Does casting a persistent spell effect on a moving platform fix the spell's orientation to the platform, or is its orientation absolute to the world around it?
Quick backstory: I have a pirate bard in my campaign that is trying to utilize certain spells on the deck of a ship and we are trying to solve some mechanical questions regarding spell orientation. The deck of the ship is essentially a moving platform.
Case 1: Unspecified persistent AOEs – Can the 10 ft. cube of control winds be anchored to a 10ft cube relative to the top deck of the ship? This concept can also be applied to other spells with an area of effect such as darkness or fog cloud. The pirate captain wants to blow his own sails and/or tactically use spells against other ships they may come across.
Case 2: The wall of force and Leomund's tiny hut spells specifically states that the effects are immovable. If it is anchored to the ship deck, does it "move" with the ship, but relativistically stationary? Or does the wall stay in position, potentially stopping or damaging the ship as the ship moves beneath the wall by hitting the aft cabin for example.
Case 3: Specified Ground Anchor Points – Do spells that specifically state the ground as its fixed point substitute the deck of the ship at its fixation point?
I included case 3 for completeness. I'm fairly certain that for spells that state a fixed point on the ground (as many wall spells do), they affix themselves to the deck and move relative to the ship. Are there any general rules regarding these situations?
Best Answer
Unless the spell text specifies, that is left to a DM ruling.
A question like this highlights how integral to this edition of the game the "rulings > rules" philosophy is in aiding a DM, or a whole table, in resolving such puzzles.
We had an extended discussion about this on the GiTP forums regarding casting Leomund's Tiny Hut on a ship. The consensus we arrived at was, to avoid absurdities, that the DM needs to pick a frame of reference. I recommend reading the whole thing, as painful as that might be.
Why is that? Because trying to shoehorn RL physics into a game where magic exists, and which isn't built as a physics emulator, opens up a can of worms that need not be opened. Poster Shining Wrath summarized it like this (LTH = Leomunds Tiny Hut):
For a spell like fog cloud, it makes sense to anchor it to a local frame of reference. You cast it at a point you can see:
I see the ship's wheel, I cast fog cloud at that point! (My aim is to confuse or confound the ships' helmsman, and perhaps a few archers in the rigging).
The fog cloud should remain centered over the ship's wheel. But ...
... if the wind is strong enough that day, that won't work since the wind disperses it. (Not due to relative motion). Rather than seeking a general rule, you need to look at the specifics of a given spell and go from there.
Select a frame of reference
As jason_c_o pointed out in a comment:
This approach fits the game's basic framework, in that the local area where the players are interacting with monsters and creatures and weather and the environment is a local frame of reference. During table top play, that frame of reference is frequently portrayed as a small grid or battle map (be they squares or hexes).
Alternately, you can overthink this and get into arguments like the ones at the linked forum discussion.
The choice, Willow, is yours.
You may be pleased to know that D&D 3.5 and Pathfinder end up using a similar approach: make a ruling. Jeremy Crawford has offered rulings on related questions regarding casting teleportation circle on a moving surface here, and also here. Thanks to @HeyICanChan and @V2Blast for their assistance and comments.