c l i n i c a l a n d e x p e r i m e n t a l optometry ... · the min min light and the fata...

Clinical and Experimental Optometry 86.2 March 2003

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The Min Min light and the Fata Morgana PettigrewOPTOMETRYC L I N I C A L A N D E X P E R I M E N T A L

Clin Exp Optom 2003; 86: 2: 109–120

Accounts of the Min Min light are a col-ourful and inextricable part of Austral-ian outback folklore that seem to have re-sisted many attempts at their explanation.In the Channel Country of westernQueensland where the majority ofsightings of the Min Min have taken place,the town of Boulia has linked its publicimage to the phenomenon and therebytried to improve its attractiveness as atourist destination. The following noticeis found in the town:

‘Min Min light, this unsolved mystery is alight that at times follows travellers for longdistances—it has been approached but neveridentified.’

Maureen Kozicka1 spent two years re-searching Min Mins before her untimelydeath and produced the only book de-voted to the phenomenon. Although sheinterviewed many who had seen the lightand spent much time in the ChannelCountry, Kozicka1 never saw the phenom-enon herself and was not able to reach anyfirm conclusions about it. I was luckyenough to witness the phenomenon on anumber of occasions while on field tripsto study the nocturnal activities of the let-ter winged kite, Elanus scriptus, in classicalChannel country; namely, the upperreaches of the Diamantina and Georginarivers in Western Queensland. The present

accounts of Min Min lights and my opticalexplanation of them are based on thoseexperiences and on some investigationsand an experiment designed to reproducethe phenomenon under field conditions.

I will begin with a brief description ofthe Min Min light phenomenon, summa-rising the key observations of others, withan emphasis on the ‘notorious’ featuresthat excite so much emotion in virtuallyall those who have witnessed it. Next, I willdescribe my own encounters with the phe-nomenon, including some field measure-ments of its distance from the observer, aswell as an experiment on the plains thatsuccessfully reproduced the major features

The Min Min light and the Fata MorganaAn optical account of a mysterious Australian phenomenon

COMMENTARY

John D Pettigrew BSc(Med) MSc MBBSFRSVision Touch and Hearing ResearchCentre, University of Queensland

Submitted: 11 September 2002Revised: 2 December 2002Accepted for publication: 3 December2002

Key words: Channel Country, Fata Morgana, inverted mirage, Min Min light, temperature inversion

Background: Despite intense interest in this mysterious Australian phenomenon, theMin Min light has never been explained in a satisfactory way.Methods and Results: An optical explanation of the Min Min light phenomenon is of-fered, based on a number of direct observations of the phenomenon, as well as a fielddemonstration, in the Channel Country of Western Queensland. This explanation isbased on the inverted mirage or Fata Morgana, where light is refracted long distancesover the horizon by the refractive index gradient that occurs in the layers of air duringa temperature inversion. Both natural and man-made light sources can be involved,with the isolated light source making it difficult to recognise the features of the FataMorgana that are obvious in daylight and with its unsuspected great distance contribut-ing to the mystery of its origins.Conclusion: Many of the strange properties of the Min Min light are explicable in termsof the unusual optical conditions of the Fata Morgana, if account is also taken of thehuman factors that operate under these highly-reduced stimulus conditions involving asingle isolated light source without reference landmarks.


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The Min Min light and the Fata Morgana Pettigrew

of the phenomenon. Finally, I will discussthe findings in the light of the hypothesisthat the Min Min is a refractive phenom-enon, in the same class as the invertedmirage, or Fata Morgana, except that itoccurs at night. This explanation may havebeen overlooked largely because the noc-turnal Min Min phenomenon involves anisolated light source and the relationshipof it to other visual landmarks cannot bedetermined, as can more readily occurwith a daytime Fata Morgana. Such a sin-gle small light source, at night, without anyreference points, may not readily revealthe diagnostic features of this kind of mi-rage in the way that would make it easilyrecognisable in a landscape during day-light. I also discuss the subjective aspectsof this phenomenon in the light of workon how visual perception deals with am-biguous or incomplete information. Theseinternal, subjective aspects vary with theindividual observer and assume greaterimportance for unfamiliar visual eventswhere there are few supporting visualclues, as occurs with the Min Min.

DESCRIPTION OF THE MIN MINLIGHT

The following summary is based loosely onthe many descriptions collected byKozicka1 and my own experience with fiveseparate sightings. These are meant to bean introduction to this varied phenom-enon, rather than a rigorous account, sothat the reader can have a background forthe specific accounts that follow.

Angular size, shape and positionViewed through binoculars, the light isoften seen to be a fuzzy, roughly circulardisc, a fraction of a degree of arc in diam-eter (that is, usually seen to be smaller thanthe diameter of a full moon, which is 0.52degrees of arc), rather than a point sourceof light like a star. The fuzzy edges are usu-ally in rapid motion, like a swarm of bees.While there are rare reports of Min Minlights high overhead for short times, espe-cially when the terrain has hollows, themajority of accounts describe the light asif it were floating not far above the hori-zon.

Brightness and colourBrightness varies greatly in different re-ports, from dim lights similar to a secondmagnitude star, to brilliant sources that areable to illuminate objects in the landscapeand to cast bright shadows of the observeronto building walls. Most reports involvewhite light but changing colour is not unu-sual, with gradual changes through thespectrum from red to green and backagain. Blue Min Mins are rare comparedwith green, yellow and red ones.

AnimationThe light seems to have a mind of its own,approaching closely at times and retreat-ing to the distance at other times. Thisaspect of the behaviour of the Min Minlight is literally hair-raising and the sourceof considerable emotion that may inhibitobservers from telling their experiencesto a stranger. Even hardened residents ofthe Outback—if interviewers are fortunateto be able to overcome the residents’ re-luctance to talk about Min Mins—usuallyadmit to being terrified by these apparentmovements of the light, which seems todance erratically in all three axes and cansometimes move smoothly and swiftly oververy large distances. On occasions, a sin-gle Min Min light has been observed tosplit into two independent lights. In manystories, the light moves suddenly just as theemotion of the observer peaks or the ob-servers themselves make a move. Somehave reported a sudden disappearancewhen a rifle shot was fired at the light.

