Various theories have been advanced to account for the occurrence of auroræ, and among others has been suggested that the phenomena is due to the presence of cosmic dust. This, however, does not appear to be at all likely: in all probability it is a purely electric magnetic phenomena. It seems likely that the phenomena occurs in other planets than our own: it is known that the obscure hemisphere of Venus appears to be often illuminated, and Winnecke says that this illumination is of a greyish violet colour—in this connection it is interesting to note that these illuminations in Venus were especially observed during the years 1721, 1726, 1759, 1796, 1806, 1825, 1865 and 1871, and that 1726, 1759, 1865, and above all 1871, were notable years for the display of the Aurora Borealis.
One of the earliest theories put forward was in the middle of the thirteenth century in an old Norwegian book called the King's Mirror, where one quaintly reads, "Some people think that this light is a reflection of the fires which surround the seas to the north and to the south; others say that it is a reflection of the sun when it is below the horizon; I think, however, that it is produced by the ice which radiates during the night the light that it has absorbed during the day" (Les Aurores Polaires, by A. Angot, 1895).
