Excerpts from a poem: need help understanding poetic English

Question

This is John Keats’s classic Petrarchan sonnet “To the Nile”, which was written two hundred years ago in a style more ancient still:

Son of the old Moon-mountains African!
Chief of the Pyramid and Crocodile!
We call thee fruitful, and that very while
A desert fills our seeing’s inward span:
Nurse of swart nations since the world began,
Art thou so fruitful? or dost thou beguile
Such men to honour thee, who, worn with toil,
Rest for a space ’twixt Cairo and Decan?
O may dark fancies err! They surely do;
’Tis ignorance that makes a barren waste
Of all beyond itself. Thou dost bedew
Green rushes like our rivers, and dost taste
The pleasant sunrise. Green isles hast thou too,
And to the sea as happily dost haste.

It is clear that the poet-narrator asks the River Nile "are you fruitful" personifying it. But I couldn't make out whether the subsequent line "Rest for space 'twixt Cairo and Decan" refers to the men in the poem or not.

(Some annotations suggest that it refers to the River Nile.)

Clarification will be appreciated.

Answer 1

Yes, the Rest clause refers to the men.

You can think of the quote as "... dost thou beguile such men to honour thee, who [given their condition], [now lie between two locations]".

That is, in the quote, it is the men who are "worn with toil" that "Rest ...".

Answer 2

... or dost thou beguile such men [to honour thee], who [, worn with toil,] rest for a space ’twixt Cairo and Decan?

First remove the clauses and phrases that merely qualify nouns. I have put them in parentheses [].

Second, replace the old usage 'twixt by between

Cambridge
'twixt
preposition, old use or literary
between

We are left with:
... or dost thou beguile such men, who rest for a space between Cairo and Decan?

It follows that it is the men who rest, not the Nile.

It may be tempting to think place refers to a geographic location between Cairo and Decan. However, decan is a star or group of stars. There are 36 such groups:

Britannica
The decans are 36 star configurations circling the sky somewhat to the south of the ecliptic.

So Keats is alluding to men who rest {for a space} between Cairo and a decan.

But the decan moves in the sky, so the space as a fixed geographical position cannot be defined.

The more likely meaning of space is:

Cambridge
space noun (TIME)
an amount of time

In conclusion, Keats seems to be alluding to men who rest for a time between Cairo and a decan (which only stays in the sky for a limited period). It is possible that Keats, who used opium, is seeking a mystical conflation of the ideas of space (Cairo) and astronomic time (decan).