As Apollo was being planned, detailed studies were made of how best to perform lunar orbit rendezvous - the bringing together of two spacecraft around the Moon, one of which had come up from the surface. As this was then considered to be difficult and dangerous, much effort was devoted to trying to optimise all the variables that affected the operation. Through this process, planners came to the conclusion that when the LM lifted off, the CSM should be in a circular orbit, 110 kilometres above the Moon. This requirement led to every Moon-bound Apollo mission being targeted to pass around the far side with a minimum altitude of 110 kilometres. This point of closest approach was called the pericynthion - a term from celestial mechanics meaning the lowest point in a lunar orbit made by a craft arriving from another body (the highest point being the apocynthion) - and it was around the pericynthion that the LOI burn was made. These two terms are rather unwieldy and refer to the particular case of an orbit achieved by a craft from outside the Moon's vicinity. It is more common to use a shorter pair of terms, perilune and apolune, which are more general in their use and mean more or less the same thing as the longer, more unwieldy terms.
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