the LM's outer aluminum skin. Despite this, the crew discovered that the spacecraft's systems were performing nominally. Following the LM inspection, the crew reviewed checklists and procedures for the following days in anticipation of their arrival and the Lunar Orbit Insertion (LOI) burn. Command Module Pilot Mattingly reported "gimbal lock", meaning that the system to keep track of the craft's attitude was no longer accurate. Mattingly had to realign the guidance system using the Sun and Moon. At the end of day two, Apollo 16 was about away from Earth. When the astronauts were awakened for flight day three, the spacecraft was about away from the Earth. The velocity of the craft steadily decreased, as it had not yet reached the lunar sphere of gravitational influence. The early part of day three was largely housekeeping, spacecraft maintenance and exchanging status reports with Mission Control in Houston. The crew performed the Apollo light flash experiment, or ALFMED, to investigate "light flashes" that were seen by Apollo lunar astronauts when the spacecraft was dark, regardless of whether their eyes were open. This was thought to be caused by the penetration of the eye by cosmic ray particles. During the second half of the day, Young and Duke again entered the lunar module to power it up and check its systems, and perform housekeeping tasks in preparation for the lunar landing. The systems were found to be functioning as expected. Following this, the crew donned their space suits and rehearsed procedures that would be used on landing day. Just before the end of flight day three at 59 hours, 19 minutes, 45 seconds after liftoff, while from the Earth and from the Moon, the spacecraft's velocity began increasing as it accelerated towards the Moon after entering the lunar sphere of influence. After waking up on flight day four, the crew began preparations for the LOI maneuver that would brake them into orbit. At an altitude of the scientific instrument module (SIM) bay cover was jettisoned. At just over 74 hours into the mission, the spacecraft passed behind the Moon, temporarily losing contact with Mission Control. While over the far side, the SPS burned for 6minutes and 15 seconds, braking the spacecraft into an orbit with a low point (pericynthion) of 58.3 and a high point (apocynthion) of 170.4 nautical miles (108.0 and 315.6 km, respectively). After entering lunar orbit, the crew began preparations