The sky is a dome 180 degrees around vertically and 360 around horizonally, the sun is fairly small in that. Yet the image shows the sun dead centre, the sun is not shown like this in all medival art to show it's daytime right? There are a number of known natural optical illusions involving the sun. It's also looking very similar to a meteor shower, there's even an impact shown on the ground.
A quick skim of that wikipedia shows "disc" shaped not "saucer" shaped. Astronomical objects are usually discs, specifically the sun and it's associated optical illusions or meteror fireballs.
A reasonable person would put these ancient phenomena down to:
#1. Meteors
#2. Optical illusions -
Optical phenomena - Wikipedia
#3. Weird lightning eg Ball Lightning
#4. Flock of birds
#5. Weird clouds
#6. Being permanently drunk since that's how you purify water
#7. Untreated illness because there's no medicine causing hallucinations
#8. Someone lying
...
#9997. Aliens
#9998. God
#9999. Time Travel
#10000. Ancient Astronauts