American Tract Society Bible Dictionary 
City of the sun,
1. A celebrated city of Egypt, called in Coptic, Hebrew, and the English version, ON, sun, light, Genesis 41:45 . The Seventy mention expressly, Exodus 1:11 , that On is Heliopolis. Jeremiah, Jeremiah 43:13 , calls this city Beth-shemesh, that is, house or temple of the sun. In Ezekiel 30:17 , the name is pronounced Aven, which is the same as On. The Arabs called it Ani-Shems, fountain of the sun. All these names come from the circumstance that the city was the ancient seat of the Egyptian worship of the sun. It was in ruins in the time of Strabo, who mentions that two obelisks had already been carried away to Rome. At present its site, six miles north northeast from Cairo, is marked only by extensive ranges of low mounds full of ruinous fragments, and a solitary obelisk formed of a single block of red granite, rising about sixty feet above the sand, and covered on its four sides with hieroglyphics.
2. Another Helioplis is alluded to in Scripture under the name of the "plain of Aven," or field of the sun, Amos 1:5 . This was the Heliopolis of Coele-Syria, now Baalbec. Its stupendous ruins have been the wonder of past centuries, and will continue to be the wonder of future generations, till barbarism and earthquakes shall have done their last work. The most notable remains are those of three temples, the largest of which, with its court and portico, extended 1,000 feet from east to west. A magnificent portico, 180 feet long, with twelve lofty and highly wrought columns, led to a large hexagonal court, and this to a vast quadrangle, 440 feet by 370. Fronting on this rose ten columns of the peristyle, which surrounded the inner temple. There were nineteen columns on each side, or fifty-four in all, only six of which are now standing, and they were seven feet in diameter, and sixty-two feet high, besides the entablature of nearly fourteen feet. This temple rested on an immense vaulted substructure, rising nearly fifty feet above the ground outside, and in this are three stones sixty-three long and thirteen feet high, lying twenty feet above the ground. The temples are of Roman origin; and in vastness of plan, combined with elaborateness and delicacy of execution, they seem to surpass all others in the world. "They are like those of Athens for lightness, but far surpass them in vastness; they are vast and massive, like those of Thebes, but far excel them in airiness and grace." (Robinson.)
Smith's Bible Dictionary 
Heliop'olis. See On .
Holman Bible Dictionary 
Watson's Biblical & Theological Dictionary 
See On .
The Nuttall Encyclopedia 
E . City of the Sun), in Egyptian On , one of the oldest and most sacred cities of Egypt; was situated about 10 m. N. of Cairo, on the eastmost branch of the Nile; it was the centre of Egyptian learning; Solon and Plato are said to have studied there, and Potiphar was one of its chief priests; the famous obelisk
tands near; and
ow on the Thames Embankment, was originally of this city. Also the name of Baalbec.