In the early Vedic times the Adityas were six, or more frequently seven, celestial deities, of whom Varuna was chief; consequently he was called the Aditya.
Adityas were the sons of Aditi. Aditi, though had eight sons, yet she approached the gods with seven, having cast away the eighth, Martanda (the sun). Thus, Martanda is one of the Adityas. Later, the number was increased to twelve, as representing the sun in the twelve months of the year. Aditya is one of the names of the sun. There (in the highest heaven) dwell and reign those gods who bear in that of the heroic poems, were entertained regarding these deities. They were twelve sun-gods, bearing evident reference to the twelve months. But for the most ancient period we must hold fast the primary signification of their name. They are the inviolable, imperishable, eternal beings.
Aditi, eternity, or the eternal, is the element and inviolable element in which by them… the eternal and inviolable element in which the Aditya dwell, and which forms their essence, is the celestial light. The Adityas, the gods of this light, do not therefore by any means coincide with any of the forms in which light is manifested in the universe. They are neither sun, nor moon, nor stars, nor dawn, but the eternal sustainers of this luminous life, which exists, as it were, behind all these phenomena.
The names of the 6 Adityas are as follows:
Out of the, Daksa is frequently excluded, and Indra, Savitr (the sun), and Dhatr are added. In the later Indian literature they are always said to be twelve. Those of the twelve Adityas are variously given, but many of them are names of the sun.
Here we must note that Sun, or Surya or Vivaswat was the chief of the gods in very early periods. The twelve solar dynasties, or personifications of the sun under a different names and sign of the zodiac in each month of the year, are also referred to Aditya. They belong to a period before the time of the Vedas, when the worship of the elements, particularly of the sun, was first enriched and extended by fancy.
Vishnu has also been called chief of the Aditya. Vishnu is always named as one of them, and as by the time when these works were written, his dignity had become enhanced in general estimation, he is declared to be the greatest of the twelve. In the Vishnu Purana and Hari Vamsa, it has been stated that those who formerly, in the chakshusha manwantara, were called the tushitas, are known as the twelve Adityas in the Vaivasvata manwantara.
Aditi, eternity, or the eternal, is the element which sustains them, and is sustained by them. The eternal and inviolable element in which the Adityas dwell, and which forms their essence is the celestial light. The Adityas, the gods of this light, do not therefore, by any means coincide with any of the forms in which light is manifested in the universe. They are neither sun, nor moon, nor stars, nor dawn, but the eternal sustainers of this luminous life, which exists, as it were, behind all these phenomena.