Casertano et al. (1999)

Casertano et al. (1999) developed a method to accurately determine the distances to globular clusters by analyzing their stellar motions. They combined measurements of how individual stars in a cluster move across the sky (proper motions) with how fast they move towards or away from us (radial velocities). By doing this, they could calculate the cluster's actual size and distance more precisely, reducing uncertainties caused by assumptions in earlier methods. Their technique improves our understanding of the structure and scale of these dense star groups, which are important for understanding the broader history of our galaxy.