry into her new home city, staged as a ceremonial welcome by her father-in-law, was a momentous ritual at the end of a conflict-ridden two-month journey from Milan to the capital of the Regno. Therefore, the spectacle in the sky was not unproblematic for the perception of Ippolita Maria Sforza, as a chronicler from Verona knows to tell: And so she entered into Naples on Saturday, 14 September 1465 and while she entered, the sun changed its natural color this day, as it did on the two successive days: First it was dark blue, then light blue, so people admired more what happened to the sun than the most honorable entry of our new lady. This was counterproductive, of course, since nothing should outshine the bride and future queen. Ippolita herself maintained intensive correspondence with her parents, but reported nothing about the celestial phenomena. The Milanese ambassador in Naples was visibly anxious to deny diffuse news about the course of events and thus to allay the concerns of his principal. In any case, the envoys of the Republic of Siena, who had attended the entrance and subsequently presented their gifts to the young couple, also reported about the blue sun.
The aforementioned chronicler from Verona is also the only contemporary who gives us an insight into competing explanations that circulated – and probably more in educated circles on the Italian peninsula, not primarily among the spectators of the princess’ entry under the blue sun: Some suspected a solar eclipse as the cause, but this was quickly dismissed, because no eclipse could last longer than three hours. Simultaneous observers in the Aegean Sea also knew what a solar eclipse was and how to calculate it, and therefore ruled out that the phenomenon was one. Other observers, who were astrometeorologically educated, suspected vapors behind the phenomenon, whose formation would have been favored by the pronounced drought of the summer and the heavy rains that followed. And, of course, the change of the sun was also interpreted as a divine sign that would announce terrible developments for the world and all Italy in its sinfulness.
But all these interpretations are listed in only one source. All other observers abstain from all evaluations and explanations, because the observed phenomenon of the blue sun eluded these interpretations: Neither in the Bible, nor in the ancient writings, nor in the contemporary knowledge of astrometeorology were the observed phenomena mentioned, particularly not a blue colouring of the sun, while eclipses were omnipresent. What could not be explained by experience or cultural reference was, surprisingly enoughy, simply described without comment. The only, but remarkable exception of this speechlessness under the blue sun, were two epigrams of humanist poets at the Neapolitan court, which apparently had entered into a kind of competition. Somewhat cryptic is the work of Antonio Beccadelli (1394–1471), called Panormita: The Titan saw Ippolita and cast down his eyes, / When in sublime form the coming goddess made herself known. / Do not scare away the rays, kind father, as if you were afraid. / This one (Ippolita) inhabits the land, you, Phoibos, inhabit the firmament. The epigram of Porcelio Pandoni (1405–1485) is much closer to the reported phenomena: Why, sad sun, do you deny the rays and hide the light? / Why does your brilliance and color fade away? / You have given way to Ippolita, whose kind eyes surpass the stars, / if there is anything greater than the stars.
Thus, closely contextualizing observation in ancient terms and borrowing from Ovid’s ‘Metamorphoses’ (Ovid, Met. 4, 200-204), the poets found words and interpretations for a phenomenon that otherwise defied explanation. They succeeded in interpreting the incident in a way that was not detrimental to the Milanese princess Ippolita, but rather reinterpreted the waning of the light on the occasion of her arrival as a respectful act of the personified sun. And so, countless particles of aerosol – whether of volcanic origin or created in the context of a great wildfire in North America – which spread like a ribbon over parts of Europe for thousands of kilometers, became an occasion to spark a poetry contest in Naples, from which the beautiful Ippolita Maria Sforza was to emerge as the undisputed winner.
