Starting with a new scene, I inserted a UV sphere with a 5028' radius (1 mile). In edit mode, I removed the bottom half making it a dome, then flipped the normals inside.
Then I added an emission shader. In cycles, it's just black, no matter how much I turn up the light/emission power. It works OK in Eevee. I also can't get point lights bright enough to hit the dome.
It works fine if the dome is 100', so this has something to do with the scale. I changed camera clipping to 30k feet, and I also set clamping in cycles to 0 for both. I wonder if there is a "light clip" setting somewhere? I also tried using the light falloff node with constant and the maximum strength it would allow.
I'd post a picture, but it's just black....
How do I get an emission shader to work on a giant object and/or from a great distance?
Here is a test file:
There is no scene camera, just the viewport camera with cycles enabled. the camera is position inside the dome.