A recently released video shows what seems to be two suns setting in China. This strange, otherworldly sight could just be a hoax, but similar optical illusions have been seen before. The only problem is we still haven't explained them.
The appearance of a second sun is caused by optical refraction, as atmospheric particles bend light in unusual ways, creating what are popularly known as mirages. The creation of the illusory second sun isn't totally uncommon, but most such mirages would find the other sun above or below the real thing. The fact that the fake sun is beside the actual sun is extremely unlikely, and astronomer Jim Kaler speculates an unusually thick clump of particles must have been responsible for this supremely odd effect.
Again, assuming this isn't a hoax, atmospheric scientist Grant Perry says this effect is particularly tricky to explain:
"This is not a common optical phenomenon that we're seeing here. I'm asking myself if this is an artifact of the lens, but if that were the case – if it's reflections of the lens elements – then the images would move in relation to each other as the camera moves. But that doesn't happen. You would have to assume it is particles of ice or something in the atmosphere aligned in such a way that they would refract the sunlight at that very small angle, but only in one direction. It would require some fairly peculiar characteristics."
Jim Kaler says this particular mirage has probably never been computer modeled or fully explained, mostly because of its extreme rarity. All we can really say is, should you ever see two suns in the sky, there is a perfectly reasonable scientific explanation. Just don't be too pushy when you start demanding specifics.