VelocityA favourite response to sightings of a MinMin in the Outback is to give it a speed-test, whether by horse or motor vehicle.Min Mins always pass these tests with fly-ing colours, apparently keeping up effort-lessly even when the observer attainsspeeds up to 120 kilometres per hour.However, note that these measurementsmake a presumption that the light is lo-cated relatively nearby, an assumptiontested below. If the light were situated at agreat distance, its ability to keep up with ahigh-speed vehicle is not so surprising. Forexample, the moon seen behind treeswould also appear to pass such a test to

anyone unfamiliar with the concept ofparallax and with the moon’s great dis-tance. Nevertheless, even when the ob-server is stationary, it is clear that the MinMin can sometimes change its apparentlocation very quickly. Some of these rapidchanges seem to be linked to the observ-er’s own actions but clear examples areknown where the movements of the lightrapidly illuminate different features of thelandscape in succession, an effect that canbe explained only if the light source itselfis changing.

DistanceReports vary widely concerning the dis-tance at which the Min Min light is per-ceived. Changes in the apparent distanceof the light are the source of the mostunnerving aspect of the light, which canappear to approach within a metre or two,only to recede rapidly to what appears tobe many hundreds of metres, sometimeslinked to the observer’s own movementsor thoughts. As the light is usually a singlesource without any nearby reference land-marks, estimates of its distance are diffi-cult and subject to error, particularly byobservers not familiar with the conceptsof parallax and angular subtense. Like themoon illusion, where the same angularsubtense is attributed to a larger physicalsize when the moon is perceived to be fur-ther away (for example, when seen in re-lation to adjacent buildings or trees on thehorizon), the disorienting aspects of MinMin lights may often result from a misper-ception of its distance.

SoundVirtually all reports of the Min Min lightindicate that it is silent, unaccompaniedby any sound.

When and where of sightingsThe open plains of the Channel Countryare the legendary best site for encounter-ing Min Min lights but reports have beenfiled from virtually every part of Australia,including seashores and over water. In theChannel Country, sightings are distributedall year round but peak noticeably in mid-winter, which favours stable air conditionsand a cold ground layer (temperature


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inversion). Sightings usually take placewhen the weather is fine with no more thana slight breeze but some of the spectacularreports of large fast movements of the lightcame from clear but windy nights. Of the140 sightings investigated by Kozicka,1 notone was associated with stormy weather.

Past historyAlthough Min Mins have an Aboriginalname and were certainly present beforethe coming of Western civilisation, indi-genes to whom I have talked consider thatMin Mins have increased in number inmodern times. Pre-existing legends aboutMin Mins include using the fear of themto help control children. I have not foundany Aborigines in the Channel Countrywho regard Min Mins in a positive way. Bythe same token, there is no report of MinMin lights causing any harm. Unlike Arc-tic cultures, which have specific words todescribe the Fata Morgana and related re-fractive atmospheric effects, I was not ableto find any Aboriginal references to, orawareness of, such refractive phenomenain the Channel Country.

EXPLANATIONS PREVIOUSLYOFFERED FOR MIN MIN LIGHTS

Luminous micro-organismsassociated with flying insects or abirdWhile bioluminescent fungi are knownfrom a variety of sites around Australia andcould theoretically contaminate a flyingbird or insect, no-one has ever caught oreven observed the proposed insect or bird.This explanation is also incompatible withthe great brilliance of the Min Min, whichon occasions can illuminate the landscapeand cast bright shadows, a feat not ob-served with even the brightest biolumines-cence. The theory of the cloud of flyingluminous insects has an initial appeal, be-cause most accounts of the Min Min in-clude a description of a bright ball withdynamic fuzzy edges. The problem withthis explanation, apart from the issue ofbrightness and the absence of any descrip-tions of such luminous flying things incontrast to the repeated observations of

the Min Min, is that one would not expecta cloud of flying insects always to main-tain a roughly circular outline, as Min Minsalmost always do. There is one vivid de-scription of a luminous bird in Kozicka,1

but this has not been replicated, in con-trast to the many hundreds of verifiedaccounts of Min Mins that defy this expla-nation.

Burning marsh gas‘The will o’ the wisp’ is a well-known phe-nomenon that has some similarities to theMin Min but lacks its brilliance and itsheight above the ground. Moreover, thisexplanation fails to account for the MinMin’s widespread occurrence far fromsources of marsh gas and its great mobil-ity at times. The flame-like appearance ofwill o’ the wisps is also different from thecircumscribed, usually disc-shaped MinMin.

Light associated with magneticanomalies/artesian basin/ionosphereAll of these possibilities are associated withthe emission of light but they are largelytheoretical, being put forward on the ba-sis that these unusual geophysical phe-nomena could account for the preferredlocation of Min Mins over the Great Arte-sian Basin. There are no direct observa-tions of light that can be attributed to thesephenomena, apart from the fact that theywould be expected to produce diffuse lighteffects (for example, the aurora) that aredistinct from the circumscribed Min Min.Moreover, Min Mins have been describedfrom every location in Australia, not justthe locations where these geophysicalphenomena are localised.

Refractive phenomenaThe proposal on which the present paperrests is not new, as landowner ColinMcKinnon had already suggested that therefractive effects of an atmospheric tem-perature inversion could explain the MinMin light (quoted in Kozicka1). I haveelaborated this suggestion in an attemptto show how refractive atmospheric phe-nomena, in combination with a lightsource, either natural (for example, planet

or star) or man-made (for example, lampor quartz-halogen headlight), along withhuman factors, could account for the di-verse puzzling aspects of the Min Min light.

DIRECT OBSERVATIONS ANDMEASUREMENTS OF THE MIN MINLIGHT

Observation 1Teriboah Creek in the Diamantina-Georgina watershed, Coorabulka Station,140 deg 8 min E 23 deg 47 min S, 11 July1992: 2153 hours; weather fine and clearwith a slight breeze; bright moonlight fromthree-quarter moon; driving across ashy-downs plains observing the behaviour ofletter winged kites (Elanus scriptus).

We noticed what we thought was eyeshine from a cat or a fox, although oddlyit was less-coloured than either predator’stapetal reflex. We estimated that it was lo-cated about 100 metres ahead of the vehi-cle. We were surprised that the bright spotof light was still there when we turned offthe headlights. We got out of the vehiclefor a better look and an emotional argu-ment ensued concerning the nature of thelight, as the three observers, (two scien-tists and one grazier with more than 20years of experience of the Channel Coun-try) disagreed violently about its nature.The arguments concerned the amount ofmovement and the distance of the light,as well as the possibility that it was respond-ing to us.

