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Certain spells and effects are defined in the rest frame of a certain unspecified observer which makes me wonder if it is the rest frame of the spellcaster or it is an absolute cosmological rest frame (like the hypothetical aether of our world). For example, is Leomund's Tiny Hut stationary with respect to the spellcaster's inertial frame of reference at the time of the casting of the spell or is it stationary with respect to an absolute frame? Another example would be the ambiguity in Glyph of Warding which specifies measurements from the position at which the spell is cast in order to determine the effective duration of the spell. Is that position relative or absolute?

If there is an absolute rest frame, do planets move with respect to this frame or are they stationary?

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It's whatever you want it to be.

As stated by Thomas, it isn't a physics simulator, it's a framework to enable enjoyable play. So if you want a Glyph cast on a moving city to activate when the city comes within range of something, sure. If you want to cast it on a chest on a ship but have it be relative to the ship as well, sure.

If there is an absolute rest frame, do planets move with respect to this frame or are they stationary?

What are planets? Who says planets exist? Who says Faerun is in a round world? Define these things however you want, but understand that some things (like ruling an immovable rod doesn't spin with the earth but does around the sun) just makes the game a bummer.

"I teleport to the next room."

"Psyche!! the room moved 42,000 miles while you cast and now you're in deep space."

That's just a jerk move.

My other point is don't be a jerk to players who expect a spell to work one way but you've implemented your own reference frame that breaks it and only tell them after they cast it. It's more than reasonable to assume that players understand how spells work on moving vehicles/cities/planets.

My recommendation is to keep it simple or you risk players breaking the game with a tiny hut that destroys the airship they're on because it's always relative to the ground, and then they fall out of the bottom.

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Frame Challenge (pun intended): The rules of D&D are not a physics simulator.

This question goes beyond what the rules are concerned with, and beyond what is necessary for adjudicating the rules. If there are any edge cases that actually depend on a substantive answer to this question (there aren’t), it will be entirely up to the DM.

That said, I use a reference frame argument in this answer about casting tiny hut upside down, but the calculus employed there is largely unnecessary most of the time (it was probably unnecessary there too).

I recall reading a thread on 4chan where a DM decided that an immovable rod had a fixed position with respect to a universal reference frame:

A while ago I got my hands on an Immovable Rod. I placed it in the air and told it to stay. The GM asks whether i'm stading to the east or west fo the rod. I say west. The GM states that I die. The rod shot forward at the same speed the earth revolves around the sun, as the rod is stuck in a universal stillpoint and the earth moved away from under it. The rod also cut its way through a large area of woods before being ejected into space.

To directly answer the question, no. The rules do not define an absolute reference frame. A DM is obviously free to do so, but if that ruling yields results like an immovable rod cutting through the earth, that DM should probably reconsider.

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    \$\begingroup\$ OK, this is close to my favorite frame challenge ever thanks to your opening line. Sir Isaac Newton would approve, methinks. 👍😆 \$\endgroup\$ Oct 25 '20 at 21:33

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