English: A series of curves which are believed to give a good general idea of the relative frequency of aurora throughout the northern hemisphere. H. Fritz’ curves, shown in the illustration, are termed isochasms. Illustration from 1911 Encyclopædia Britannica, article Aurora Polaris.
This image comes from the 13th edition of the Encyclopædia Britannica or earlier. The copyrights for that book have expired in the United States because the book was first published in the US with the publication occurring before January 1, 1929. As such, this image is in the public domain in the United States.
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