We all agreed that the light appeared tobe floating above the horizon. In the fieldof view of 10X binoculars, it was about 0.5to one degree above the horizon; this esti-mation was difficult to make because thehorizon was indistinct in the dark.Through binoculars, the light was a fuzzydisc, about 0.1 to 0.2 degree in diameter,with an outline that gave the impressionof dynamic activity, as if it were a ball ofbright swarming insects that could not beresolved. The light was variable in bright-ness, from around the brightness of a firstmagnitude star to perhaps five timesbrighter than that, changing unpredict-ably but gently over seconds. The light hadan eerie quality, apparently moving closer


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or further away, as well as bobbing around,up and down and from side to side. Asthese movements were one major sourceof disagreement among the three observ-ers, I suspected that they might be con-nected with the observers’ own head andeye movements and suggested that we getback in the vehicle. There, we could ob-serve the light against the reference frameof the vehicle (there were no trees or otherreference objects visible on the horizon),with a head-rest to stabilise head move-ments. When we did this, most of the ap-parent movements of the light ceased (asdid the heated argument). The variationsin brightness could still give the impres-sion that the light was approaching andreceding, but the stability provided byviewing from a headrest reduced some ofthe uncanny aspects of the light and madethe variation in intensity the likely key fea-ture that was making it appear to approachand recede.

By carefully manoeuvring the vehicle itwas possible to take a bearing on the light,using the magnetic compass mountedabove the dash, in combination with thesharp vertical contour provided by theVHF radio antenna attached to the bullbar. By making repeated measurements,we were able to achieve an accuracy ofaround one degree for the bearing of thelight. The bearing of the light was 130degrees true (120 degrees magnetic). Wethen drove the vehicle across the plain,orthogonal to the direction of the light (arelatively simple proposition in the plainscountry where the only obstacles are oc-casional small channels that can be drivenacross, with care). After a distance of aboutone kilometre, we could not detect anychange in the bearing of the light. Thiswas reassuring confirmation about the sta-bility of the light’s location but was discon-certing in that it implied a great distance.Even though our compass limited the ac-curacy of bearings to around one degree,the light would have to be more than 60 kmaway to have an undetectable shift in bear-ing with a one-kilometre change in thesighting location. We drove a further fourkilometres to put a base on the triangle offive kilometres. The bearing at five kilo-metres was now 129 degrees true. Within

Figure 1. Location of observations and demonstration of Min Min lights in the ChannelCountry of Western Queensland.

Observation 1. Two bearings of the light were taken using a car compass; the two pointswere separated by five kilometres. Note that the bearings converge on a fairly straight sectionof the Diamantina Development road that has both a direction and a distance from theobservation site consistent with the road train known to be moving northwest along the roadat that time, on the night. What is not consistent with this explanation is the great distance(more than 300 km) and the intervening line of hills that would normally preclude line ofsight between the site of Observation 1 and the road east of Windorah. Temperature inversionand a Fata Morgana will enable these two distant sites to be made visible to each other.

Observation 2. Similar to Observation 1 but with a closer source light in the form of avehicle at around 20 km.

Observation 3. Min Min light demonstration from a vehicle 10 km away, over a rise thatnormally precluded line of sight, during a temperature inversion.

Observation 4. Daytime Fata Morgana (shown in Figure 3) was observed in the same locationas Min Min demonstration Observation 3 at sunrise immediately following, during the sametemperature inversion.

Observation 5. Early observation of planet Venus that ‘did not set’ and conformed to manyfeatures of the Min Min phenomenon.


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of approximately180 m and 200 m abovesea level, respectively, with the Emu Bushhills near Mount Butler, elevation 303 mbetween. Even if there had been com-pletely flat country between the observa-tion site and the section of road con-cerned, it should have been impossible tosee this far over the horizon from a vehi-cle. The presence of the intervening hillsunderlines the mystery by further rulingout a line-of-sight observation (Figure 2).

Observation 2Ingledoon 2 Bore, Davenport Downs Sta-tion: 140 deg 28 min E 24 deg 20 min S;15 July 1992. On the night following Ob-servation 1, under similar weather condi-tions, we were able to make very similarobservations on another light to the souththat appeared for 30 minutes and the lo-cation of which proved to coincide with aroad that would normally be invisible be-cause of intervening hills. At one point,this light split into two sources that movedaway from each other, along a line that wasparallel to the horizon, at about two de-grees per second until the southerly light(on our left) disappeared. Through bin-oculars, we could see that the remaininglight kept moving very slowly northwards,consistent with a vehicle on the Monkira-Coorabulka road, except for the fact thatit was visible over the top of interveninghills.

Observation 3Ingledoon #2 Bore, Davenport Downs Sta-tion: 140 deg 28 min E 24 deg 20 min S;15 July 1992, between 2300 hours and 0100hours. The weather was calm and clearafter a warm day. I drove a vehicle 10 kmnorth of the camp, over a slight rise into awatercourse on the upper reaches of Nail’sCreek at 140 deg 40 min E 23 deg 55 minS. Because of the intervening hill and thelower elevation of the creek, the campsiteand the watercourse 10 km away are notnormally visible to each other. When Ipointed the vehicle in the direction of thecamp, six observers there saw a light, float-ing above the horizon. One of the six ob-servers had been present at Observation 1and said that the appearance of the lightwas similar in this case, a disc approxi-

Figure 2. Over-the-horizon refraction by the inverted mirage (Fata Morgana).During a temperature inversion, there is a gradient of increasing refractive index toward

the ground layer of air. This gradient can carry light rays around the Earth’s curvature(exaggerated here to make the point clear). Because we are accustomed to light rays travellingin straight lines, the light source emanating from over the horizon at A appears to be floatingabove the horizon for the observer at B. Images of light sources can be transmitted fairlyfaithfully in this way hundreds of kilometres around the globe, so there will be neitheraccompanying sound nor any cues from nearby objects to help in their identification. Therefractive index gradient acts like a very large light guide so that there is less dissipation anddispersion of the light as well as the possibility of magnification. If the light source is perceivedas close (a very common perception), it will not respond in the usual way to changes in theobserver’s viewpoint, giving rise to bizarre apparent distortions like those seen in PatrickHughes’ ‘reverspective’ paintings where there are conflicting cues about distance. Lightsources, both natural (for example, planets and bright stars) and man-made (for example,quartz halogen headlights), can give rise to the Min Min light phenomenon under conditionsof Fata Morgana at night when the usual diagnostic features of this mirage phenomenon areabsent. Because the inversion is usually changing constantly, the refracted light source willhave a shimmering edge (the usual description of a Min Min) and will fluctuate irregularly inbrightness with a resulting change in the apparent distance (as there are no other cues to thedistance of such a light source except brightness).

the accuracy of our estimate of the bear-ing, we calculated that the light was morethan 300 km away. This astonishing valuewas so far away from our initial 100 metreestimate that we double-checked it, withthe same result (within the error of ourmethod). This is shown in the map of Fig-ure 1.

