Do spell effects move if the object they are cast on moves?

Question

let's suppose that I cast a Silent image of a wall on an airship. Let's suppose that I don't move it with concentration.

Does the image of the wall move with the airship or it will progressively go far?

Answer 1

Make a house rule early in the campaign

The game doesn't cover this. I suggest picking one of the following.

1. Reality is comprised of 5-ft. squares. The squares impose themselves on creatures and objects it contains. Using this model, the universe is a fixed background and everything moves independent of it. Thus, when a caster creates an effect targeting point in space, it stays at that point, regardless of surround conditions. This is elegant and initially appealing but, as Wyrmwood's answer mentions, gets wonky, especially considering planetary rotation, galactic spin, planar cosmology, and a host of headaches. However, it has the advantage of making reality incredibly predictable and making teleportation mathematically precise.

   In the example, casting the spell silent image while on a moving airship requires the caster to concentrate continuously on the spell to move the image as the airship moves, and when the image exceeds the the spell's area of effect, the image continues to exist, but at the limit of area's effect, perhaps floating in space after the airship's exceeded the bounds of the spell's area.

2. Things in reality are comprised of 5-ft. squares. Sort of like the video game Minecraft, everything in reality is mildly pixelated, and things bring with them their own 5-ft. squares. Anything big enough to have a map imposes that map on other maps. This is sloppy, but it enables effects to function like most imagine they should actually function.

   In the example, casting silent image while on a moving airship requires no special rulings. The airship moves and, when doing so, takes with it the spell's affected area.

As an aside, in my campaign, this came up not with silent image cast while on an airship but with Mordenkainen's magnificent mansion cast while on a sailing vessel. The table eventually agreed that the first house rule was too hard and too weird and went with the second.

Answer 2

I don't think the rules say anything about this, however, they also don't say anything about a spell moving with the rotation of the earth either. I think a good ruling would be to use frame of reference, that is, if you cast it in the air, near the airship, it would not move with the ship without your effort, but if you cast on the airship itself, it would.

Answer 3

The Stronghold Builders Guidebook offers an approach for this question using the Architecture rules.

The magic-item-as-architecture item creation rules and formula give the cost and method for a magical effect being permanently embedded into architecture.

The cognizant point of the rule with regard to this question is that it explicitly allowed for otherwise non-mobile effects to become mobile and be centered upon the architecture in question (when combined with the stronghold options for mobility, like walking castles or flying towers, etc.), so long as it was embedded into a structure of some kind.

Given that the default rule of 3.5 is to include all 3.0 material which was not explicitly updated in a later source is automatically considered to be 3.5 (though a few groups house-rule it otherwise, which is fine), then if your group/DM uses the default, you are covered.

Some groups would prefer a more explicit ruling, and if this is not sufficient for RAI, then it could certainly be used as a basis for a houserule.

Answer 4

If you cast it on something that moves, it should move with it. Take the spell Light for example, you can cast it on the end of your wizard staff and use it like a torch. The light hovers around/sticks to the pole and moves with it. I would assume that Silent image works the same. If you make a Silent Image of a wall, and you cast it on the floor boards of the ship, if that ship moves, the floor boards move and the silent image attached to the floor also moves.