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The explosion was registered by an earthquake-measuring station some 4,000 kilometres away in the city of St

Petersburg. Earthquake tremors were also recorded by more distant stations around the world.

Disturbances in Earth's magnetic field - similar to ones produced by nuclear explosions in the atmosphere - were recorded 970 kilometres south of the explosion site by the Irkutsk Magnetic and Meteorological Observatory. The magnetic storm lasted more than four hours. Subsequent analysis of these records showed that the epicentre of the 'earthquake' coincided with the location of the explosion (latitude 60 degrees 55 minutes north, longitude 101 degrees 57 minutes east) and confirmed the accurate time of the event (0014 gmt; 7.14 a.m. local time).

Two weeks after the explosion, the Smithsonian Astrophysical Observatory and Mount Wilson Observatory in the United States recorded a marked decrease in the air's transparency. It has been suggested that this was due to the loss of vast amounts of material from the fireball as it passed through the atmosphere. Calculations showed this loss to be several million tonnes, a hundred times more than the normal yearly fall of meteorite matter on Earth.

On the evening of the explosion, bright, colourful and prolonged dusks were noticed across the Continent as far as Spain. Photographs of the sky taken on that night at the Heidelberg Astronomical Observatory were badly clouded because of the bright sky. A Hamburg photographer who took a picture of the sky at 11 p.m. described it as 'volcanic dust', as the memory of the 1883 Krakatoa volcano, modern history's most violent eruption, was still fresh in people's memory. After sunset in Antwerp the northern horizon appeared to be on fire. A report in Vart Land, a Stockholm evening newspaper, described a 'strange illumination' on the night of 30 June.

The nights were also unusually bright over the British Isles. On 2 July the London Times published a letter from a Miss Katharine Stephen of Huntingdon about 'the strange light in the sky' which she and her sister had observed between midnight and 12.15 a.m. on 1 July. 'It would be interesting', she requested, 'if anyone could explain the cause of so unusual a sight.' The next day a Holcombe Ingleby of Brancaster wrote about the 'curious sun effects at night' which had the appearance of a dying sunset of exquisite beauty. 'This not only lasted but actually grew both in extent and intensity till 2.30 this morning', he continued. 'I myself was aroused from sleep at 1.15, and so strong was the light at that hour that I could read a book by it in my chamber quite comfortably.' In the same issue the newspaper also reported that in Dublin 'a very remarkable afterglow prolonged the daylight to such an extent that it was possible to read a newspaper in the open air'.

On 4 July The Times made an attempt to explain 'the remarkable ruddy glows which have been seen on many nights lately'. The newspaper remarked that these glows had been seen over an area extending as far as Berlin, and pointed out that there was a considerable difference of opinion as to the nature of these glows: 'Some hold that they are auroral; their colour is quite consistent with this view ... [others hold that] the phenomenon was simply an abnormal twilight glow ... We may recall the circumstances of the wonderful glows which were seen in this country in the autumn of 1883, and which were due to the dust scattered in the upper atmosphere by the terrific outburst at Krakatoa at the end of August. Those glows had many points in common with the recent ones.' The Times noted that 'distance is no obstacle in vast cosmical phenomena of this kind, which are absolutely world-embracing' and it was possible that the dust may have come 'from some unreported volcanic eruption in some little-known region of the world'.

On 3 July, The New York Times reported that 'remarkable lights were observed in the northern heavens on Tuesday and Wednesday nights, the bright diffused white and yellow illumination continuing throughout the night until it disappeared at dawn', which the newspaper attributed to 'important changes on the sun's surface, causing electrical discharges'. Two days later the newspaper published another report from its London correspondent. The report, 'like dawn at midnight:

london sees sky blue and clouds tipped with pink at that hour', said that several nights through the week were marked by strange atmospheric effects. 'Following sunsets of exceptional beauty and twilight effects remarkable even in England', the report continued, 'the northern sky at midnight became light blue, as if the dawn were breaking, and the clouds were touched with pink, in so marked a fashion that police headquarters was rung up by several people, who believed a big fire was raging in the north of London.'

At Edinburgh Observatory the night sky was noted as being very striking, 'practically daylight'. This practical daylight, The Scotsman reported, caused shadows to be cast in rooms with windows facing north. The saying 'make hay while the sun shines' took on a new meaning when farmers in the north of England worked in the fields all night getting in their hay before an impending storm broke.

