On Saturday, 24 July 2021, shortly before his 80th birthday (b. 9.9.41), Noel M. Swerdlow passed away. He has been one of the world’s leading historians of astronomy.
Swerdlow received his doctorate on "Ptolemy's Theory of the Distances and Sizes of the Planets: A Study of The Scientific Foundations of Medieval Cosmology" under the supervision of Asger Aaboe at Yale. Since then, Swerdlow joined the informal group of scholars, who collaborated in their work as part of a history of the exact sciences in the spirit of Otto Neugebauer. Swerdlow took on the studies of the late Renaissance and modern periods from Regiomontanus through Copernicus, Tycho, to Kepler. Swerdlow drew on his profound understanding of the geometrical details of epicyclic planetary models from their Ptolemaic roots to Kepler. He studied them as the evolution of empirically controlled geometrical devices of celestial motions.  Swerdlow combined his great technical competence with a relentless historical criticism of sources, which did not allow any speculation without a corresponding textual basis. Swerdlow did not accept historical reasoning without such a textual basis, neither in private nor public controversy. In 1973, Swerdlow published his groundbreaking study on Copernicus' Commentariolus. Based on this expertise, Neugebauer entrusted his fragmentary manuscript on Copernicus' De revolutionibus to Swerdlow, who completed it into the two-volume masterpiece "Mathematical Astronomy in Copernicus' De revolutionibus".  Most of his academic life he spent at the Department of Astronomy and Astrophysics at the University of Chicago. After his retirement he moved to the California Institute of Technology, where he continued his research. 
For many years his advice guided many scholars at MPIWG.  To some of us he has become a close friend. We mourn the loss of a great scholar and a wonderful person.

Gerd Grasshoff and Jürgen Renn, Berlin, 29. July 2021