Periapsis or Pericenter? A brief rant.
  Jonathan McDowell, Nov 2006.

What is the generic word for the closest approach in an orbit?
Jean Meeus, whose expertise in celestial mechanics I greatly respect but
with whom I respectfully disagree on this point, says:

"[periapsis] is a wrong term, because literally it means the point
closest to an apsis, which is ridiculous, because what is meant is the
apsis itself!"

Roger comments:
> "apsis" means an extremum (that is, a minimum or maximum), and "peri"
> means near, so periapsis would mean "near a minimum or maximum"

Why wouldn't it mean "the near extremum" (and apoapsis "the far
extremum")? Just the argument by analogy with periastron?  Almost all
non-astronomical English compounds with peri- use it in the quite
different meaning 'around' or 'surrounding' -  cf. perimeter,
peripatetic, peripheral, so it's not like there's no latitude here. What
would the correct Greek be? How would Pericles have said "the close
extremum"? (Some have suggested that the correct Greek would be
'perapsis/apapsis').

However, the question is not whether periapsis is the correct Greek word, but
whether it is the correct English word. There are many cases of
irregular formation of English words from classical languages. For
example, in my youth pedants (and I am proud to be a pedant myself)
complained that the word "television" was malformed because (for some
unexplained reason) it was wrong to combine a Greek prefix 'tele' with a
Latin suffix 'vision'. However, I think it is unlikely that Sky&Tel
will ban that word. Similarly, I believe that 'periapsis', while an
imperfect formation from the classicist's point of view, is now a
standard English word whose meaning is clear to the vast majority of
those who use it. It is well within late-20th-century word-formation
practice which is much looser than pre-1950 rules.

The only synonym in active use is 'pericenter' (the OED quotes Newcomb
1902). I feel that pericenter/apocenter are both less clear and less
euphonious than periapsis/apoapsis -  the former might easily be read as
referring to the center of the ellipse rather than its focus.

The periapsis/apoapsis form may have arisen, or been most strongly
adopted, at JPL in the early 1960s and spread into the US astronautical
community. The earliest use I have found (after a very superficial
search) is in NASA Special Publication 59 from 1965, a report on the
Mariner 2 mission to Venus - the first successful planetary flyby. The
Venus-centered trajectory is characterized by its 'periapsis'. The
Mariner 4 Mars flyby final report also uses the word, but JPL documents
from 1962 describing Ranger lunar flybys   used the pericenter form.
Periapsis and apoapsis are used in a standard satellite astrodynamics
textbook from the US Air Force Academy in 1971, `Fundamentals of
Astrodynamics' by Bate, Mueller and White.

I conclude that while periapsis may continue to be deprecated as a
technical term within purely astronomical celestial mechanics, and
discouraged by the Sky&Tel style guide, it is sufficiently widespread
and long-established within the field of spacecraft astrodynamics that
it should be accepted as correct technical English (albeit bad ancient
Greek).

  - Jonathan McDowell