What forms of FTL travel/communication are you allowing here and what are the travel times associated with them? The reason I ask is because that affects what infrastructure options are available and the benefits of centralised/decentralised government: Governing a republic with week long travel times is a lot easier than one with year long travel times, and needs a different approach to government.

@JoeBloggs I was envisioning handwaving away the specifics by simply installing a special engine on certain ships that allows FTL travel - for once, I'm abandoning all forms of scientific explanation. I actually don't plan to have FTL communication yet; messages would have to be carried via probes. Travel times for ships would be on the order of days to travel distances about 1000 light-years, and it would take about two months to traverse the galaxy.

@HDE226868 have a look at Scalzi's Skip Drive, it sounds like something that would fit your handwavey requirements and also would've already been established :)

@dot_Sp0T Well, that's certainly . . . peculiar. Creative, though.

Does it have to be a planet?

@rangerike1363 I suppose not; AndyD273's answer made it a spaceship.

********IMMEDIATELY READ THIS********* A capitol is a building which is the seat of government. A capital is a city which is the seat of government. The question asks for a capitAl. Please correct yourself when you post before my head explodes.

Place capital there where it had lot's of sense a millenia ago, where such place was selected. It would make a good and realistic story that such place is no longer optimal.

@kingledion what about Das Kapital? Does that make your head warm and cozy?

I really wanted to answer this question with "At the start of the Galactic sentence..."

The travel times for your FTL drive don't make sense to me. Travelling distances of 1000 light years in days means either 500 or 333-33 light years per day. Traversing the galaxy, say from its centre to the edge gives a FTL speed of 833.33 light years per day, while traversing edge to edge is 1666.67 light years per day. This assumes constant FTL speed. Your certain engine will need to accelerate ships to attain these travel times. I assume you will have factored this into your FTL travel model.

Wouldn’t it be best for such a capital to be distributed, possibly redundantly so that the seat of any committee or such can be switched (such as, rotated monthly, or whenever a crisis requires it)? This also eliminates the “single point of failure” problem, while addressing distances and… well, politics of not favouring a single region.

Somewhere in the galactic core, I would say. Anyway, very interesting and thoughtful answers on this question.

Was the Star Wars empire supposed to encompass a whole galaxy? Or even a significant chunk of one? I know it was in a glalaxy far far away, but I didn't know it filled a galaxy far far away. Iain M Banks' culture series was content to have the galaxy "shared" by ... hundreds, thousands (?) of "empires", with plenty of unclaimed space in the crappy cold bits too. To my mind, a truly galaxy spanning empire is story ruiningly nonsensical. And who is going to be the other actors/rivals/enemies of this empire? Another galaxy!?

@GrimmTheOpiner. Isaac Asimov's Foundation series did a pretty good job of addressing that.

"I have some ideas for the planet itself" Who says you need a planet? Why not make it a large spacefaring construct?

"placing the planet near the supermassive black hole is likely not a good idea." - it is excellent place to place it. It is f galactic empire, and stronghold have to be strong, and SMBH is good place to convert matter into energy which you might need if not to fight aliens then to rebels. If you do not take this sweet spot empire will be owned by one who do choose the place. It is not a question of convenience but of survival.

Well, all I can say is that if you want me to support your future initiatives, it had better be in my senatorial district.

How fast is your communication? And in turn, why do you need a Capital? Couldn't you have Skype for Empires conferences?

@kingledion Good thing you put that in capitol letters so people would notice.

How old is this republic? Is this "majority" of systems spread across the galaxy or concentrated in a chunk (as in Star Wars)? Does the republic cede systems as they move away? Galaxies don't have fixed 'geography'. Stars drift relative to eachother over millennia. After some tens of millions of years, any neat arrangement the republic had will be completely scattered. Whether your capital is in the centre of the republic or cut off on the opposite side of the galaxy depends on those questions. This is one of the many things Star Wars lore gets dead wrong its ancient pangalactic societies.

It could be something really daft like a mobile version of Starbase Yorktown that circumnavigates the galaxy every few years :D

@a4android Yes, I indeed factored that in. The details are unimportant, I think.

@Mast I originally wanted it to be a planet, but other answers have suggested otherwise, which is fine by me.

@Mooz Long-distance communication is limited by the speed of light, via electromagnetic radiation (namely, radio waves). That makes telecommuting not too easy.

@CircleSquared The republic is relatively young, perhaps a decade at the most. There may have been a previous political structure, but I have yet to establish what that could have been.

@HDE226868 If that preceding order lasted only for decades to centuries, and if all prerepublican history which still noticeably affects the modern day doesn't go back more than 1 or 2 millennia, then you can treat relative locations as being mostly the same. There will still be noticeable drift for any interstellar society, but it can treated more like a minor administrative matter. As an example: look at how Barnard's Star moved across our sky over 20 years. imagine.gsfc.nasa.gov/features/cosmic/images/barnard2005.gif

@HDE226868 This is a personal hobbyhorse. Too often FTL travel is simply arbitrarily FTL travel without proper performance parameters. You seemed like someone who would work out these details, so I had to ask. Certainly not important to this question, at least, directly.

@a4android your calculations are based on the dimensions of our galaxy. OP stated a galaxy like ours. It may therefore be a bit smaller to allow for the maths to tie in a bit better

C1: I've always thought that Trantor already was the capital planet, so you must be in some other galaxy.

C2: As others have asked, a little more clarity on galaxy size could help, as would limits of both FTL travel and communication. (I mean, it took the Millennium Falcon nearly "12 parsecs" just to transit "the Kessel Run".) Along with physical elements, what exactly is the political purpose of the "republic"? Is it to end war? To subjugate conquered systems? To extract tributes? More? Other? Its fundamental purpose would be significantly affected by physical limits. A galaxy is BIG.

@kingledion Or you could just edit the question yourself...? :)

The first logical answer that comes to mind is right in the galactic center. -- The closest place to everywhere.

@DarrenH Quite so.I used the lower, most commonly used dimensions for our galaxy. Strangely enough, our galaxy is similar to a galaxy like ours. In fact, the numbers work better for a galaxy that's a bit bigger. The OP is more than capable of realizing what scales I was working on, and could have easily corrected them. I used a convenient set of numbers for an old-fashioned back-of-the-envelope calculation.