The light was visible for about 20 min-utes and then suddenly blinked off. Whenlater plotted onto a map of the area, wefound that the bearing and distance coin-cided with a section of straight road to theeast of Windorah. This section of road had

a general heading toward our observationsites on the plains (Figure 1). In additionto the surprisingly large distance, we notedthat there were a couple of lines of reliefbetween the observation site and the ap-parent source of the light. One of theselines of hills was more than 100 metreshigher than the observation site and theapparent source (Hills near Mount But-ler, Figure 1). A later check with Windorahresidents revealed that a road train hadarrived from the east at around the timeof our observations. This section of roadand the observation site were at elevations


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mately 0.2 deg in diameter with a blurred,dynamic outline. By radio, we could verifythat switching the headlights on and offresulted in the disappearance and reap-pearance of the ‘floating’ light. By cover-ing each headlight in turn with an opaquecover, we could verify that there was a sin-gle, roughly circular light source withfuzzy, dynamic edges regardless of whetherone or both headlights were involved. Theintensity of the light varied, even when theheadlight source was known to be un-changing and there were also slightchanges in apparent position, mostly in thevertical direction that were estimated tobe around 0.5 degree by using binocularsand a steady reference. Its colour alsochanged, from a vivid red through orange,yellow and green. All observers agreed thatthe unusual light floating above the hori-zon that they observed must have beenproduced by the lights of my vehicle, al-though five were baffled and excited be-cause the vehicle should not have beenvisible at all, because of both the distanceand the intervening rise.

Observation 4This observation was made in daytime, themorning after Observation 3. Although itis not an observation of the Min Min light,it does illustrate the extraordinary opticaleffects of the atmospheric conditions atthe time and emphasises that they can pro-duce mystifying alterations that can be dif-ficult to understand even when there arefamiliar reference objects. How muchmore difficult it must be to correctly in-terpret effects on a single light source,without any reference marks, under suchatmospheric conditions. The weather wasperfectly clear and calm.

At sunrise there was a striking alterationto our view of the landscape that seemedto originate in the northeast and then ap-peared to ripple across the plains as theywere warmed by the rising sun. The altera-tion was a progressive revelation of thecoolabah trees (Eucalyptus microtheca) inthe channels where they normally lie in-visible from this camp. First, a tree out-line would pop up from the creek, thenthe one next to it and so on down the creekuntil the whole line of trees in the creek

Figure 3. Fata Morgana phenomenon observed at sunrise the morning after demonstrationObservation 4 of Min Min lights.

Figures 3a–3e show a succession of views, top to bottom, earlier to later. The usual viewfrom this site is shown in 3e, where the horizon is a featureless, slightly elevated plain. On themorning in question, a range of hills (Tent Hills) was clearly visible for about 10 minutes(a–d) just after sunrise as the temperature inversion of the previous night was enhancedbriefly by the warming action of the sun. The hills become visible and are vertically elongated,with a horizontal axis of symmetry, the hallmark of the inverted mirage. Small ‘holes’ of skythen appear in the mirage (b) and expand, dissecting the ‘plateau’ into a number of ‘mesas’(c) that multiply (d) and then sink beneath the horizon (e).

It is instructive to compare this daytime mirage with the Min Min lights that weredemonstrated at night, a few hours before, at the same site.

By imagining the path of light from a single point in the range of hills, one can begin toappreciate better the strange behaviour that a single light source would take at night. Thiscould move and split in a bizarre fashion that would be very puzzling without the corroboratinginput from lots of other points that we receive in a daylit scene. It is also very difficult toestimate the distance of such a mirage, so it would be likewise impossible for a single lightsource at night, the distance of which would almost always be underestimated.

The difficulties of recognising that Min Min lights are a nocturnal aspect of Fata Morganaand temperature inversions arise partly from the fact that a single isolated light source doesnot provide the suite of diagnostic features (such as vertical elongations, horizontal axis ofsymmetry, shimmering outline, dynamic dissections et cetera) that help one to recognise thephenomenon during the day.


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was made visible over a few seconds. A simi-lar effect became visible in creeks to thesouth, until there were dozens of lines oftrees in their channels, all invisible a fewminutes before. Through binoculars, eachtree had a pointed top, instead of the usualrounded crown that characterisescoolabahs. There was a constant shimmerand changeability of the altered landscapethat made it difficult to be certain aboutall details of this change but it appearedthat the pointed profile was producedbecause each tree now had a mirror im-age above it, reflected about the horizon-tal plane, so the top of the new view ofeach tree was its trunk rather than itscrown.

As we were marvelling at the phenom-enon in the channels and trying to photo-graph it, we noticed an even more bizarrescene to the north. Instead of the slightrise that normally dominates the view tothe north, there was a plateau of brightlycoloured rock that occupied a sweep of20 to 30 degrees of the northern horizon.The plateau appeared to be made of lay-ers of brown and red rock and throughbinoculars, we could see that it had a hori-zontal axis of symmetry. Just as we noticedthis mirror symmetry, tongues of blue skystarted to ‘eat into’ the mesa. Both hori-zontally along the line of symmetry andvertically in a few places, small ‘holes’ ofsky appeared and became dissecting blue

blades that invaded the plateau so that itwas broken into smaller ‘buttes’ that con-tinued to diminish in size (Figure 3). Aswe tried to integrate all the changes andphotograph them, the buttes sank smoothlyand vertically below the horizon. Thesechanges are shown in Figures 3 and 4.

On an orientation tour later, we foundthat the plateau that we had observed wasproduced by a low lying rocky hill (TentHills, approximately 20 km north) that isnot normally visible unless one drives atleast about 10 km from the camp wherewe made the observations.

