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Author
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Topic: Perilune or pericynthion (low lunar orbit)
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Blackarrow
Member Posts: 3328
From: Belfast, United Kingdom
Registered: Feb 2002
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posted 06-23-2021 01:40 PM
  
My understanding is that the low point in a lunar orbit of a spacecraft that has arrived from Earth is, technically, the "pericynthion." The equivalent for a spacecraft launching from the Moon into lunar orbit is "perilune."However, there is clearly a lot of confusion, even with NASA and some Apollo astronauts. I have seen both terms used. For instance, after Apollo 8 entered lunar orbit, Frank Borman used "pericynthion" but Capcom Mike Collins referred to "perilune." I have seen a much-respected textbook on Apollo suggesting that "perilune" is often used instead of "pericynthion" because it is neater and shorter. I might add that for the uninitiated, it would be easier to understand the link to the Moon. Is there any good reason NOT to use "perilune" for an Apollo that has arrived in lunar orbit from Earth? |
randy
Member Posts: 2448
From: West Jordan, Utah USA
Registered: Dec 1999
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posted 06-23-2021 03:02 PM
I always thought they were interchangeable. |
Headshot
Member Posts: 1007
From: Vancouver, WA, USA
Registered: Feb 2012
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posted 06-23-2021 05:36 PM
There was a big conflict in the late '60s about these terms along with periselene added to the mix. I believe NASA tried to justify its choice by stating it referred to an object in orbit around the moon that was not launched from the lunar surface. Astronomers snorted at that weaseling stating the origin of an orbiting body has nothing to do with it. I do not recall if the IAU got involved. If they did, I am certain whatever they chose ticked off a lot of people. |
Blackarrow
Member Posts: 3328
From: Belfast, United Kingdom
Registered: Feb 2002
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posted 06-23-2021 06:17 PM
quote:
Originally posted by Headshot:
I believe NASA tried to justify its choice...
I'm not sure what you're saying. Did NASA choose "pericynthion?" If so, not everyone got the memo. I have just checked my "Mission Director's Summary Reports" for Apollo 11 through Apollo 17 and I note that each contains a list of lunar orbital parameters identified as "Apolune/Perilune" — that specifically includes the immediately-post LOI orbital parameters. |
Headshot
Member Posts: 1007
From: Vancouver, WA, USA
Registered: Feb 2012
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posted 06-23-2021 07:27 PM
Actually, after checking a number of NASA LRO papers and websites, it looks like NASA has dropped the issue altogether and now just gives orbital dimensions like "30km by 212km" with no reference to perilune, pericynthion, or periselene. From the Apollo-era reference you cited which includes terms apolune and perilune, perhaps NASA adopted the perilune nomenclature, but then dropped the distinction altogether after Apollo ended. Cannot wait to see what they will do for Artemis.Just as info, NASA's definitions (from the Apollo 8 Glossary, NASA Release 68-208, December, 1968) were as follows: Pericynthion - Point nearest moon of object in lunar orbit - object having been launched from body other than moon.Perilune - The point at which a satellite (e.g.spacecraft) in its orbit is closest to the moon: differs from pericynthion in that the orbit is moon-originated. |
Blackarrow
Member Posts: 3328
From: Belfast, United Kingdom
Registered: Feb 2002
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posted 06-23-2021 08:51 PM
Yes, those definitions appear in the glossary of the Apollo 11 press kit, so that's what NASA was telling the world's press before the mission. Yet when Sam Phillips and George Mueller signed off on the Apollo 11 Mission Director's Summary Report on 24th July, 1969, the tables and the text relating to post-LOI orbits use "perilune" and "apolune." I'm pretty sure Apollo 11 was launched from "a body other than the Moon."At this point I think we can agree with Randy's comment that the terms were (are) interchangeable. I would add that "perilune" is shorter! |
Buel
Member Posts: 770
From: UK
Registered: Mar 2012
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posted 06-29-2021 04:55 PM
Apollo PAO Doug Ward offered this: Perilune is clearly the correct term for the low point of an orbit around the moon. However, old habits die hard and I am sure the term pericynthion persisted whether referring to the moon or earth. What term does JPL use with respect to the various planetary and lunar orbits? |
minipci
Member Posts: 412
From: London, UK
Registered: Jul 2009
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posted 06-30-2021 09:49 AM
In "Apollo by the Numbers", on the Lunar Subsatellites table on page 304, Apogee and Perigee are used. So it looks like NASA might have been a bit confused! |
Jim Behling
Member Posts: 1645
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 06-30-2021 01:50 PM
Apoapsis and periapsis are the generic terms |
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