in the service module made that burn, known as descent orbit insertion (DOI), with the lunar module still attached to the CSM. The initial orbit Apollo 15 was in had its apocynthion, or high point, over the landing site at Hadley; a burn at the opposite point in the orbit was performed, with the result that Hadley would now be under the craft's pericynthion, or low point. The DOI burn was performed at 082:39:49.09 and took 24.53 seconds; the result was an orbit with apocynthion of and pericynthion of . Overnight between July 29 and 30, as the crew rested, it became apparent to Mission Control that mass concentrations in the Moon were making Apollo 15's orbit increasingly elliptical—pericynthion was by the time the crew was awakened on July 30. This, and uncertainty as to the exact altitude of the landing site, made it desirable that the orbit be modified, or trimmed. Using the craft's RCS thrusters, this took place at 095:56:44.70, lasting 30.40 seconds, and raised the pericynthion to and the apocynthion to . As well as preparing the lunar module for its descent, the crew continued observations of the Moon (including of the landing site at Hadley) and provided television footage of the surface. Then, Scott and Irwin entered the lunar module in preparation for the landing attempt. Undocking was planned for 100:13:56, over the far side of the Moon, but nothing happened when separation was attempted. After analyzing the problem, the crew and Houston decided the probe instrumentation umbilical was likely loose or disconnected; Worden went into the tunnel connecting the command and lunar modules and determined this was so, seating it more firmly. With the problem resolved, Falcon separated from Endeavour at 100:39:16.2, about 25 minutes late, at an altitude of . Worden in Endeavour executed a SPS burn at 101:38:58.98 to send Endeavour to an orbit of by in preparation for his scientific work. Aboard Falcon, Scott and Irwin prepared for powered descent initiation (PDI), the burn that was to place them on the lunar surface, and, after Mission Control gave them permission, they initiated PDI at 104:30:09.4 at an altitude of , slightly higher than planned. During the first part of the descent, Falcon was aligned so the astronauts were on their backs and thus could not see the lunar surface below them, but after the craft made a pitchover maneuver, they were upright