 Jim Lovell 's main job as Command Module Pilot was as navigator . Although Mission Control performed all the actual navigation calculations , it was necessary to have a crew member serving as navigator so that the crew could return to Earth in case of loss of communication with Mission Control . Lovell navigated by star sightings using a sextant built in to the spacecraft , measuring the angle between a star and the Earth 's ( or the Moon 's ) horizon . This task was difficult , because a large cloud of debris around the spacecraft , formed by the venting S @-@ IVB , made it hard to distinguish the stars . 
