from the FAQ:
"Q: "Please explain sunrises/sunsets."
A: It's a perspective effect. Really, the sun is just getting farther away; it looks like it disappears because everything gets smaller and eventually disappears as it gets farther away. "
Light does not just stop travelling. If the sun was the size specified, it would still appear as a sizeable lighted ball during the night-time.
Also, given the required trajectory of the sun's movement, the radius of the circle of the track of the sun's motion would not be that big.....not far enough to make the sun fade substantially.