Inspired by this question, I'm curious why halachic literature calls twilight בין השמשות, which means, literally, "between the two suns"? The sun has set, and there is just one sun, anyway. What do they mean by saying "two suns"?
I understand that the twighlight period is considered safek (doubtful) day or night. But, night is the absence of the sun. What does it mean by referring to "two suns" and being "between" them?