Not hard to explain: they were simply drawing the glow discharges they saw in the sky. The growing body of theory–research isn't well known yet; for an introduction, see Peratt's groundbreaking paper from 2003:

Characteristics for the Occurrence of a High-Current, Z-Pinch Aurora as Recorded in Antiquity

http://plasmauniverse.info/downloadsCosmo/PerattTPSv31-2003c...