This is a strange question that sometimes pops up into my mind when reading spell descriptions - and sometimes even my players ask them 'What happens if I cast XXX while I'm on a moving cart/ship/whatever ?'
It's true that the rules already imply that such spells are 'immobile' only relating to the heavenly body where they are cast - we all know that Earth moves into the space at an amazing speed, and that if we think that a spell like Globe of Invulnerability, Rope Trick or Prismatic Sphere is totally fixed into a point in space relating to the Universe itself, all those spell would be simply unplayable (a Prismatic Sphere would easily be on the other part of the globe and beyond just after being cast, due to revolution and rotation - killing A LOT of creatures on its apparent route) (as a side note, since Universe is moving by itself - expanding and so on - I really wonder where such spell would go, by such strict reading of the word 'immobile').
However, would you define 'immobile' a spell cast on a sailing ship, or on a moving cart? That is, a Wizard casting Lesser Globe of Invulnerability while on a ship would still be protected if he doesn't move from his very spot, or the Globe (being 'immobile' on regards of the planet and not on regards of the ship) would be left behind? And what about a Rope Trick? I can easily imagine a party of stowaways who jump aboard a ship during the night and casts Rope Trick among the riggings of the mast - only to be left into OPEN SEA the following day because, well, the ship moved and the spell not...
What is your opinion?
I've just always assumed immobile to be in relation to the planet itself. However, I have always wanted to use Wall of Force as a weapon against a moving ship, or a flying dragon.
What is your opinion?
In our game it is a bit circumstantial.
Generally immobile means immobile relative to the closest substantial surcafe. I.e. If you are standing on the ground immobile is relative to the planet; if you are on a ships deck immobile is relative to the ship. However, And this is the circumstantial bit, if you are in a canoe the effect is immobile relative to the planet, not the canoe. The canoe is simply not substantial enough. The same goes for a coach or flying carpet. How large something has to be in order to be the relative center of a spell or other effect depends on the objects mass and composition.
I try to be consistent and inform players about it when it becomes relevant, but I do not believe a rule set in stone can be made.
The above is how I interpret/apply it.
Thank you for your feedback, both of you. One of the things that currently bother me is that some immobile offensive spells (like Wall of Fire, Blade Barrier, or Prismatic Sphere) could easily be abused with either the interpretations.
If we assume that the immobile spell is fixed in space regarding the planet, a caster could easily ‘drop’ such spells while moving around into a moving vessels – and damaging his chasers. I can easily see a Wizard who drops a Wall of Fire on the first wagon of the Lightning Rail (Eberron Campaign Setting – basically, it’s an elemental-powered train) and wipes off all the passengers while the train moves away, because the Wall of Fire stays in place on the railroad and the train moves on.
However, if we assume that the spell is fixed in place regarding a (considerable large) vessel, such spells could be even more dangerous and easily being abused. I can clearly figure the image of a Wizard who sits on the top of the ram of a Galley and casts Prismatic Sphere to create an unstoppable ship that can easily destroy anything on its path…
My interpretation: "immobile" means immobile with respect to local gravity. This is usually a planet's surface, but in a zero-gravity environment, it could be the spellcaster's own body (for instance).
I can easily see a Wizard who drops a Wall of Fire on the first wagon of the Lightning Rail (Eberron Campaign Setting – basically, it’s an elemental-powered train) and wipes off all the passengers while the train moves away, because the Wall of Fire stays in place on the railroad and the train moves on.
I don't see how it's any different for a wizard who casts a spell while standing by the Lightning Rail tracks who casts Wall of Fire; it would have the same effect.
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I can clearly figure the image of a Wizard who sits on the top of the ram of a Galley and casts Prismatic Sphere to create an unstoppable ship that can easily destroy anything on its path…
I would so like to have that come up in a game...
Not really supported by the rules though.
I'd say that from the two, anchored to the planet is less open to abuse. At least in a chase situation, the follower would have a chance to avoid the static spells that the leading mage was dropping for them to run into.