There are 3 ways to play this, the spell compensates for momentum, angular velocity/Coriolis effects and you arrive where you intend to, so pretty much D&D as normal bar the odd misty step/dimension door/gate off a cliff as a readied action. The path of least hassle.Building on @Tonguez 's answer, previous editions were consistent that momentum is preserved when teleporting:
From Jeremy Crawford in 2010 covering the history of Teleportation effects (the oft-quoted "teleportation does not set a prone creature upright"), effects are terminated simply because you teleport. It does stand to reason this doesn't get you out of falling (removing an effect of all your kinetic energy and momentum) any more than it does being prone.
- From the 3.5e FAQ: "If you’re plummeting toward the ground when you cast teleport to reach a safe spot, you’d still be “falling” and would therefore take damage as appropriate to the distance you actually fell before teleporting."
- From the 4e Rules forum FAQ: "Is momentum conserved when teleporting? The designers lean towards yes"
- From an article written on the subject by a 4e designer (Stump the Game Lizards, a former D&D forum):
- ... If you throw a stone through a portal, it comes out the exit still flying through the air. I can't imagine anyone saying that the stone just drops to the ground with all of its momentum somehow absorbed by the portal. (If that were the case, you couldn't even step through. Force is force.) The same thing applies if you leap through; you come out the far end mid-leap...
- ... This type of teleportation, then, is intuitively correct—which means momentum is conserved...
- ... An important addendum to what's written above is that momentum should be conserved when jumping through a portal relative to the portal. If I step into a magic circle on a flying boat and emerge in a magic circle on the ground, I don't have the boat's momentum; I have my momentum relative to the circle I stepped into.
In the real world, as has been pointed out, because the planet and universe is moving, if teleporting erased your momentum, you'd either (1) instantly be a stain upon a surface, or (2) shoot out into space at supersonic speeds.
However 5E is silent upon the matter. Sage Advice confirmed there is no official rule, and 5E leaves it to the DM whether they consider logic from prior editions to have precedent at their table.
So, take your pick.
The physics geek version where you need to know (before teleporting) your exact latitude and longitude and height above sea level (including destination) and relevant orbital and stellar motion data and do the math for the time of transfer to occur. Definitely a ritual with sever consequences of an error.
Or only allow teleport to a destination that is a permanent circle where all the physics is handled by compensator in the transport buffers.
Your choice.