  Relativistic twin paradox can have important implications for Mach's
principle. It has been recently argued that the behavior of the time asynchrony
(different aging of twins) between two flying clocks along closed loops can be
attributed to the existence of an absolute spacetime, which makes Mach's
principle unfeasible. In this paper, we shall revisit, and support, this
argument from a different viewpoint using the Sagnac delay. This is possible
since the above time asynchrony is known to be exactly the same as the Sagnac
delay between two circumnavigating light rays re-uniting at the orbiting
source/receiver. We shall calculate the effect of mass $M$ and cosmological
constant $\Lambda$ on the delay in the general case of Kerr-de Sitter
spacetime. It follows that, in the independent limits $M\rightarrow 0$, spin
$a\rightarrow 0$ and $\Lambda\rightarrow 0$, while the Kerr-dS metric reduces
to Minkowski metric, the clocks need not tick in consonance since there will
still appear a non-zero observable Sagnac delay. While we do not measure
spacetime itself, we do measure the Sagnac effect, which signifies an absolute
substantive Minkowski spacetime instead of a void. We shall demonstrate a
completely different limiting behavior of Sagnac delay, heretofore unknown,
between the case of non-geodesic and geodesic source/observer motion.
