The name On appears in Numbers 16:1 as a Reubenite who joined the conspiracy of Korah against Moses and Aaron. Separately, On (also transliterated Aven or Heliopolis) is an Egyptian city mentioned in Genesis 41:45 (the city of Joseph's father-in-law Potiphera) and Ezekiel 30:17 (judged alongside other Egyptian cities). The Hebrew name of the Egyptian city means "strength" or "vigor."
The Egyptian city of On (Heliopolis in Greek) was the center of sun worship in ancient Egypt, home to the great temple of Ra. It was also where Joseph was given his wife Asenath, the daughter of a pagan priest (Genesis 41:45). This detail — the patriarch of Israel marrying into the family of a sun-priest in the city of sun worship — is not glossed over by Scripture. God's purposes advanced even through circumstances that violated His ideals. Joseph's story demonstrates that God's sovereignty operates not only through clearly holy situations but through the complex moral realities of human life in a fallen world.