  The rotation velocity of the spiral pattern of the Galaxy is determined by
direct observation of the birthplaces of open clusters of stars in the galactic
disk as a function of their age. Our measurement does not depend on any
specific model of the spiral structure, like the existence of a given number of
spiral arms, or the presence of a bar in the central regions. This study became
possible due to the recent completion of a large database on open clusters by
our group. The birthplaces of the clusters are determined by two methods, one
that assumes that the orbits are circular, and the other by integrating the
orbits in the Galactic potential for a time equal to the age of the clusters.
We selected in the database a sample of 212 clusters for which proper motions,
radial velocities, distances and ages are available, or of 612 clusters that
have ages and distances available. We tested different assumptions concerning
the rotation curve and the radius $R_0$ of the solar orbit. Our results confirm
that a dominant fraction of the open clusters are formed in spiral arms, and
that the spiral arms rotate like a rigid body, as predicted by the classical
theory of spiral waves. We find that the corotation radius $R_c$ is close to
the solar galactic orbit ($R_c/R_0 = 1.08 \pm 0.08$). This proximity has many
potentially interesting consequences, like a better preservation of life on the
Earth, and a new understanding of the history of star formation in the solar
neighborhood, and of the evolution of the abundance of elements in the galactic
disk.
