  Inspired by the work of Zagier, we study geometrically the probability
measures $m_y$ with support on the closed horocycles of the unit tangent bundle
$M=\text{PSL}(2,\mathbb{R})/\text{PSL}(2,\mathbb{Z})$ of the modular orbifold
$\text{PSL}(2,\mathbb Z)$. In fact, the canonical projection
$\mathfrak{p}:M\to\mathbb{H}/\text{PSL}(2,\mathbb Z)$ it is actually a Seifert
fibration over the orbifold with two especial circle fibers corresponding to
the two conical points of the modular orbifold. Zagier proved that $m_y$
converges to normalized Haar measure $m_o$ of $M$ as $y\to0$: for every smooth
function $f:M\to \mathbb R$ with compact support $m_y(f)=m_0(f)+o(y^\frac12)$
as $y\to0$. He also shows that
  $m_y(f)=m_0(f)+o(y^{\frac34-\epsilon})$ for all $\epsilon>0$ and smooth
function $f$ with compact support in $M$ if and only if the Riemann hypothesis
is true. In this paper we show that the exponent $\frac12$ is optimal if $f$ is
the characteristic function of certain open sets in $M$. This of course does
not imply that the Riemann hypothesis is false. It is required the
differentiability of the functions in the theorem.
