[windows_tips]

You don't need solar cells to heat water to boiling, just glass or a metal like aluminum or silver.

With glass you can build a lens to concentrate light. With a metal you can build a concentrating mirror.

[codeisawesome]

Good points. But lens building is a complex endeavour that requires tooling at the manufacturing level (especially computing power!) - no matter the choice of available material, yes? Doing it with high precision at a civilisation-powering scale seems like an impossible ask of a species that was still considering much of their Home planet to be “new” (Americas).

[windows_tips]

Galileo was making lenses hundreds of years before the time we are talking about. Also, eye and magnifying glasses existed then.