When the word "灯" is used by itself, it has several pronouciations ひ、ともしび and あかり. Those are all 音読み.I think you can pronouce "灯がともる" in both ways "あかりがともる" and "ひがともる".

This is a good question, 灯がともっていたhi ga tomotteita, but now its sounds redundant.