If you cast a spell that targets a location while on a moving platform (say, a ship, or a train, or a flying castle) does it move with the platform?
Does it matter what kind of spell? For example, most cloud spells I would intuitively expect to stay in a the same position relative to the air, which means indoors they'd stay in one place, and outdoors they'd likely get carried away rapidly.
I'm pretty sure that if you've got a moving battle map, most DMs will have areas move with the map because anything else is annoying, but RAW doesn't actually address this afaik.
That would depend on what the spell is and what you're casting it on. A few examples.
Darkness can be cast either on an area or on an object that isn't being worn or carried. If it's cast on an area, the moving platform may move out of said area. If it's cast on the moving platform, it would move with the platform, because it states that it becomes an emanation originating from the object.
Fog Cloud and Stinking Cloud specify only a spot within range. Because they say nothing about objects, they would stay put regardless of what happens around them.
Cloudkill and Incendiary Cloud both specify they move 10 feet away from you each turn. They don't say anything about objects, so they function mostly like Fog Cloud and Stinking Cloud above, but the direction that's "away" from you might change from turn to turn. This could create some interesting interactions.
Basically: Each spell that creates an AoE will tell you whether or not it would move with a moving platform, and other details about how it may move. The RAW addresses this by being specific about object interactions when it's relevant.
Now hold on. Physics disagrees with a lot of that. Physics says that any physical thing is subject to relative speed and air resistance. For example, if you're inside a fully enclosed train car and drop a ping pong ball, the ping pong ball will fall at the same relative speed as the train, and to you, a passenger inside the train, it will appear to fall straight down. Likewise, a Fog Cloud would "stay put" inside the same fully enclosed train car.
However, if you're on an open-air platform, where the surrounding air is motionless while you are in motion, then if you dropped the same ping pong ball, the air resistance will dramatically slow the ping pong ball, causing it to appear to you, a passenger on the platform, to fall "backwards." Likewise, a Fog Cloud would be acted on by the same air resistance, causing it to appear to move backwards as the platform moves forwards. Technically, the relative starting speed of the cloud and the friction of the platform would cause the cloud to temporarily move forward, but you would barely notice. It bears mentioning that the speed of the platform might create enough air resistance to mimic the effects of a "strong wind," which would immediately disperse such spells, but that would be up to the DM to decide.
As to purely metaphysical effects, I think it's ultimately a DM call. I'd argue that metaphysics pretty closely mimics actual physics, so a Darkness spell targeting an area would be targeting the relative space as opposed to the static space, and that it would travel with you regardless of the movement of the platform, but that's largely because it's more fun than telling your party caster that they can't use any of their spells. Also, photons (and, thusly, the deprivation of photons) can't be subjected to any kind of resistance, so I'd argue it'd still work on an open-air platform.
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If you cast a spell that targets a location while on a moving platform (say, a ship, or a train, or a flying castle) does it move with the platform?
Does it matter what kind of spell? For example, most cloud spells I would intuitively expect to stay in a the same position relative to the air, which means indoors they'd stay in one place, and outdoors they'd likely get carried away rapidly.
I'm pretty sure that if you've got a moving battle map, most DMs will have areas move with the map because anything else is annoying, but RAW doesn't actually address this afaik.
That would depend on what the spell is and what you're casting it on. A few examples.
Darkness can be cast either on an area or on an object that isn't being worn or carried. If it's cast on an area, the moving platform may move out of said area. If it's cast on the moving platform, it would move with the platform, because it states that it becomes an emanation originating from the object.
Fog Cloud and Stinking Cloud specify only a spot within range. Because they say nothing about objects, they would stay put regardless of what happens around them.
Cloudkill and Incendiary Cloud both specify they move 10 feet away from you each turn. They don't say anything about objects, so they function mostly like Fog Cloud and Stinking Cloud above, but the direction that's "away" from you might change from turn to turn. This could create some interesting interactions.
Basically: Each spell that creates an AoE will tell you whether or not it would move with a moving platform, and other details about how it may move. The RAW addresses this by being specific about object interactions when it's relevant.
Now hold on. Physics disagrees with a lot of that. Physics says that any physical thing is subject to relative speed and air resistance. For example, if you're inside a fully enclosed train car and drop a ping pong ball, the ping pong ball will fall at the same relative speed as the train, and to you, a passenger inside the train, it will appear to fall straight down. Likewise, a Fog Cloud would "stay put" inside the same fully enclosed train car.
However, if you're on an open-air platform, where the surrounding air is motionless while you are in motion, then if you dropped the same ping pong ball, the air resistance will dramatically slow the ping pong ball, causing it to appear to you, a passenger on the platform, to fall "backwards." Likewise, a Fog Cloud would be acted on by the same air resistance, causing it to appear to move backwards as the platform moves forwards. Technically, the relative starting speed of the cloud and the friction of the platform would cause the cloud to temporarily move forward, but you would barely notice. It bears mentioning that the speed of the platform might create enough air resistance to mimic the effects of a "strong wind," which would immediately disperse such spells, but that would be up to the DM to decide.
As to purely metaphysical effects, I think it's ultimately a DM call. I'd argue that metaphysics pretty closely mimics actual physics, so a Darkness spell targeting an area would be targeting the relative space as opposed to the static space, and that it would travel with you regardless of the movement of the platform, but that's largely because it's more fun than telling your party caster that they can't use any of their spells. Also, photons (and, thusly, the deprivation of photons) can't be subjected to any kind of resistance, so I'd argue it'd still work on an open-air platform.