 Set die sabbati quartodecimo semptembris hora .XX. in transfiguratione solis in colorem azurri [..] Sed in ipso mense septembris .XIIII. indictione infra octavam nativitatis beate virginis Marie totum aer sursum factum nubilosus et caliginosus per plures dies et noctes in tantum quod sol in ortu suo videbatur velut luna non radians, et in meridie non naturaliter set paulisper effundebat radios suos ita quod in oppositis vero faceret umbram, qui cum declinaret ad occasum aspicientibus latebat in totum. Sed eadem ebdomada die veneris .XIII. eiusdem magis oscuratus est et in sequenti die sabati .XIIII. in mane apparuit quasi inter nebulas intrusus crocei coloris usque ad sextam, ita quod vultus etiam hominum sese adspicientium viderentur eiusdem crocey coloris seu zallany; et circa meridiem usque ad vesperos dictus sol apparuit coloris azuri seu brevis parum radians inter nebulas vel caligines interpositas, et omni die in vesperis latebat quasi in totum, et hoc duravit quasi per decem dies cum silentio et tranquilitate aeris sine flatu alicuius venti, nec aliqua nubes apparebat super terram vel in montaneis nisi in aere, quasi caligines sine motu (Costantino Corvisieri [ed.], Notabilia temporum di Angelo de Tummulillis da Sant’Elia [Fonti per la Storia d’Italia 7], Livorno 1890, p. 133).
 Eodem anno  a dì 14 di settembre, in die santae crucis, lo sole fece molte mutationi; quanno pareva verde e quanno azzurro et alcuna volta giallo, tutto lo dì (Diario della città di Roma di Stefano Infessura scribasenato, a cura di Oreste Tommassini [Fonti per la Storia d’Italia 5], Roma 1890, p. 69).
 A dí 13 de setembre, che fo vènere, circa el mezo dí, el sole deventò celestro, et stecte cosí tucto el sabato, che fo a dí 14; e cosí ando sotto la sera. A dí XX de setembre 1465, fo lo eclisis a hore xxiii et durò fine al tramontare del sole: scuraro li tre quarti del sole (Cronaca di Ser Guerriero da Gubbio dall’ anno MCCCL all’ anno MCCCCLXXII, a cura di Giuseppe Mazzatiniti, Città di Castello 1903 [RIS² 21/4], p. 80.
 Anno Christi MCCCC65. […] El mese de settenbre fu chalinzoso, ché ‘l sole mostrava pocho spiandore, era de colore como è la notte la luna, cum colore azuro e smorto, l’ochio del sole biancho como latta (Corpus Chronicorum Bononiensium, a cura di Albano Sorbelli, 4 vols., [RIS² 18/1], Città di Castello 1938–40, here vol. IV, p. 341).
 Up des hilgen Cruces avende Exaltacionis do was dey sunne an dem hemele so bla ind dey lucht so gelbleck, […] und dat dey rynck van der sunnen und dey schyn was alle blaewyt und was ok eyne wijle tydes roit und wyt (Karl Lamprecht [ed.], Die Chroniken der westfälischen und niederrheinischen Städte. Dritter Band: Soest und Duisburg [Die Chroniken der deutschen Städte 24], Leipzig 1895, p. 51).
 Darna upp ein fridage vor des hiligen Crützes dage in dem rechten midden dage, do vvas de sunne so blauwe umme getzirkelt alse ein blauw korne blome, unde gaff nenye schyn van sick (Leibniz, Gottfried Wilhelm [ed.], Chronicon Brunsvicensium Picturatum dialecto saxonica conscriptum, in: Ead. (ed.), Scriptorum Brunsvicensia illustrantium, vol. 3. Hannover 1711, pp. 277–425, here p. 411).
 More commonplace is the phenomenon of particularly red sunsets due to air pollution, for example, but the processes for green and blue suns are much more complex, see Wullenweber, Nellie et al.: On the phenomenon of the blue sun, in: Climate of the Past 17 (2020), pp. 969–983, https://doi.org/10.5194/cp-17-969-2021
 Cordy Tymstra, The Chinchaga firestorm. When the moon and sun turned blue. Edmonton, Alberta 2015.
 Werner Jenne, Das Phänomen “Blaue Sonne”, in: Tellus 3 (1951), pp. 129–134.
 Katrin Kleemann, A Mist Connection. An Environmental History of the Laki Eruption of 1783 and Its Legacy. (Historical Catastrophe Studies / Historische Katastrophenforschung, 3), Berlin 2023, pp. 112–131; George James Symons (ed.), The Eruption of Krakatoa and Subsequent Phenomena. London 1888.
