The Middle of What?

[Bêthberry]

Not that a source ever fully defines or explains things, yet they do sometimes demonstrate how a writer's imagination is sparked. (I suggest, in response to davem's question, that Tolkien was exceptionally able to enter into the imaginative space of the Anglo Saxon world through its poetic remains.)

Here's where Tolkien found the name in its Old English form, and here also Tolkien found the initial idea for his Earendil:

Originally Posted by couplet in Crist

"Eálá Earendel engla beorhtast
Ofer middangeard monnum sended."

Originally Posted by couplet in Modern English

"Hail Earendel brightest of angels,
over Middle Earth sent to men."

Old English poem Crist by Cynewulf

Modern English translation

I seem to recall that the Carpenter biography gives this info, but I don't have the bio at hand at the moment.