In Norse mythology, Heimdall was the watchman of Asgard, stationed at the top of the rainbow bridge, and possessed of a mighty horn -- the Gjallarhorn -- that was so powerful it could be heard throughout the entire Norse cosmos of nine worlds.

The wonderful illustrated children's rendition of Norse mythology by Ingri and Edgar Parin D'Aulaire, Norse Gods and Giants (1967), describes him this way:
Heimdall was an excellent watchman. His clear blue eyes were so keen that he could see to the end of the world. So sharp were his ears that he could hear everything, even the sound of the wool growing on the sheep down in Midgard. And he needed no more sleep than a bird. 56
They also explain why the rainbow bridge was so important to Asgard's defense: "Flimsy as the rainbow bridge looked, it was the strongest of all bridges. The Aesir had made it with great care to keep the frost giants away, for the red in it was glowing fire that burned the icy feet of jotuns and trolls" (35).

In Hamlet's Mill, Giorgio de Santillana and Hertha von Dechend explain why Heimdall is such an important figure, saying:
the character of Heimdal raises a number of sharp questions. He has appeared upon the scene as "the son of nine mothers;" to be the son of several mothers is a rare distinction even in mythology, and one which heimdal shares only with Agni in the Rigveda, and with Agni's son Skanda in the Mahabharata. [ . . . ] The nine mothers of Heimdal bring to mind inevitably the nine goddesses who turn the mill. The suspicion is not unfounded. Two of these "mothers," Gjalp and Greip, seem to appear with changed names or generations as Fenja and Menja. Rydberg claims Heimdal to be the son of Mundilfoeri. The story is then astronomical. Where does it lead? Thanks to the clues provided by Jacob Grimm, Rydberg and O.S. Reuter, and thanks to many hints hidden in the Rigveda, Atharva Veda and at other unexpected places, one can offer a probable conclusion: Heimdal stands for the world axis, the skambha. 158.
Later, they elaborate slightly further and declare: "Heimdal stands for the equinoctial colure which 'accompanies' the slowly turning, wholly abstract and invisible axis along the surface of the [celestial] sphere" (159). (De Santillana and von Dechend prefer the spelling "Heimdal;" other spellings include "Heimdallr").

The technical details of the colures and the other mechanisms of the celestial sphere seem mysterious at first, but they are critical to understanding the phenomenon of precession and the coded language embedded in the mythology of the ancients, and are carefully explained piece by piece in the Mathisen Corollary. Also explained is the reason why the intersection of the equinoctial colure and the celestial equator creates the two equinoxes, the shifting of which result in the changing ages (from the Age of Taurus to the Age of Aries, and from the Age of Aries to the Age of Pisces, as well as the ongoing shift from the Age of Pisces to the Age of Aquarius).

The association of the equinoxes with fire is quite clear throughout the mythologies of the world, and the reason has to do with the fact that they represent the "doors" or gateways through which the sun travels. This fact explains the symbology of the flaming bridge which Heimdall guards, and the end of the world corresponds to the end of the Age, and Heimdall's association with the end of the Age is embodied in the horn that he will sound when the forces of fire ascend the rainbow bridge, split the world-tree, and devour the old sun.

The turning of the equinoctial colure is a function of precession and creates the inexorable shift of the world-ages, and de Santillana and von Dechend provide supporting evidence for the association of Heimdall with this shifting colure in addition to his association with the flaming rainbow bridge, including the fact that his names Hallinskidi and Heimdal are associated with the ram (a fact that Jacob Grimm said was "worthy of remark"), the zodiac constellation that is traditionally considered the head or first of the twelve zodiac symbols, and the fact that another of his mysterious names, Vindler, comes from the word "to turn" (it is related to the English verb "to wind," as in "winding up a string").

Heimdall's association with fire may cause some readers to think of another mythological figure, Prometheus from Greek mythology, who gave fire to mankind (without permission) and was then punished for eternity. The connection is valid, and de Santillana and von Dechend spend even more time discussing the celestial and cosmological significance of Prometheus than they do of Heimdall. Sufficient for this discussion to show the connection is the tradition that Heimdall once came down to Midgard and dwelt among men, giving them advice and creating the various social classes or walks of life. The similarity to Prometheus (and Heimdall's association with fire) is quite pronounced in the famous painting of Heimdall bringing forth the gifts of the gods to mankind by Nils Asplund (1874 - 1958) which hangs in the University of Gothenburg (Göteborg) in Sweden:

The astronomical and precessional connections hidden in the mythology of the world are a little-understood but extremely important set of clues about the ancient history of mankind.