One or more midcourse corrections might be required should the navigator's triangulation measurements show Apollo to be veering off course. A sextant is used to determine the angles between the vehicle, a star, and a common reference point on Earth or Moon. Such readings are taken with reference to three different stars, giving the crew the data they need to pinpoint their position in space with accuracy. As Lunar gravity begins to supercede Earth's gravity, the vehicle begins to accelerate once again. Once established in a 60 nautical mile lunar parking orbit , the LM crew powers up their landing vehicle.
Every system is checked out carefully in an effort to eliminate surprises during the final descent.
The astronauts are not seated in the LM during these maneuvers. Engineers reasoned that sitting down was immaterial to comfort in a microgravity environment, and so designed a system of belts and cables to support them in a standing position. The descent engine reaches full throttle about thirty seconds after ignition, with the onboard computer in control as the vehicle begins its graceful arc toward the surface. The Commander eventually takes over manual control to guide his vehicle and its crew around the rugged terrain. He must drop the LEM onto a level landing site with no sideward motion whatsoever, lest its four spidery landing legs collapse from the lateral stress.