Old Sun

The Old Sun provided light to The World until (in Prophetbot's words) "it went out". Prophetbot mentions in passing how the Old Sun used to sit at the top of the Tower and provided light to The World. Based on the fact that the Sun's "pedestal" at the top of the Tower is a light bulb socket and the fact that Prophetbot mentions how the Old Sun "went out", it is assumed that the Old Sun (like the new sun) was a light bulb.