Observation 5On 26 July 1990, we were waiting for acolleague at a rendezvous on the plains inSallen Creek. Around 2100 hours, we no-ticed a red light to the west. Thinking thatit must be the tail light of Lindsay’s vehi-cle, we set out across the plains in the 4WDto pursue. After a very bumpy ride on theMitchell grass tussocks, we realised that wewere getting no closer to the light andstopped to check it with binoculars.Through binoculars, we saw a light hover-ing above the horizon that changed col-our from green to yellow to red and thathad a fuzzy outline. The light was in theappropriate location for a setting planet,(Venus and Jupiter were close to the sunat that time and would have been visibleto the west in the evening) but seemed atfirst to be getting no closer to the hori-zon, so we did not consider this interpre-tation. After another 10 minutes or so, thelight disappeared over the horizon sud-denly and we realised that we must havebeen watching Venus in some odd atmos-pheric conditions. At the time, I made noconnection to the Min Min light but yearslater realised that this may have been myfirst experience of the phenomenon.

DISCUSSION

Some of the findings presented here mightseem to stretch scientific credibility. Forexample, it can be calculated that even amodern bright, quartz halogen headlightwould be too dim to be seen from 300 kmaway under normal conditions at night.Such a calculation would make the usual

Figure 4. ‘Horizontal splitting’ of Fata Morgana: same observation site as Figure 3 but viewedfurther to the west. Arrow indicates same rock in each view. This horizontal axis of symmetryis diagnostic of the Fata Morgana.


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assumption that light energy was beingradiated according to the inverse squarelaw, along with some atmospheric attenu-ation and gives a brightness around 0.006cd/m2 which is too close to the brightnessof the night sky to be visible.2 Similarly, atsuch distances the light source would bevery much over the horizon and thereforeunable to reach the observer, if light is trav-elling in straight lines as we expect it todo, even if there were no intervening hills(the distance of the horizon is 3.57√h kilo-metres where h is the height above theground in metres). At distances of hun-dreds of kilometres, a light source shouldalso have had a much smaller angularsubtense, so that it should have appearedas a point source rather than the disc thatwas observed.

How could the light be brought over thehorizon and without the usual dissipationof intensity and reduction in size? An ex-planation is offered by the rare but well-described, atmospheric phenomenoncalled the Fata Morgana or inverted mi-rage.3 In contrast to a ‘normal’ mirage,where light is reflected from a hot, rare-fied ground layer of air (for example, sothat we see the blue sky reflected in thesurface of a hot road or desert), an in-verted mirage occurs when light is re-fracted by a cold, dense ground layer ofair, which acts to guide light over the hori-zon, sometimes for hundreds of kilome-tres, with the possibility of magnificationand a reduced degree of dispersion anddissipation.4 The observations of Min Minlights reported here, as well as most ofthose summarised in Kozicka’s study,1 areconsistent with a light source that has beenrefracted by the Fata Morgana, a phenom-enon that is produced by the refractiveindex gradient between a ground layer ofcold, dense air and the warmer, less denseair above it. The name comes from theMorgan fairy, who was reputed to be ableto conjure cities floating on the sea. Thisis a case where a myth has a solid basis inphysics. The phenomenon is better knownin the Northern Hemisphere, where seaice or very cold sea and relatively calmweather conditions often favour the appro-priate layering of air. In some northernlanguages there is a specific word for the

phenomenon that reflects its commonoccurrence, for example, hilldring in Nor-wegian. The magnitude of the gradientvaries considerably according to the atmos-pheric conditions.

The features of the Fata Morgana kindof refractive effect include the following,all of which can be seen to apply also tothe observations of the Min Min.

Refraction over the horizonIn the most celebrated cases, Irish sea cliffshave been observed clearly in the middleof the North Atlantic, many hundreds ofkilometres away from their real origin.4

Apart from the fact that they were far overthe horizon, these cliffs should have beentoo small to be visible. A 100-metre cliffwill subtend only a fraction of a minute ofarc in height at that distance, instead ofthe many minutes of arc observed. Thisfamous example of the Fata Morgana re-calls the difficulties in explaining thepresent observations of the Min Min atmore than 300 km in Observation 1. An-other celebrated example, from Antarc-tica, concerns unexpected sunrises.Shackleton,5 among many others, whoexperienced this subsequent to his earlyaccount, described how his party’s firstsunrise after an Antarctic winter occurredthree days earlier than calculated becauseof atmospheric refraction. The reality ofthe refractive explanation was confirmedby the fact that the sun failed to rise at allon the following two days. This FataMorgana effect was a source of some cheerto Shackleton’s stranded men, at thethought that their fastidious meteorologisthad calculated his dates wrongly.

If the ‘ground layer’ of air is very coldand still, transmission of light can be re-markably faithful, as in the Irish sea cliffsexample described above, where the greenof grass and brown of rock were distin-guishable even though the image hadbeen transmitted nearly 1,000 km. Simi-lar fidelity is also apparent in the descrip-tions of icebergs over the horizon madeby Shackleton below, where one also notesthe changeable and distorting effects. Inother cases, there is more diffusion of thetransmitted image, such as may be occur-ring in Min Min observations of a single

fuzzy ball with dynamic edges, even whenthere were two nearby sources of light suchas two headlights. Under Fata Morganaconditions one cannot assume that lightwill obey the inverse square law, as it is notradiating perfectly from the light sourcebut is being guided within the refractiveindex gradient, which may focus the lightto some extent and thereby lead to thepropagation of a much brighter sourcethan expected on the basis of normal dis-sipation. There are many reliable reportsof the Min Min casting a shadow and illu-minating its surroundings. These becomemore understandable if the Fata Morganaeffect is acting as a crude light guide anddelivering a light source with little of theusual attenuation to an unsuspecting ob-server tens or even hundreds of kilome-tres away.

The over-the-horizon feature explainsthe most commonly observed location ofMin Mins, like those described here, wherethe light is seen floating above the hori-zon (Figure 1). The over-the-horizon ef-fect would explain how a Min Min lightcould be produced by a light 340 km awaywith 100 m of relief in between, as in Ob-servations 2, as well as the other observa-tions, such as 5, where the light sourcewould not be visible under ordinary atmos-pheric circumstances.

