Can a permanent Teleportation Circle be made on a moving vehicle?

Question

There is a similar question about this for 3.5e, but can a permanent teleportation circle be created on a large moving vehicle? For example, a sailing ship, or a wide portable floor?

The spell says:

You can create a permanent teleportation circle by casting this spell in the same location every day for one year. You need not use the circle to teleport when you cast the spell in this way.

To further illustrate the question, here are sub-questions:

• What happens if I cast the spell on the vehicle for a year (and it is unmoving), and then the vehicle moves? Can it still be used as a destination point?

• Can I create a permanent TC on the vehicle if it is never in the same place twice in the first place?

Answer 1

Yes

The spell says:

As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle of your choice whose sigil sequence you know and that is on the same plane of existence as you.

D&D 5e co-creator Jeremy Crawford clarified on Twitter that the destination of teleport (which may be a sigil from teleportation circle) may move. At the time of the post tweets were considered official rulings by WotC, but is no longer the case. It remains a logical reading of the rules, and the designers intent:

The teleport spell requires a destination. The nature of that place isn't specified. It can be the deck of a moving ship, for instance.

Likewise, the surface of the circle is important, not its point in space. So, the surface may also move while performing the teleportation circle spell every day for a year, and after it has become permanent. It has no restrictions that the permanent sigil doesn't move:

When you cast teleportation circle, you create the circle on the ground. The circle is bound to that surface, not to a point in space.

What happens if I cast the spell on the vehicle for a year (and it is unmoving), and then the vehicle moves? Can it still be used as a destination point?

Yes, the target is the sigil, not the point in space the spell was cast.

Can I create a permanent TC on the vehicle if it is never in the same place twice in the first place?

Yes. Jeremy's post shows that.

But, even more from a real world perspective, all motion is relative. On Earth, we tend to think of places we travel to as stationary -- but they are not. They are rotating around the Earth's axis and revolving around the sun. To the car/planes we travel in, they are standing still (relative to themselves) and Earth is moving beneath them...

Importance in Astral Plane and Spell Jammer

If this were not the case, then there couldn't be permanent teleportation circles in the Astral Sea nor in Wild Space because everything is moving/drifting about. The strict reading where you took "ground" to mean "solid earth" would prevent sensible use of a number spells from being used in the Astral Plane, on Astral ships, or other vehicles and structures. Like you should be able to use Tensors Floating Disk on the deck of a airship, or in a city that is on the back Astral Whale.

Answer 2

No

The key line of the teleportation circle spell description is this:

As you cast the spell, you draw a 10-foot-diameter circle on the ground

You can't cast it on a vehicle.

Jeremy Crawford says in a tweet (https://twitter.com/JeremyECrawford/status/941033897982271488):

The DM decides how generously to interpret words like "ground." Unless we redefine or focus a word, we use it in its idiomatic English sense, knowing that some words are open to creative interpretation.

The idiomatic English sense is defined as (https://www.lexico.com/definition/ground):

The solid surface of the earth.

So it is up to the DM to decide how generous they are with where this spell can be cast. However it seems clear that there is no interpretation of idiomatic English that means the floor of a vehicle is the ground, however generous you are. House rules are of course, as ever, absolutely fine, but probably should be acknowledged as such to avoid arguments at different tables.