I see what you mean, though I imagine that some calculation is done to keep the orbital paths in their correct positions relative to time when a player enters a system, probably during the frameshift jump? My mind was thinking that it was all updated in real-time after the galaxy was generated but what you say makes more sense from a server resource perspective. I hadn't thought about it till now, though I dare say that market / economy information couldn't be handled in a similar way?
The two are very different.
The positions of planets are calculated according to an extremely simple algorithm which needs
one variable - the current time. All of this happens on your computer when you enter the system, and then is updated in the background as you go along. The systems don't really even
exist unless you look at them - it's all generated dynamically as you look at it: one of Frontier's great successes with Elite Dangerous is to have fully automated "just in time" generation of systems for an entire galaxy - down to the surface features of planets - in a way that
looks like it was all there in advance, and all remains there when no-one's looking. But it's just a very clever illusion.
The market / economy information is handled entirely on Frontier's servers, because the quantity of goods available in a market at any given time depends on what every other player is doing as well. That
said, the amount of information involved really isn't that large on a computer scale, and it still only needs calculating if a player docks at that market, so I wouldn't expect that to be a technical barrier to implementation. The existing system already stores market data for hundreds of commodities over hundreds of thousands of markets, and processes in near real-time all the trade transactions on top of that which adjust supply and demand. Changing the algorithms that on a per-station basis determine what happens to the markets with no players around isn't the difficult bit: making the result fun is.