Vertical reflection and stretchingof the mirageMin Mins are commonly reported to splitinto a pair or more lights, which may be aresult of this aspect of the Fata Morganaphenomenon. We did not observe any ver-tical elongation or splitting of the lightsbut the ‘mesa’ and coolabah observationsof mirror images in daylight (Observation4) illustrate this effect and emphasise howdifficult it would be to interpret the be-haviour of a single light source,uncorrelated with others, in the presenceof these kinds of distortions. Observation2 involved a horizontal splitting that canbe related to the horizontal dissection ofthe Fata Morgana observed in Observation4. Under extreme circumstances of thiskind, it would be possible to see lightsahead that had originated behind, as ob-served for the Min Min described by Dr


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Steve van Dyck (cited by Kosicka1). A 180-degree refraction would be required toexplain this observation, again stretchingcredulity given the gentle curves usuallydescribed by light paths in mirages. Nev-ertheless, the observer was an experiencedscientist and the terrain may have facili-tated the formation of a giant refractive‘bubble’ or lens, with greater than usualcurvature of the refractive index gradient.This speculation could be tested with moredetailed observation of the three-dimen-sional shape of temperature inversions inthe sandhill country where this unusualMin Min was sighted.

Colour changesSpectral changes are common in FataMorgana, resulting, for example, in a‘green flash’ at sunset or sunrise.3 Whileblue Min Mins are rare, presumably be-cause of greater scatter at short wavelengths, green, yellow and red variants arecommonly observed, as in the presentobservations.

Great sensitivity to temperaturechangesSlight changes in the gradient of air den-sity and refractive index can produce dra-matic changes in the refractive effects. Inthe present observations, shimmer andmovement of the lights and presumablywaves of changes in the daylight scene arecaused by temperature fluctuations as theearth cools (night) or is warmed (as in thedaylight example). This aspect could helpexplain the uncanny ability of the Min Minto move suddenly or to split as the refrac-tive layers change with warming or cool-ing or wind.

The following lyrical description of ice-bergs during a Fata Morgana, fromShackleton5 illustrates all these aspects:over the horizon refraction, vertical‘stretching’, colour changes and tempera-ture changes.

‘The distant pack is thrown into tower-ing, barrier-like cliffs, which are reflectedin blue lakes and lanes of water at theirbase. Great white and golden cities of Ori-ental appearance at close intervals alongthese cliff tops indicate distant bergs, somenot previously known to us. Floating above

these are wavering violet and creamy linesof still more remote bergs and pack. Thelines rise and fall, tremble, dissipate, andreappear in an endless transformationscene. The southern pack and bergs,catching the sun’s rays, are golden, but tothe north the ice masses are purple. Herethe bergs assume changing forms, first acastle, then a balloon just clear of thehorizon, that changes into an immensemushroom, a mosque, a cathedral. Theprincipal characteristic is the verticallengthening of the object, a small pressureridge being given the appearance of a lineof battlements or towering cliffs. The mi-rage is produced by refraction and is in-tensified by the columns of comparativelywarm air rising from several cracks awaynorth and south.’

Animation and human factorsThe above account has focused on thephysical aspects of the phenomenon butit is important to recognise that many ofthe remarkable aspects of the Min Minlight may be a result of contributions fromthe observer’s side. This applies particu-larly to the common observation that MinMins appear to be responsive to the ob-server. Kozicka1 even has a chapter de-voted to the many ways that were inventedto handle the Min Min: ‘How to get rid ofit’. Certainly the hair-raising aspects of MinMin are usually connected to its apparentability to respond to the observer. How canthis feature be encompassed in the presentaccount?

An answer to this question requires arecognition of the ambiguities present inall perceptual judgements and the largecontribution required by outgoing brainactivity to supplement passive sensoryprocessing (‘efference copy’ or ‘corollarydischarge’ in neurophysiological terms or‘expectations’ in common parlance).

A single isolated light source is difficultto pin down in space when there are noobvious visual landmarks nearby. As a re-sult, one’s own eye movements and headmovements may contribute motion to theMin Min, regardless of whether this is lat-eral motion or motion in depth. This ef-fect varies between individuals, becausethe ability to subtract the self-induced

motion signal (efference copy) from theretinal motion signals, to calculate realmotion, is not constant, either from indi-vidual to individual6 or from situation tosituation. In keeping with this, sitting withone’s head still against a rest tends to elimi-nate or reduce some of the lateral motionsof the Min Min, as described above forObservation 2. The problem of motion isa common source of the strong emotionsexperienced by observers of the Min Minlight. In support of this interpretation,careful observation with the head heldsteady against a support reduced the ‘un-canny’ aspects of the lights we observed.In fact, sometimes a calm observer willwitness the phenomenon without realisingthat it is a Min Min. For example, I remem-ber one case where some workers at acamp in the Channel Country referred toseeing ‘Peter’s light’ on a few nights, inreference to a field scientist from Queens-land National Parks whom they knew wascamped about 10 kilometres away (at Ob-servation site 2, 3). A low intervening hillwould normally block any light comingfrom that direction but this was not real-ised by the workers because the undula-tions in that country are gentle. Becausethe light was in the appropriate directionand there was no-one else in this uninhab-ited area, they correctly assumed that itwas Peter’s light but failed to appreciatethat the light would have had to travel avery unusual curved path to reach them.Ironically, one of the same workers ex-pressed a regret that she had never seen aMin Min despite some time spent outdoorsat night in the Channel Country.

Min Min lights will have no strong cueabout their distance from the observer.They may be too far away to generate bin-ocular cues, such as disparity between thetwo eyes. There are usually no nearby con-tours to provide relative motion cues. Theyhave an indeterminate size or unfamiliarshape that prevents or confuses size con-stancy cues. Perspective and texture gra-dient cues are usually not available becausereference surfaces such as the ground andthe horizon are barely visible. Therefore,it is likely that changes in brightness mightbe interpreted as changes in distance, evenif these fluctuations are related to atmos-


