Natural Questions Chapter 2

Videamus nunc, quemadmodum fiat is fulgor, qui sidera circumuenit. Memoriae proditum est, quo die urbem diuus Augustus' Apollonia reuersus intrauit, circa solem uisum coloris uarii circulum, qualis esse in arcu solet. Hunc Graeci G-halo uocant, nos dicere coronam aptissime possumus. Quae quemadmodum fieri dicatur, exponam. Cum in piscinam lapis missus est, uidemus in multos orbes aquam discedere et fieri primum angustissimum orbem, deinde laxiorem ac deinde alios maiores, donec euanescat impetus et in planitiem immotarum aquarum soluatur; tale quiddam cogitemus fieri etiam in aere: cum spissior factus est, sentire plagam potest; lux solis aut lunae uel cuiuslibet sideris incurrens recedere illum in circulos cogito. Nam umor et aer et omne, quod ex ictu formam accipit, in talem habitum impellitur, qualis est eius, quod impellit; omne autem lumen rotundum est: ergo et aer in hunc modum lumine percussus exibit. Ob hoc tales splendores Graeci areas uocauerunt, quia fere terendis frugibus destinata loca rotunda sunt. Non est autem, quod existimemus istas, siue areae siue coronae sunt, in uicinia siderum fieri. Plurimum enim ab his abstint, quamuis cingere ea et coronare uideantur: non longe a terra fit talis effigies, quam uisus noster solita imbecillitate deceptus circa ipsum sidus putat positam. In uicinia autem stellarum et solis nihil tale fieri potest, quia illic tenuis aether est. Nam formae crassis demum spissisque corporibus imprimi solent, in subtilibus non habent, ubi consistant aut haereant: in balneis quoque circa lucernam tale quiddam aspici solet ob aeris densi obscuritatem, frequentissime autem austro, cum caelum maxime graue et spissum est. Nonnumquam paulatim diluuntur et desinunt, nonnumquam ab aliqua parte rumpuntur et inde uentum nautici expectant, unde contextus coronae periit: si a septemtrione discessit, aquilo erit, si ab occidente, fauonius. Quod argumentum est intra eam partem caeli has fieri coronas, intra quam uenti quoque esse solent: superiora non habent coronas, quia ne uentos quidem. His argumentis et illud adice, numquam coronam colligi nisi stabili aere et pigro uento; aliter non solet aspici. Nam qui stat aer, impelli et diduci et in aliquam faciem fingi potest; is autem qui fluit ne feritur quidem lumine (non enim resistit nec formatur, quia prima quaeque pars eius dissipatur): numquam ergo ullum sidus talem sibi efligiem circumdabit, nisi cum aer erit densus atque immotus et ob hoc custodiens incidentem in se rotundi lineam luminis. Nec sine causa; repete enim exemplum, quod paulo ante' proposui: lapillus in piscinam aut lacum et alligatam aquam missus circulos facit innumerabiles; at hoc idem non faciet in flumine (quare? Quia omnem figuram fugiens aqua disturbat): idem ergo in aere euenit, ut ille, qui manet, possit figurari, at ille, qui rapitur et currit, non det sui potestatem et omnem ictum uenientemque formam ex eo turbet. Hae, de quibus dixi, coronae cum dilapsae sunt aequaliter et in semet ipsae euanuerunt, significatur quies aeris et otium et tranquillitas; cum ad unam partem cesserunt, illinc uentus est, unde finduntur; si ruptae pluribus locis surît, tempestas fit. Quare id accidat, ex his, quae iam exposui, intellegi potest. Nam si facies uniuersa subsedit, apparet temperatum esse aera, et sic placidum; si ab una parte intercisa est, apparet inde aera incumbere: et ideo illa regio uentum dabit. At cum undique lacerata et concerpta est, manifestum est a pluribus partibus in illam impetum fieri et inquietum aera hinc atque illinc assilire: itaque ex hac inconstantia caeli tam multa temptantis et undique laborantis apparet futura tempestas uentorum plurium. Hae coronae noctibus fere circa lunam et alias stellas notantur, interdiu raro, adeo ut quidam ex Graecis negauerint omnino eas fieri, cum illos historiae coarguant. Causa autem raritatis haec est, quod solis fortius lumen est et aer ipse agitatus ab illo calefactusque solutior: lunae inertior uis est ideoque facilius a circumposito aere sustinetur; aeque cetera sidera infirma sunt nec perrumpere aera ui sua possunt: excipitur itaque illorum imago et in materia solidiore ac minus cedente seruatur. Debet enim aer nec tam spissus esse, ut excludat ac summoueat a se lumen immissum, nec tam tenuis aut solutus, ut nullam uenientibus radiis moram praebeat. Haec noctibus temperatura contingit, cum sidera circumiectum aera luce leni non pugnaciter nec aspere feriunt spissioremque, quam solet esse interdiu, inficiunt.

