I've read a couple of excellent books on the Norse myths in English, Gods and Myths of the Viking Age by H. R. Ellis Davidson and The Norse Myths by Kevin Crossley-Holland. In Davidson's book, she discusses the root of the Heimdall myths as a parallel to Celtic myths about Manannan. There is a theory that Heimdall is a chief god of an earlier myth cycle (or maybe a Vanir), connected with the sea. The Nine mothers are the nine waves of the ocean, pictured as "ewes", and the Heimdall is "the ram", the ninth wave. Heimdall's horn is part of that image, apparently, and he is pictured as either the father of the nine waves or as the child of the nine waves, or both. The text connects the nine waves to Longfellow's "ninth wave" in the Coming of Arthur.
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I've read a couple of excellent books on the Norse myths in English, Gods and Myths of the Viking Age by H. R. Ellis Davidson and The Norse Myths by Kevin Crossley-Holland. In Davidson's book, she discusses the root of the Heimdall myths as a parallel to Celtic myths about Manannan. There is a theory that Heimdall is a chief god of an earlier myth cycle (or maybe a Vanir), connected with the sea. The Nine mothers are the nine waves of the ocean, pictured as "ewes", and the Heimdall is "the ram", the ninth wave. Heimdall's horn is part of that image, apparently, and he is pictured as either the father of the nine waves or as the child of the nine waves, or both. The text connects the nine waves to Longfellow's "ninth wave" in the Coming of Arthur.