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pheric conditions instead of distance. In-tense ‘looking’ can be shown to affect thevergence signals that activate the visualcortex of behaving monkeys in a way thatexaggerates the changes in activity pro-duced by fixating a closer object. This ef-fect is only quasi-predictable because itdepends on the personality of the mon-key and the conditions of the experi-ment.7,8 As with variations in efferencecopy associated with lateral motion, thereare likely to be individual variations in theextent to which an individual human at-tributes motion in depth to a target whenthere is no other visual information avail-able about this kind of motion. Generally.it is not appreciated that isolated cues(such as stereopsis or motion parallaxalone) do not provide visual depth that isveridical. Our sense of depth from suchcues is so strong that we may fail to appre-ciate that the information about the dis-tance of objects is only relative and that ithas to be ‘truthed’ by another system (suchas the tactile system or by head move-ments) before we can achieve veridicaldistance. The artist Patrick Hughes takesadvantage of this fact by the use of whathe calls ‘reverspective’ in his paintings.9

These paintings are real three-dimensionalobjects (for example, a truncated pyra-mid) that project out toward the observerbut the projections are painted with per-spective cues that make them appear toproject in exactly the opposite direction(make your own, using http://www. p e r c e p t i o n w e b . c o m / p e r c 0 9 9 9 /wade.pdf). When they are viewed at a dis-tance, or with one eye closed, so that stereoinformation does not dominate, thepainted perspective cues give the impres-sion that the solid objects are hollow. Thiswill not be surprising to those of us famil-iar with a three-dimensional cast of a face.In the right kind of lighting, these hollowfaces are not seen to be hollow but con-vex instead. The real surprise with suchfigures comes when one moves while‘holding in mind’ the reversed view. Forboth the Hughes ‘reverspective’ paintingsand for the hollow face, there is a very dis-turbing distortion as the brain tries to clingto its false perspective construction aboutdepth relations despite the conflicting

information coming from motion parallax.The face appears to turn and follow, eventhough we ‘know’ that this is impossible(hence the incorporation of such effectsin entertainment parks such as Disney-land’s ‘Haunted House’), while theHughes reverspective painting undergoesa similar impossible transformation (as ifit has suddenly been made into stretch-able rubber) that can be nauseatingly realto many viewers. Cues from one’s past ex-perience with real faces and solid objectstend to dominate the veridical ones in thisbattle of conflicting alternatives.

Similarly, an observer of a Min Min light,with finely-honed senses after years ofworking in the testing visual conditions ofthe Outback, will be likely to apply this pastexperience to the phenomenon. If thelight is judged to be a short distance awaywhen it is actually hundreds of kilometres,the changes in the light produced by theobserver’s movement clearly will not con-form to expectations. If the observer’sbrain adheres for a time to the first per-ception, head movement will produce avery disturbing sensation, like that seen inthe Patrick Hughes paintings, because theexpected changes caused by motion par-allax will not occur for a light source thatis very far away. It is important to pointout that these distortions can be just asdisturbing when first seen by a visual sci-entist, even one completely familiar withthe underlying psychophysics, such asmyself. It is only after long experience withthe phenomenon and ‘truthing’ from othersources of information that an observer cancome to an acceptance of such mismatchesbetween constructions and reality.

For this reason, observers with a longexperience of photography or astronomy,for example, may be less taken in by theextreme aspects of the Min Min phenom-enon. Most of us do not experience anysurprise when the moon ‘keeps up’ behindthe trees as we speed along in a car. Thealarm at a Min Min doing the same thingis based on the assumption that the MinMin is not far away. Because the Min Mincan be located hundreds of kilometresaway ‘over the horizon’, the key contribu-tion from the individual observer thatmakes the phenomenon so dramatic is the

observer’s assumption about its location.If one assumes that the light originatesnearby, its ability to maintain its headingwith respect to trees in the foreground aswe speed is surprising. On the other hand,if it is far away, this would be no more sur-prising than it would be for the moon tomaintain its bearing.

Answers to frequently-askedquestions about Min Min lightsbased on the Fata Morgana

WHY THE CHANNEL COUNTRY?

Hot days and cold nights favour the for-mation of temperature inversions in theChannel Country, while the channelsthemselves provide gentle hollows for theaccumulation of cold air and the flat plainspermit visibility right to the horizon.

WHY ARE SOME SO BRIGHT?

Although natural light sources such asfires, bright stars and planets may havebeen responsible for the origins of the MinMin legend among the indigenous popu-lation, the advent of the quartz halogenheadlight has undoubtedly contributed tomore recent stories of bright shadows gen-erated against buildings. The surprisingmaintenance of brightness over great dis-tances, when one would normally expectdissipation according to the inverse squarelaw, can be explained by the refractive ef-fect of the ground layer. A refractive in-dex gradient can act to ‘trap’ light, in afashion akin to a light guide, so that lightcan be carried long distances with less ofthe usual reduction in size and brightnesscaused by distance.

WHAT ABOUT THOSE CASES WHERE

ONE CAN ABSOLUTELY RULE OUT A

LIGHT SOURCE IN THE VICINITY?

Because light can be carried hundreds ofkilometres with minimal distortion in afavourable Fata Morgana, one has to lookmore widely than the local vicinity toexclude possible light sources.

HOW DOES THE LIGHT FOLLOW AN

OBSERVER?

None of the usual cues to distance is avail-able in a refracted, isolated, small light


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source, so the perceived distance can varywidely and be subject to internal humanfactors. The act of scrutinising an isolatedlight source can make it appear to moveas a result of tracking and vergence eyemovements if there are no nearby refer-ence targets.

WHY IS MID-WINTER A PREFERRED TIME

FOR MIN MINS?

Cold stable air is necessary for the refrac-tive effects of a temperature inversion.These conditions occur most often in mid-winter in outback Queensland.

HOW CAN THEY MOVE SO QUICKLY?

There are two independent possiblesources of rapid movement: from internalhuman factors such as eye movements, ofwhich the saccadic variety are very fast andfrom optical changes in the refractive lay-ers, which can occur rapidly because ofwind or temperature change. One way thatone can infer the kinds of rapid opticalchanges that might take place with a sin-gle light source is to look at a series ofphotos of a Fata Morgana landscape in day-light, where the same feature can betracked from frame to frame as it movesabout and sometimes splits (see http://www.jackstephensimages.com/Merchant/photographicgallery/Fatamorgana/fatamorganapage.html)

HOW CAN THEY BE RESPONSIVE TO

THE MOOD AND ACTIONS OF THE

OBSERVER?

Apparent direction and distance are am-biguous for isolated stimuli and changewhen the willed effort of the observer alsochanges. Firing a rifle will have an effect onthe observer, for example, that will suddenlychange the observer’s viewpoint and there-fore the perceived location of the light.

WHY NO ASSOCIATED SOUNDS?