Voyons maintenant comment se forment les cercles lumineux qui entourent quelquefois les astres. On rapporte que le jour où Auguste revint d’Apollonie à Rome, on vit autour du soleil un cercle empreint des couleurs variées de l’arc-en-ciel. C’est ce que les Grecs nomment Halo et que nous pouvons très-justement appeler Couronne. Voici comme on en explique la formation : qu’on jette une pierre dans un étang, on voit l’eau s’écarter en cercles multipliés, dont le premier, fort rétréci, est successivement enveloppé par d’autres de plus en plus larges, tant qu’enfin l’impulsion se perde et meure dans la plaine immobile des eaux. Il faut supposer dans l’air des effets analogues. Quand ce fluide condensé est susceptible de percussion, les rayons du soleil, de la lune, d’un astre quelconque, le forcent, par leur action, à s’écarter circulairement. L’air, en effet, comme l’eau, comme tout ce qui reçoit une forme d’un choc quelconque, prend celle du corps qui la frappe. Or, tout corps lumineux est sphérique ; donc l’air qui en sera frappé prendra la forme ronde. De là le nom d’Aires donné par les Grecs à ces météores, parce que les lieux destinés à battre le grain sont ronds généralement. Du reste, il n’y a pas la moindre raison de croire que ces cercles, quelque nom qu’on leur donne, se forment dans le voisinage des astres. Ils en sont fort éloignés, bien qu’ils paraissent les ceindre et leur faire une couronne. C’est près de la terre que se dessinent ces apparitions ; et l’œil de l’homme, toujours faible et trompé, les place autour des astres mêmes. Rien de pareil ne peut se former dans le voisinage du soleil et des étoiles, où règne l’éther le plus subtil. Car les formes ne peuvent absolument s’imprimer que sur une matière dense et compacte ; sur des corps subtils elles n’ont pas de prise ou ne tiennent pas. Dans nos bains mêmes, on observe un effet semblable autour des lampes, au milieu de cet air dense et obscur, surtout par le vent du midi, qui rend l’atmosphère lourde et épaisse. Ces cercles parfois se dissolvent et s’effacent insensiblement, parfois se rompent sur un point, et les marins attendent le vent du côté du ciel où la rupture s’est faite : l’aquilon, si c’est au nord ; si c’est au couchant, le zéphyre. C’est une preuve que ces couronnes prennent naissance dans la même région que les vents. Au delà, les vents ne se forment plus, ni par conséquent les couronnes. À ces preuves ajoute que jamais ces météores ne s’engendrent que dans un air immobile et stagnant ; le contraire ne se voit pas. En effet, un air tranquille peut recevoir une impulsion, prendre une figure quelconque ; un air agité se dérobe à l’action même de la lumière, car il n’a ni forme ni consistance ; les molécules frappées les premières sont aussitôt disséminées. Ces cercles donc qui couronnent les astres n’auront jamais lieu qu’au sein d’une atmosphère dense et sans mouvement, et par là propre à retenir le faisceau conique de lumière qui vient la frapper. Et en effet, reviens à l’exemple que je citais tout à l’heure. Une pierre jetée dans un bassin, dans un lac, dans toute eau dormante, y produit des cercles sans nombre ; ce qu’elle ne fait pas dans une eau courante. Pourquoi ? Parce que toute figure est brisée par la fuite de l’eau. 11 en est de même pour l’air : tranquille, il peut recevoir une forme : impétueux et agité, il se dérobe et brouille toutes les empreintes qui veulent s’y appliquer. Quand les couronnes se dissolvent également sur tous les points, et s’évaporent sans déplacement, c’est une marque que l’air est tranquille ; et ce calme universel annonce de l’eau. Se rompent-elles d’un côté seulement, le vent soufflera du côté de la rupture ; se déchirent-elles en plusieurs endroits, il y aura tempête. Tous ces accidents s’expliquent par ce que j’ai exposé plus haut. Car, que l’ensemble du phénomène se décompose à la fois, cela démontre l’équilibre, et, partant, le calme de l’air. Si la fracture est unique, c’est que l’air pèse de ce côté, et que de là doit venir le vent. Mais si le cercle est déchiré et morcelé de toutes parts, évidemment il subit le choc de plusieurs courants qui tourmentent l’air et l’assaillent tous à la fois. Cette agitation de l’atmosphère, cette lutte et ces efforts en tous sens signalent la tempête et la lutte imminente des vents. Les couronnes ne paraissent guère que la nuit autour de la lune et des autres astres ; de jour elles sont si rares, que quelques philosophes grecs prétendent qu’on n’en voit jamais ; ce que toutefois l’histoire dément. La cause de cette rareté, c’est que le soleil, ayant trop de force, agite, échauffe et volatilise trop l’air : l'action de la lune, moins vive, est plus aisément soutenue par l’air ambiant ; il en est de même des autres astres, également incapables de le diviser. Dès lors leur figure s’imprime et peut s’arrêter sur cette vapeur plus consistante et moins fugace. En un mot, l’air ne doit être ni tellement compacte qu’il éloigne et repousse l’immersion de la lumière, ni tellement subtil et délié, qu’il n’en retienne aucun rayon. Telle est la température des nuits, alors que les astres, dont la lumière douce ne vient pas heurter l’air d’une façon brusque et violente, se peignent dans ce fluide, plus condensé qu’il ne l’est d’ordinaire pendant le jour.

