not a noob but

http://wiki.spaceempires.net/index.php/Moons_%28SEV%29

SpaceEmpires.net | Space Empires Wiki

"Moons" in SE:V are the smaller planets in adjacent hexes to the larger planets. Unfortunately they are not labeled with letter prefixes as they were in SE:IV nor are they in the same sector. However, you can mod for the moons to be in the same sector, but there are some display issues with doing so.

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Space Empires Depot | SE:V Balance Mod

How does combat turn out if there is two or more planets in one sector? Will the two models be on top of each other?

I remember once playing a mod with moons in the same sector (might have been an old version of Balance Mod, or maybe IRM or Golden Eclipse - those are all the mods I can remember playing! ), and I don't recall planets being inside each other in combat; they behaved as in SE4, where they are offset a bit from each other. As for the system map, Kwok's right - the planet and the moons sort of "flicker" back and forth as you move the camera about the system.
At least you get the colony icon above the moon - you didn't get that in SE4!

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The power of the ancients is now yours to command. You are truly the Master... of the obvious

Moons in the same sector should act kind of like asteroid groups, where you see the moons floating around the main planet, and they move a bit into nearby sectors.

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A project a day keeps the deadline away.

Actually... you COULD mod that in... it would just require a bit of trickery...
Create copies of each of the "tiny" planet entries in XFileClasses_Stellar.txt, renaming them from "Planet Foo Bar NNNN" to "Moon Foo Bar NNNN", and set the following fields in the copies to arbitrary values that are different for each entry (within reason - I think the default hex diameter is something like 40 units so you don't want them TOO far out from the planet nor do you want the planetary systems looking like molecular models!):

Starting Position Offset X                           := 0

Starting Position Offset Y                           := 0

Starting Position Offset Z                           := 0

Next, create copies of all the "tiny" planet entries in StellarObjectTypes.txt, and rename them from "Planet Foo NNNN Tiny" to "Moon Foo NNNN Tiny". Change their "Xfile Class Name" entries from the old Planet ones to the new Moon ones you just created.

Now find all the "moons" in SystemTypes.txt (you can recognize them by "Obj N Position" fields that look like "Offset X Ring Y") and replace the Obj N Position field values with just plain "Same As X" where X is the object number that was after the word "Offset". Also change the Stellar Ability called for to "Normal Moon".

You'll have to of course create the "Normal Moon" entry in StellarAbilityTypes.txt, but that can be done by simply copying and pasting from the "Normal Planet" entry

If all goes well, you should now have moons in the same sector as planets, but floating off to the side/top/bottom/whatever a bit! The only time when 2 moons will be in the same position is if they use the same xfile entry (which basically means they look the same), and there are enough xfile entries to make that a rare enough occurrence.

Sorry I can't test this out to make sure it works but I can't actually get se5 running on Linux... maybe I should go back to Windows... :/

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The power of the ancients is now yours to command. You are truly the Master... of the obvious