 The VIII Cominza lo sole ad perdere lo suo lume et dura cossì ammalato qualche X dii (Diario nepesino, a cura di G. Levi, in: Archivio della Società Romana di Storia Patria 7 (1884), pp. 115–182, here p. 153).
 Nun was sy vor das jar jn her zu mengen mal mit ir varb entstelt gewesen[…], ye das sy ir würküng der früchten halb das jar nit hett als andre jar (Sandra Wolff (Hg.), Die “Konstanzer Chronik” Gebhart Dachers: “By des Byschoffs zyten volgiengen disz nachgeschriben ding vnd sachen … ”; Codex Sangallensis 646: Edition und Kommentar. Ostfildern 2008, p. 696)
 The historical findings in Martin Bauch, The Day the Sun Turned Blue. A Volcanic Eruption in the Early 1460s and its Possible Climatic Impact – a Natural Disaster Perceived Globally in the Late Middle Ages? In: Gerrit Jasper Schenk (ed.): Historical Disaster Experiences. Towards a Comparative and Transcultural History of Disasters Across Asia and Europe (Transcultural Research – Heidelberg Studies on Asia and Europe in a Global Context). Heidelberg 2017, pp. 107–138 speak for the years 1464/65; ice core evidence points to a major eruption in 1458, cf. Michael Sigl et al, A new bipolar ice core record of volcanism from WAIS Divide and NEEM and implications for climate forcing of the last 2000 years, in Journal of Geophysical Research: Atmospheres 118 (2013), pp. 1151–1169; Northern Hemisphere temperature reconstructions from tree-ring data prefer the year 1453, cf. Esper, Jan et al: Northern Hemisphere temperature anomalies during the 1450s period of ambiguous volcanic forcing. In: Bulletin of Vulcanology 79/41 (2017), pp. 1–9.
 See Matthew Toohey, Martin Bauch, Lauren Marshall, Francesco S. R. Pausata, Anja Schmidt, Michael Sigl, Clarifying the role of volcanism on mid-15th Century climate using historical sources, proxy archives, and climate-modelling / EGU General Assembly 2018, Session IE2.3/AS3.10/CL4.22/GMPV6.4/NH2.2 “Characterizing, understanding, predicting the radiative effects and the climatic impacts of major volcanic eruptions”.
 Feria quarta in die Sancti Bartholomei in Proschowice, plerisque aliis locis, sole vergente ad occidum, imago Crucifixi rubra primum, deinde pallida, ad extremum nigra, gladium habens exertum, in aere apparuit, et horis plus quam duabus, ab occidente in meridionem tendendo, magnum spectantibus miraculum praebuit (A. J. Długosz, as quoted by Małgorzata Hanna Malewicz, Zjawiska przyrodnicze w relacjach dziejopisarzy polskiego średniowiecza (Monografie z dziejów nauki i techniki, 123), Wrocław 1980, p. 104).
 do keiser Friederich der dritt hoit regiert, […] die sone ist vil tag blahe gesehen worden vnd ein creutz ist gesehen worden inn dem mone vnd vil anders wunders an dem himmel (Matthias von Kemnat, Chronik Friedrichs I. des Siegreichen, in: Conrad Hofmann [ed.], Quellen zur Geschichte Friedrich’s des Siegreichen. Erster Band: Matthias von Kemnat und Eikhart Artzt, Munich 1862, pp. 1–141, here p. 87).
 Jeryldene M. Wood, Ippolita Maria Sforza. The Renaissance Princess Who Linked Milan and Naples, Jefferson 2020.
 et così adì sabbato XIIII settembre MCCCLXV entrò in Napoli et ne lo intrare che fece, el sole per quel giorno, secondo e terzo successivi, mutò colore dal naturale, in forma che era morello scuro o sia azuro, in modo che magior admiration fu del caxo del sole, che de lo intrare così onoratamente la nova dona (Giovanni Soranzo [ed], Cronaca di Anonimo Veronese 1446-1488 [Monumenti storici pubbl. d. R. Deputazione Veneta di Storia Patria. Terza Ser. 4], Venezia 1915, p. 226
 Ippolita Maria Sforza, Lettere, a cura di M. Serana Castaldo (Gli arsili. Rari, inediti, bizarri, 6), Alessandria 2004.
