# Is GEO redundant (Geo- and Earth)? Would we call a Lunasychronous Lunar orbit LLO?

I think the title of the question Is it possible to establish a synchronous lunar orbit without using Lagrange points? is absolutely clear; it's an orbit around the Moon that is synchronous to the rotation of the Moon.

We call an Earth orbit synchronous to the rotation of the Earth GEO for *Geosynchronous Earth Orbit.

For an orbit around the Moon, synchronous to the Moon's rotation, using the same logic, would we call it a Lunasychronous Lunar Orbit or LLO?

If so, would there be some redundancy in this?

• For Earth we have geo- and sun- synchronous orbits (are there more? Any orbit being described as moon-synchronous?). No idea if there are more such orbits around the Moon. Mar 31, 2019 at 8:52
• @jkavalik TESS' orbit is "duolunosychronous" with the Moon. I just made up that word, but it's period is double that of the Moon's period (without reference to Earth's rotation or otherwise) I can't think of anything else along the lines of what you're thinking though. I wonder if I should change the question to "What kinds of synchronous orbits have names?"
– uhoh
Mar 31, 2019 at 8:58
• @uhoh the TESS orbital period is actually half the Moon's rotational period. See here. Mar 31, 2019 at 9:29
• @OscarLanzi ya I mis-spoke. It's twice the orbital frequency not twice the orbital period. I've explained elsewhere that it's in a 2:1 orbital resonance with the Moon. Also, It's only a coincidence that it's also resonant with the "rotational period", due to tidal locking.
– uhoh
Mar 31, 2019 at 9:33
• This is a question I had too, but it didn't matter enough to ask; thank you for asking it for me :) +1. Apr 1, 2019 at 20:28

No, it's not redundant. The abbreviation GEO expands to Geosynchronous Equatorial Orbit.

A geosynchronous orbit that coincided with the Earth's equatorial plane would then also be geostationary.

• How does that compare to your comment? Also, the Wikipedia article's source for that is a an Arianespace page that doesn't exist and is only archived. i.stack.imgur.com/WGucb.png So I'm not sure how universal that interpretation for GEO is. I wonder if there is a more authoritative source somewhere?
– uhoh
Apr 1, 2019 at 15:16
• @uhoh GIS Geography says geostationary and geosynchronous orbits are not the same. The latter is a satellite synchronized with the rotation of the Earth; the former is the same idea but in equatorial orbit. Planetary.org also differentiates between geosynchronous and geostationary, saying that all geostationary satellites are geosynchronous, but not all geosynchronous satellites are geostationary. Apr 1, 2019 at 17:37
• @Hobbes I've added two sentences which extends your answer a bit farther along the lines of the question. With the edit I think I can accept this as long as we can find a reliable link that's not deprecated. Thanks!
– uhoh
Apr 1, 2019 at 18:09
• @Snoopy I didn't even notice that, wow that's another thing about this terminology that confuses me. As far as I know the three letters GEO is only ever used to refer to geostationary, and geosynchronous does not have a generally accepted three letter abbreviation. Maybe I'm wrong, but that's the way it was explained to me. The 2014 Planetary society blogpost you link to was written by a journalist and I don't know where he got that from. I've not seen GSO used for geostationary, and in this SE site GEO generally means geostationary.
– uhoh
Apr 1, 2019 at 18:10
• @Snoopy though in comments below this post you can see that in 2016 I was still pretty confused about the terminology
– uhoh
Apr 1, 2019 at 18:21

According to Wikipedia, "synchronous orbit" with no prefix on "synchronous" is used generically, and "geosynchronous" refers specifically to Earth. From this point of view "geo" and "Earth" do appear redundant. Maybe the nomenclature gurus wanted to make a catchy acronym ("GEO") that itself looks like it references Earth.

There is also a specific term for Mars:

"A synchronous orbit around Earth that is circular and lies in the equatorial plane is called a geostationary orbit. The more general case, when the orbit is inclined to Earth's equator or is non-circular is called a geosynchronous orbit. The corresponding terms for synchronous orbits around Mars are areostationary and areosynchronous orbits."

The corresponding case for the Moon would be called "selenosynchronous" from the Greek term for the Moon, but based on the question referenced by the OP it might also be called "nonexistent".

• the name GEO is in line with LEO, MEO, HEO. Mar 31, 2019 at 9:25
• hmm, I wonder if geo- is only for what the orbit is synchronous with, and Earth is only for what the orbit is around? In that case they would not necessarily have to refer to the same body, and so would not be redundant? I think this is what (at)jkavalik was referring to?
– uhoh
Mar 31, 2019 at 9:28
• I get that last idea. Its like referring to a sun synchronous orbit more fully as a sun synchronous Earth orbit. There could be a Geo synchronous lunar orbit using the same terminology (if such a thing exists) that is locked in RAAN towards the Earth. Mar 31, 2019 at 9:47
• @uhoh- terms like geology and terrain don't have to be translated for each celestial body.
– amI
Apr 1, 2019 at 19:04

I think to consider GEO as Geosynchronous Equatorial Orbit is a backronym. To me, GEO is a short form for "geostationary orbit", that is, the orbit objects on which are seen as stationary from the Earth.

In this way, GEO is not a redundant acronym.

Of some note also is there is no such thing as a Lunar-synchronous orbit. Or rather, Earth is at that point. The best that could be done is a L1 or L2 halo orbit, such as the Queqiao relay satellite assisting the Chang'e 4 mission.

• This is of course correct and the answer to the linked question.
– uhoh
Apr 1, 2019 at 22:29