To accomplish this, we need:
- Perfect shape match.
- Perfect ratio of size to distance.
- Perfect angular placement.
My head is not a circle, so it is not possible to do this with my head. Getting the ratio of size to distance right is very difficult. So is getting the right angular placement.
For the moon-sun, the sphere just arises because they are both sufficiently large.
The ratio of size to distance is a freak accident. There is no good explanation for why the sun and moon are at essentially the same ratio.
The angularity is surprising too. As I understand it, there is no good explanation for why the moon ever intersects with the placement of the sun. I could have been a different way. Though it is possible I am wrong here (@PdotdQ).
The fact that size/distance of moon = size/distance of sun is pretty surprising. I’m not sure that applies to any other planet-moon combination in the solar system.