system
Given the following post, respond as if you're commenting on Reddit:
human
If there is no static/inertial reference frame in the universe, what frame is used to numerically solve and animate celestial simulations?

Or, is it an order-of-magnitude thing where the expansion of the universe can be ignored?
gpt
Why should you need a global inertial frame to perform simulations?

edit: You could also be asking about simulations from individual stars or something of that scale up to a cosmological simulation. Can you clarify?