At distances of kilometres to hundreds ofkilometres, the origins of the Min Min lightwill be inaudible, even if they are associ-ated with a sound source such as a vehicle.

ApologyI know that there will be great resistanceto the acceptance of this and any other

explanation of the Min Min light frommany in the Outback who are cynicalabout attempts by city slickers to reducethe magic and wonder of the phenom-enon. I have some sympathy with this re-action but would plead that my approachto putting the phenomenon on a moreunderstandable basis does not necessarilyexplain it away but rather may enhanceone’s experience of it. My detailed knowl-edge of the exquisite visual system of birdsof prey has not taken away any of the won-der and enjoyment of watching these birdsin their natural habitat. On the contrary, Iwould say that increased understandinghas enhanced my enjoyment. Knowingmore about the underlying principles canimprove one’s ability to interpret subtlefeatures in the natural world. Likewise, myaccount of the Min Min may enhance view-ing of it, if only by defining more preciselythe atmospheric conditions under whichit is likely to take place. If I had knownwhat I know now about the Fata Morganawhen Maureen Kozicka was alive, I mayhave been able to help her achieve herdesire to witness one.

Future researchThe refractive index gradients producedby a temperature inversion are crucial tounderstanding the fluctuations of a MinMin light, along with the intrinsic humanfactors I have mentioned. Refractive indexgradients have become an important topicin vision science, especially since the ‘fivedioptre surprise’ where early calculationsof the power needed in a replacement lenswere in error because of the failure to rec-ognise the greater effective power of thenatural gradient index lens compared withthe replacement of the same curvatureand same average, but uniform, refractiveindex.10 Determining the precise natureof the refractive effects involved in outbackAustralian Fata Morgana phenomenawould require one to determine the pro-file of the atmospheric density changesduring an inversion, using a probe on ahelium balloon, for example. This couldbe done concurrently with observations ofthe kind I have described here, with thepossibility of accounting in more detail forthe bizarre aspects of the phenomenon,

along with precise details of the changesin refractive gradients that I believe to beresponsible for the sudden appearancesand disappearances of the Min Min light.

ACKNOWLEDGEMENTS

My visits to the Channel Country werefunded by an Australian Research Coun-cil Special Research Centre. I thank themany colleagues who participated in thesestudies and the friends in the Outback whoinitiated my interest in Min Mins by shar-ing their experiences with me. Two anony-mous referees provided helpful commentson the paper.

REFERENCES1. Kozicka MG, The Mystery of the Min Min

Light. Cairns, Queensland: Bolton Imprint,1994: 136.

2. Middleton WEK. Vision Through the At-mosphere. Toronto: University of TorontoPress, 1952.

3. Lynch DK, Livingstone W. Light and Colorin Nature, 2nd ed. Cambridge: CambridgeUniversity Press, 2001.

4. Minnaert MGJ. Light and Color in the Out-doors. 1st ed 1937; translation of 5th ed.Berlin: Springer-Verlag, 1993.

5. Shackleton EH (1919) South. 400 pp.North Books; ISBN: 1582871159; (April,2000).

6. Behrens F, Grusser OJ. Smooth pursuit eyemovements and optokinetic nystagmus elic-ited by intermittently illuminated station-ary patterns. Exp Brain Res 1979; 37:317-336.

7. Trotter Y, Celebrini S, Stricanne B, ThorpeS, Imbert M. Modulation of neural stere-oscopic processing in primate area V1 bythe viewing distance. Science 1992; 257:1279-1281.

8. Papathomas TV. Experiments on the roleof painted cues in Hughes’s reverspectives.Perception 2002; 31: 521-530.

9. Hughes A. In: Pettigrew J, Sanderson KJ,Levick WR, eds. Visual Neuroscience. Cam-bridge: Cambridge University Press, 1986.

10. Viguier A, Clement G, Trotter Y. Distanceperception within near visual space. Percep-tion 2001; 30: 115-124.

Notes and websites on Fata Morgana andMin Min lights1. h t t p : / / m i n t a k a . s d s u . e d u / G F /mirages/mirintro.html

This excellent website by Andrew TYoung has a detailed description of theoptical phenomena of mirages, including


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the Fata Morgana (inverted mirage), withdetailed ray-tracing diagrams, an accountof the controversy over whether miragesreflect or refract light, a detailed classifi-cation of the three different inverted mi-rages (which I have placed into the FataMorgana category but which Young distin-guishes, giving the Fata Morgana title onlyto cases in which there is a stack of multi-ple mirages), the related ‘green flash’ phe-nomenon et cetera.2. Arctic Fata Morgana

http://www.jackstephensimages.com/M e r c h a n t / p h o t o g r a p h i c g a l l e r y /fatamorgana/fatamorganapage.html

Shows photos of Thule Rock in thearctic, with multiple images of the rock insome cases and a dramatic variation withtime of the shape of the rock.3. South Dakota Fata Morgana

http://www. crh.noaa.gov/unr/iwe/2000/1219/

Shows ‘towers’ and ‘mesas’ like thosedescribed here.4. Accounts of the Min Min

http ://www.cas t leofspir i t s .com/minmins.html

This site has excerpts from MaureenKozicka’s book, as well as some additionalsightings.

It is interesting that some of the tough-est and roughest people I have met, likeshooters and drovers, had no qualmsabout admitting that they cried or weptwhen they encountered the lights.5. Min Min High Overhead

http://www.strangenation.com.au/Casebook/minminlight.htm

This is a detailed description of an en-counter with a Min Min by a truck driver.At the end of the encounter, the lightmoves from a position above the horizonto a more elevated position over the ob-servers and then heads off to the horizonat great speed. Although the observersthink that the sighting could be attribut-able to the ‘phosphorescent bird’ expla-nation, it should be borne in mind thatrefraction can also produce extreme de-viations, particularly if there is a pocket ofcool air in a hollow and the light encoun-ters the edges of the hollow. In this case,the light moved overhead when the observ-ers were in a slight hollow, after rain. The

observers describe rain puddles on theground, which may have had a cool layerthat was responsible for the extremedeviation of the light.

Authors address:Professor John D PettigrewVision Touch and Hearing ResearchCentreRitchie Research LabsResearch RoadUniversity of QueenslandQLD 4072AUSTRALIA

c l i n i c a l a n d e x p e r i m e n t a l optometry ... · the min min light and the fata...

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