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The question has arisen in my mind on whether when a mage has cast a Magnificent Mansion (5e) while on an object that is in motion (boat, airship, giant alpaca, etc), would the doorway to the Mansion then remain static in place?

The relavent wording from the spell:

You conjure an extradimensional dwelling in range that lasts for the duration. You choose where the entrance is located. The entrance is 5 feet wide and 10 feet tall [...]

I could see that this would indicate that the mage has chosen the location to be the giant alpaca and as such that is where the door would remain located. Or that the mage has chosen the place in space that the door is located and as such the door would remain there despite the alpaca continuing to move forward. Or perhaps something else might occur...?

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Your DM decides

D&D is not a physics simulation. D&D is a game that makes simplifying assumptions to support gameplay. In practical terms, this normally means that immovable effects are immovable relative to the frame of reference of the game action. Often, if you use a battlemap, they do not move around on that battlemap.

For example, if you are moving around on a flying carpet the zips across overhead, then the entrance could be stationary relative to the the overall area, not the carpet. If you are all on a sailing ship drawn on the map, and a sea monster attacks, the entrance could be stationary relative to the ship. Or your DM could rule it will be stationary to the ocean, and the ship will sail on, leaving you stranded in the mansion.

As the rules do not really explicitly speak on this, it will be up to your DM to adjudicate what the frame of reference is (a conclusion also supported by the many linked related questions).

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  • \$\begingroup\$ Appreciate the answer. So many questions ultimately fall into this category I suppose but that is the primary job of a DM! Adjudicate and maximize the fun. \$\endgroup\$
    – VerasVitas
    Mar 16 at 16:48
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Everything is moving

Unless you choose the one extremely specific frame of reference where it isn’t.

The planet revolves and orbits the star. The star orbits the galaxy. The galaxy moves in rather complicated ways with respect to the local cluster and so on.

Or, maybe your world is more fantastical and takes place on a disc with a central volcano that spews new land while old land around the edges breaks off an floats away? And civilizations have to migrate to the Centre or collapse. Hang on ... I have to write this stuff down for my next campaign.

Anyhoo, this spell is way less problematic than the ones that create immobile stuff without telling you what they are immobile with respect to. Here the rules say you “choose” where to put it. For mine, that means you can choose the frame of reference.

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    \$\begingroup\$ "Everything is moving" is certainly a modern Physics interpretation, sure. It is not obvious that any particular game world operates under these rules, however. If I'm riding a giant eagle toward something, my arrows fired at it don't do more damage than if I am flying away. A magic spell's definition of what is "moving" doesn't have to correspond to the RW any more than a transformation spell has to obey conservation of mass. \$\endgroup\$
    – Kirt
    Oct 17, 2020 at 17:02
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    \$\begingroup\$ @Krit AFAIK both Forgotten Realms and Eberron are also moving but not via the same mechanism that planets move in our universe. \$\endgroup\$ Oct 20, 2020 at 23:13
  • \$\begingroup\$ @Kirt your arrows don’t do damage because of momentum or energy or any physical concept - they do damage because the rules say they do. \$\endgroup\$
    – Dale M
    Oct 21, 2020 at 0:32
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    \$\begingroup\$ @DaleM I think that is my point - the rules are not making an attempt to model RW physics principles. \$\endgroup\$
    – Kirt
    Oct 21, 2020 at 3:49

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