 Thus in his letter of 16 September 1465 to Francesco Sforza, which was expressly intended to relativize, if not revise, other reports: avvisare vostra signoria de quello è seguito, remetendone il scrivere de questi altri vostri che ne scrivono più difusamente (Veronica Mele, La creazione di una figura politica. L’entrata in Napoli di Ippolita Maria Sforza Visconti d’Aragona, duchessa di Calabria, in: Quaderni d’italianistica 33/2 (2012), pp. 23–72, Appendice, No. 10, pp. 70–71, here p. 71.)
 Voliamo etiam che V. M. S. sappi come sabbato a dì xiiij, el dì che entrò la Sposa in Napoli, el sole si mutò in colore azurro chiaro che ogni homo dice non avere mai più vista simile cosa. Speriamo che ancho costì si sia dimostrato simile (Alessandro Lisini, Le feste fatte in Napoli nel 1465 per il matrimonio de Ippolita Sforza Visconti con Alfonso, duca di Calabria. Da lettere del tempo, Siena 1898, p. 39).
 Alcuni veduto tal novità nel sole dissero eclipsato el sole per la interposition de la terra, altri la luna interposita tra el sole e noi, che facilmente tal opinion si getta a terra, quum sit che nullo eclipse naturaliter possi durare pi_u de 3 hore o circa (Tummulillis, as footnote 1, p. 227).
 For at high noon the sun, which shone undimmed and cloudless, darkened in a sudden change, and became dim, and its appearance changed, and it took on the color of dark copper, and became altogether gloomy and black, not in the usual way as in solar eclipses (for it was not an eclipse at that time), but in another unprecedented way, as if a mist or a dense dark cloud were covering it (Kritobulos von Imbros: Mehmet II. erobert Konstantinopel. Mehmet II. erobert Konstantinopel. Die ersten Regierungsjahre des Sultans Mehmet Fatih, des Eroberers von Konstantinopel 1453. Das Geschichtswerk des Kritobulos von Imbros, übersetzt, eingeleitet und erklärt von Diether Roderich Reinsch, [Byzantinische Geschichtsschreiber 17], Graz/Vienna/Cologne 1986, p. 277; translation M.B.).
 molti altri forono, che volevano tale offuschatione essere processa da vapori ellevati da la terra, prima dissichata per la extrema secha de la estate et poi apresso per la exuberante moltitudine de piogie, che abundaro subito ne lo autonno. altri volsero che fusse vapori grossi, vischiosi, humidi, ellevati fino a la megia region del aire etc..; altri de altra opinione, dicendo molte estate per adrieto essere state piu seche che la preterita et per consequens autumni più humidi ch’el presente, unde magior copia de vapori naturaliter doveriano esser concreati (Tummulillis, as footnote 1, p. 227).
 et successive fatto qualche segno, che minime s’e` veduto tale offuscation del sole fuss più presto signo divino che naturale, minatiante cose terribilissme per totum orbem et più per questa povera Italia per li peccati de quella (Ibidem).
 Donatella Coppini, Un eclisse, una duchessa, due poeti, in: Alessandro Perosa (ed.), Tradizione classica e letteratura umanistica, vol. 1 (Humanistica 3,4), Roma 1985, pp. 343–373.
 Panhormita ad Ducissam Calabriae, / Vidit ut Hypolitam, contraxit lumina Titan, / Maiorem formam fassus adesse Deam, / Non est ut timeas, radios Pater alme resolve, / Ista colit terras, tu Phaebe, cole polos (Ippolita Maria Sforza, as footnote 19, p. LXXXIX).
 Quid radios, sol moeste, negas et lumina condis? / Quo nitor ille tuus cessit et ille color? / Cessisti Hippolitae, cuius pia lumina vincunt / Sidera, vel siquid sidere maius est (Ippolita Maria Sforza 2004, as footnote 19, p. LXXXIX).
Dieser Artikel wurde redaktionell betreut von Stephan Ebert (Gastredakteur) und Björn Gebert.