At niu was presented two different view of the sky as seen in high latitudes respectively close to the equator:

A spinning top respectively a tombola wheel. Although the Polynesians live close to the equator I believe they regarded the sky dome as a spinning top - or at least also as a spinning top. One of the meanings of niu is spinning top.

A third view is to say that the sky is formed like an hourglass, an alternative presented at hahe:

The picture is a drawing in Hamlet's Mill where it is presented in order to suggest that the precession of the equinoxes was what the North-West Africans tried to depict in the picture below:

'The internal motion of the cosmic tree' ... 'In the firmanent that motion marks the rotation of the stars above the earth and below the earth, around the fixed poles indicated by the axis formed by the elements in the middle of the cosmic tree'.

The description does not necessarily refer to the precession of the equinoxes. A more reasonable explanation is the wobbling motion described by the sky over a year. The more so as the 'circles' evidently form spirals, tightening towards the 'earth' (Terre).

These tightening spirals is what otherwise is referred to as the whirlpool, I suggest. And when Sun follows the spiral path from the top where he is born in midwinter he will come gradually closer to 'earth'.

His crucial point is midsummer where the path is tightening and coming close to the earth. Then the 'whirlpool' will swallow him and turn him upside down - he will dive into the 'water' (the region below 'earth' and a mirror image of the 'sky').