238 THE ERUPTION OF KRAKATOA IN 1883 



Optical Phenomena 



No effect connected with the Krakatoa eruption has been so widely 

 observed and speculated upon as the wonderful appearance the sky 

 presented during the two years that followed the outbreak. Many 

 theories have been offered to explain the phenomena, and while most 

 of these agree that the dust formed in"such enormous quantities during 

 the eruption was the primary cause, some contend that the two were in 

 no way connected. It is impossible to give here even a short account of 

 these numerous theories, or the reasons why many have been rejected. 



Meager accounts exist of similar phenomena accompanying pre- 

 vious great eruptions. During the 250 years, from 1500 to 1750 inclu- 

 sive, there were 65 in which eruptions were recorded, nine of which were 

 of exceptional magnitude, " and in three of these latter years Denmark 

 was affected by appearances, two of which certainly were of the char- 

 acter of the afterglows, while the third is doubtful." 



From 1750 to 1886 the number of "years of eruption" was 90; 

 25 of the outbreaks being of great magnitude, and of these 16 appear 

 to have been followed by "unusual redness in the sky, or brightness 

 after sunset." Thus it will be seen that in the past like causes have 

 produced like effects, and the lack of information on the subject is 

 probably the fault of observers rather than the want of opportunity for 

 observation. The optical effects of the eruption of Krakatoa, however, 

 were exceptional, and had previous phenomena been so marked and so 

 widely spread over the earth, they would have received more attention 

 than the records imply. The effects of the outburst at Krakatoa were 

 the unusual twilight glows, the blue, green, and other appearances of 

 the sun, the sky haze, and the large corona round the sun and moon 

 — all these phenomena were noticed first in the vicinity of Krakatoa. 



The dust, after being ejected from the volcano, was carried west- 

 ward around the earth at the equator, certainly twice, and "perhaps 

 continuously." From this band the particles appear to have spread 

 north and south, and so the phenomena were witnessed throughout 

 the world. The twilight glows traveled westward at the rate of about 

 72 miles an hour, while the colored suns moved at the rate of about 70 

 miles. The unusual twilight glows are supposed to have been caused 

 by the reflection of the sun's rays from the glassy particles of the pow- 

 dered pumice that were suspended about 16 miles above the earth. 