The night brightness slowly diminished and disappeared after a few days, but scientists continued to speculate on the cause of these 'nocturnal glows'. W.F. Denning, an astronomer from Bristol, wrote in the weekly journal Nature on 9 July: 'I have never seen June nights so dark, and Milky Way so gorgeously displayed in the heavens ... nor have I ever noticed the sky so bright as it appeared on the nights of June 30 and July 1.' Bohuslav Brauner of the Bohemian University in Prague also wrote in the same issue: 'The peculiar light phenomenon at midnight on June 30 . was also observed by me at Prague . It is reported that magnetic disturbances were experienced on the telegraphic lines, but I saw no trace of the characteristic auroral bands or columns.' The next week Denning wrote again, this time discounting his earlier explanation that the night glows were due to aurora borealis (displays of coloured lights in the northern skies): 'Whatever the true nature of the recent exhibition may have been, it is certain that something in the air exercised the capacity of reflection in a very high degree.'

In its August issue, the magazine of the Royal Observatory at Greenwich provided a vivid account of the night glows: 'At 9.30 p.m. at Greenwich, on June 30, the sky along the north-west and north horizon was of a brilliant red, in fact there was what is usually termed a

"brilliant sunset", the only peculiarity being that the brightness stretched more to the north than is usual, and endured, so that at one o'clock in the morning it extended well across the north of the horizon, and the northern sky above was of a brightness approaching that of the southern sky at the time of Full Moon.' The unsigned article went on to say that observations failed to give any evidence that it was an auroral display but 'the light, indeed, was sufficient to take photographs of terrestrial objects'. An excellent photograph taken shortly before midnight with an exposure of about one minute showed the domes of the Naval College of the Royal Observatory with the College's training ship Fame in the foreground.

On the other side of the Atlantic, Scientific American reported on 29 August that 'sky glows', called by some of the European astronomers aurora displays, were now the subject of interesting discussion in astronomical circles, especially among the scientists of Europe. 'For some time a peculiar strong orange-yellow light over the horizon, the color of which was more orange in its lower parts and more yellow in its higher parts, has been observed all over northern Europe and the United States', the report continued. 'Clouds or spiral streams of various tints were brilliantly outlined across the sky, so luminous that few stars could be seen, and the Milky Way was hardly distinguishable.' The report quoted Denning and Brauner from Nature and added that 'both say they saw no trace of the characteristic auroral bands or columns in this phenomenon'.

The scientists at the Dublin meeting of the British

Association for the Advancement of Science in September 1908 did not know that the microbarograph invented in 1903 by two prominent members, W.N. Shaw (later Sir Napier Shaw) and W.H. Dines, had, in fact, recorded disturbances caused by the Tunguska explosion. The microbarograph automatically records sudden small changes in atmospheric pressure, but does not show changes due to ordinary rising and falling of the barometer. During a discussion on wave motion, as a curious example of atmospheric wave motion, Shaw presented six graphs recorded at six different locations in England on 30 June 1908 at about 0514 gmt (that is, five hours after the Tunguska explosion). Each graph showed a series of air waves during a period of about one hour. Each wave had four clear peaks, as if there had been four disturbances in Earth's atmosphere during that period. Shaw noted that the peaks lasted for about fifteen minutes and they were then 'violently interrupted by a sudden though slight disturbance' for a similar interval. Scientists at the meeting thought that this curious phenomenon was due to a large atmospheric disturbance in some unknown part of the world.

Like the night glows, the six graphs were to remain one of the unexplained mysteries of science for two decades. No one, except some observers in Siberia, was aware that a mysterious fireball had exploded in the Siberian sky.

The British Antarctic Expedition of 1907-09, led by Sir Ernest Shackleton, was wintering at the Cape Royds Station in the Antarctic when the Tunguska fireball exploded. Did Shackleton's party observe aurora australis

Figure 1: Six microbarographs recorded at different locations in England on 30 June 1908 at about 0514 gmt. (From F.J.W. Whipple, 'The great Siberian meteor', in the Quarterly Journal of the Royal Meteorological Society, vol. 56, 1930.)

(coloured lights in the far southern skies) at the time of the explosion? There is no evidence of this; however, there is a record of an exceptional aurora seven hours before the explosion. Was this aurora in any way related to the fireball?

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