Let us now see how the brightness is produced that sometimes envelops the heavenly bodies. History has put on record that, on the day of the late Emperor Augustus’ entrance into Rome on his return from Apollonia, a parti-coloured circle, such as is wont to be seen in a rainbow, appeared round the sun. The Greeks call this a Halo; our most appropriate name for it is a Crown. Let me explain how it is formed. When a stone is thrown into a pond, the water is observed to part in numerous circles, which, very narrow at first, gradually widen out more and more until the impulse disappears, lost in the surface of the smooth water beyond. Let us suppose something of the same kind to occur in the atmosphere. When condensed it is capable of receiving an impact: the light of sun, moon, or any heavenly body encountering it forces it to recede in the form of circles. Moisture, be it observed, and air, and everything else that takes shape from a blow, is driven into the same form as that possessed by the object that strikes it. Now every kind of light is round. Therefore, the air when struck by light will assume this form. Accordingly the Greeks gave the name Threshing-floor (i.e. Halo) to a brightness of this kind, because spaces set apart for threshing corn were, as a rule, round. Be the better name threshing-floors, or be it crowns, there is no reason to suppose that they are formed in the neighbourhood of the heavenly bodies. They are a very long distance from them, though as seen from the earth they seem to touch and encircle them. In reality such an image is formed not very far from the earth, but the wonted frailty of human vision is deceptive, and we imagine the ring is formed close round the heavenly body itself. But no such thing could possibly occur in the neighbourhood of the sun and stars, as there is nothing but thin ether there. It is only when bodies have become rough and dense that shape can be impressed upon them. In subtle bodies there is no point on which form can lay hold or to which it can adhere. A phenomenon of the same nature as the halo may often be witnessed in baths, because the atmosphere is thick and dark: it is most frequent when the wind is in the south, when the air is heaviest and most dense. Halos sometimes are dissolved gradually and fade away, sometimes they are broken up on one side. In the latter case seafaring men look for wind in the direction in which the circle of the crown has been broken. If the parting is on the north, there will be a north wind, if on the west, zephyrs will follow. This is a proof that these crowns are formed in the region of the sky in which the winds are usually formed. The upper regions of air have no crowns because they have no winds either. An additional proof of the connection of winds and halos is afforded by the fact that the halo is never formed unless the atmosphere is at rest, and the wind, as it were, inactive. Under other circumstances it is not usually observed. The atmosphere when it is at rest may be fashioned to any pattern by being driven or drawn in any direction. But when it is in motion, light cannot even strike it. It takes no shape and offers no resistance, because the part first affected is always dissipated by the motion. Therefore it is that no heavenly body can ever be surrounded by a figure of the kind referred to unless when the atmosphere is dense and motionless, and so preserves the ray of round light that strikes upon it. Nor is it without good reason. Recollect the analogy mentioned a little ago. A pebble thrown into a pond or lake or any other circumscribed piece of water produces innumerable circles; but it has not the same effect if thrown into a river. And why so? Because in the latter case the water as it hurries on prevents the formation of any definite figure. So in the atmosphere the same thing happens; when it is stationary, it may receive a pattern; when it rushes in rapid motion, it evades all control, warding off every blow and every form as it approaches. When these crowns, of which I have spoken, have disappeared uniformly on all sides, and vanished in their own tracks, it is an indication of equilibrium in the atmosphere: there is perfect quietness and you may then look out for rain. When they break up at one side, it means wind in that quarter. If they burst at several points, a storm is brewing. The reason of this may be gathered from the explanations I have now given. If the ring fade all round, it is evident that the atmosphere is equable, and therefore calm. But if it is broken through on one side, evidently there must be an inclination of the air in that direction: hence that quarter will produce wind. But when the halo is rent and torn on all sides, plainly an attack is being made on it from several quarters at once, and a disquieted atmosphere is assailing it on this side and on that. So this disturbance of the heavens, the repeated effort and striving in all directions, betokens evidently that a storm is coming up with sudden shiftings of the wind. These crowns may be observed generally by night round the moon and other stars, but very seldom by day; in fact, so rarely in the latter case, that certain of the Greeks have denied that they appear at all by day. But history proves that they do. The cause of the infrequency of their appearance by day is that the sun’s light is stronger then, and the atmosphere itself when stirred and warmed by it is less dense. The moon’s power, on the other hand, is feebler, and is therefore more easily resisted by the surrounding air. The rest of the heavenly bodies are equally weak, and unable by their own force to burst through the atmosphere. So their shape is impressed and retained in the more solid and less yielding medium. For, in order to produce the phenomenon, the atmosphere must neither be so thick as to exclude or dissipate the light that streams in on it, nor yet so thin and rare as to furnish no hold to the rays that fall upon it. This particular consistency is obtained at night: the sluggish air is at that time struck with the faint light from moon or stars without violence or rudeness, and, being thicker than it is wont to be by day, is tinged thereby.