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 .