zemanim - Why is twilight called "Bein Hashmashot"?

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?

Answer

1. 
   
   

   According to Kipa here the reason is that it is after the sun ("Shemesh of the day") finishes lighting up the sky and before the moon ("Shemesh of the night") lights up the sky. Therefore, this time period is between the "two suns".

   
   

   This name makes perfect sense, as the "sun of the day" is not fully gone, but the "sun of the night" hasn't fully arrived, so it is a Safek if it is the time of the old Shemesh (of the day), or the new Shemesh (of the night).

   
   


2. 
   

   According to Wikishiva here, it is between when the sun disappears, and when it's light goes away:

   
   

   
   

   ביאור השם "בין השמשות" הוא: הזמן שבין שקיעת גוף השמש ובין העלם אור השמש, כלומר: בין שתי "שמשות"‏

   
   

   
   

   This would also make sense - the setting of the sun could be the end of the day, and darkness could be the beginning of the night.