Sol was the solar deity in Ancient Roman religion. It was long thought that Rome actually had two different, consecutive sun gods. The first, Sol Indiges, was thought to have been unimportant, disappearing altogether at an early period. Only in the late Roman Empire, scholars argued, did solar cult re-appear with the arrival in Rome of the Syrian Sol Invictus picked up from pagan worship of Mithras.
The Latin sol for "Sun" is the continuation of the PIE heteroclitic *Seh2ul- / *Sh2-en-, cognate to Germanic Sol, Sanskrit Surya, Greek Helios, Lithuanian Saulė. also compare Latin "solis" to Etruscan "usil". Today, "sol" is still the main word for sun in romance languages. "Sol" is used in contemporary English by astronomers and science fiction authors as the proper name of the Sun to distinguish the Sun from other stars, which are the suns of their own planetary systems.
Various Roman philosophers speculated on the nature of the sun, without arriving at any consensus. A typical example is Nigidius, a scholar of the first century BC. His works have not survived, but writing five centuries later, Macrobius reports that Nigidius argued that Sol was to be identified with Janus and that he had a counterpart, Jana, who was Moon. As such, they were to be regarded as the highest of the gods, receiving their sacrifices before all the others. Such views appear to have been restricted to an erudite elite — no ancient source aside from Macrobius mentions the equation of Sol with Janus — and had no impact on the well-attested cult of Sol as independent deity.