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The rotation velocity of the spiral pattern of the Galaxy is determined bydirect observation of the birthplaces of open clusters of stars in the galacticdisk as a function of their age. Our measurement does not depend on anyspecific model of the spiral structure, like the existence of a given number ofspiral arms, or the presence of a bar in the central regions. This study becamepossible due to the recent completion of a large database on open clusters byour group. The birthplaces of the clusters are determined by two methods, onethat assumes that the orbits are circular, and the other by integrating theorbits 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 thathave ages and distances available. We tested different assumptions concerningthe rotation curve and the radius $R_0$ of the solar orbit. Our results confirmthat a dominant fraction of the open clusters are formed in spiral arms, andthat the spiral arms rotate like a rigid body, as predicted by the classicaltheory of spiral waves. We find that the corotation radius $R_c$ is close tothe solar galactic orbit ($R_c/R_0 = 1.08 \pm 0.08$). This proximity has manypotentially interesting consequences, like a better preservation of life on theEarth, and a new understanding of the history of star formation in the solarneighborhood, and of the evolution of the abundance of elements in the galacticdisk.Comment: 9 pages, 5 figures, submitted to Ap

Direct Determination of the Spiral Pattern Rotation Speed of the Galaxy