Not P but Moby-Dick (63)

[Mike Jing]

>From Chapter 81:

It was his death stroke.  For, by this time, so spent was he by loss of
blood, that he helplessly rolled away from the wreck he had made;  lay
panting on his side, impotently flapped with his stumped fin, then over and
over slowly revolved like a waning world;  turned up the white secrets of
his belly;  lay like a log, and died.

What does "a waning world" mean here?