  Greene's rational function $\Psi_P({\bf x})$ is a sum of certain rational
functions in ${\bf x}=(x_1, \ldots, x_n)$ over the linear extensions of the
poset $P$ (which has $n$ elements), which he introduced in his study of the
Murnaghan-Nakayama formula for the characters of the symmetric group. In recent
work Boussicault, F\'eray, Lascoux and Reiner showed that $\Psi_P({\bf x})$
equals a valuation on a cone and calculated $\Psi_P({\bf x})$ for several
posets this way. In this paper we give an expression for $\Psi_P({\bf x})$ for
any poset $P$. We obtain such a formula using dissections of root polytopes.
Moreover, we use the subdivision algebra of root polytopes to show that in
certain instances $\Psi_P({\bf x})$ can be expressed as a product formula, thus
giving a compact alternative proof of Greene's original result and its
generalizations.
