Just an idea I came up with after reading De Revolutionis Orbis Coelestis.

Suppose Nightmare Moon didn't have the power to actually bring about an Eternal Night (or just wasn't insane enough to do that knowing how it would mess up the world) and instead used her power over dreams and some hypnotism/mind control to make everypony think that it was always night outside?

The result being that ponies would walk around in broad daylight convinced that the sun was missing or had been replaced with the moon (albeit a very bright and luminescent moon). Anypony who resists this indoctrination would be unable to convince their fellows that the sun still exists and might attract the attention of Nightmare Moon or any of her enforcers.

What would a rational pony do in such a situation?

1784605
Start stockpiling food?

be generally annoyed at nightmare moon and shake their hooves in her general direction?
also, wear more sunscreen than most other ponies, and try to convince other ponies that sunscreen has to be worn also at night :-/
?

1784891
Hard to say if they'd need it at all. Rainbow Dash looked like she was sunbathing in Too Many Pinkie Pies. If anything, I'd translate the old farming calendar to a new one, that way everyone would still have food.

It depends on what the rational pony's goal was. If the pony just wanted to survive, that's one thing, but if they wanted to actually try and fight against Nightmare Moon's regime, that'd be different. I'm assuming this pony didn't fall for the hypnosis somehow, because if they did, if Nightmare Moon was competent, they'd never realise it unless it was brought to their attention. (I mean, what pony, rational or not, would think "I'm being hypnotised into thinking the moon is still up" when there's a perfectly reasonable-sounding explanation for eternal night already?)

The first question is...why. What advantage does Nightmare Moon get from convincing everypony that she's brought eternal night that she doesn't get from actually BRINGING eternal night? The main thing I can think of is the aura of total omnipotence. Assuming Equestria works somewhat like Earth, if eternal night went on for a week or two, things would get SERIOUSLY fucked up. Nightmare Moon could take credit for this not happening, saying it was her power and hers alone that allowed the plants to grow and for ponies to not freeze to death.

So, how do we challenge this aura of omnipotence? The answer is to create an experiment that shows conclusively that the sun must still be out during the day. Presumably, Nightmare Moon is smart enough to also use her hypnosis to prevent you from feeling the sun on your skin, or else the whole thing would blow up very quickly. Ponies probably don't sunburn, at least not easily, given they have coats. Plant behaviour (even Zap Apples) don't work, because Nightmare Moon's already taken credit for that.

The first (weak) experiment I can think of is to shine a mirror or foil thing into a pony's eyes. The pony wouldn't be able to see it through the hypnosis, but would find their eyes hurting from the light, which makes no sense if it's actually night-time. If you explained what was going to happen first, that could work, though that would probably only work on someone inclined to trust you; it wouldn't be enough to convince someone adamant that you were wrong.

Anyone got any better ideas to continue this train of thought?

What would a rational pony do in such a situation?

Find five friends and blast Nightmare Moon with the Elements of Harmony.

What would a rational pony do in such a situation?

Realize that the moon is the same as the old sun, replace the word "sun" by "moon" in their thinking, and move on with their lives.