How were lunar occultations of the Pleiades used in Charles Sanders Peirce’s early survey work?

Charles Sanders Peirce utilized observations of lunar occultations of the Pleiades to determine the longitudes of American survey points relative to European ones.1

In 1861, Peirce reentered the U.S. Coast and Geodetic Survey as a computing aide to his father, Benjamin Peirce, who had been tasked with this specific project. Much of the astronomical work required for these calculations was conducted at the Harvard Observatory.21