Heimdallur relates to the first element of 5, the first höfuðskepna (main-created), space.
When Óðinn Vili Véi start the momentus scramble of looking for knowledge in pure knowledge, Heimdallur in Himinbjörg, Himalaya, the abode of the divine, seems to grasp the opportunity of space being possible.

Heimdallur, the bright one (dallur), is related to sound (and the sense of hearing).
His sounding is by Gjallarhorn, the horn (musical instrument) of river Gjöll. Heimdallur hears the grass grow and the wool grow.

Sound is connected to memory, Mímir the all-remembering. Shruti smriti. By remembering to and fro, forth and past, Mimir relates to space. Without space, no to and fro and forth and past can be.

Mímir is total memory of all, his name from Sanskrít smriti, smara

This heim- in Heimdallur is from Sanskrit kham, which means akash, the space element;
note that dellingur is the bright one; but dallur as a big vessel is later time meaning, nýíslenska (so using dictionaries for translating ancient terms is dangerous).

Heimdallur has 9 mothers. Who are they?  If we see space as 10D (/10 dimensions), knowing that one of them is time, these remaining 9 become the mothers of creation’s first-born element, höfuðskepna:  Heimdallur as space.

See also Óðinn Vili Véi

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Listen to pronunciation: Heimdallur Himinbjörg Gjallarhorn, Mímir