Manwantara is an astronomical period of time measurement which is equal to 71 times the number of years contained in the Four Yugas. Manwantara is the age of Manu, the Hindu progenitor of mankind. This is the duration of the Manu, the divinities and the rest, which is equal to 852,000 divine years, or to 306,720,000 years of mortals.

Manwantara is ruled by a particular Manu who in turn is created by Lord Brahma. It is Manu of a particular period which creates the world and all the creatures of that particular period. Each Manwantara comprises the life span of Manu. After the death of one Manu, Brahma creates another Manu to carry on the cycle of creation in this universe. During every Manwantara, Lord Vishnu takes on a new avatar, a new Indra and the seven Rishis or the Sapta Rishis are appointed.

Manwantaras in Different Ages
In Vishnu Purana, while discussing the concept of Manwantaras, is mentioned that the first Manu was Swayambhuva. Then came Swarochisha, then it was Auttama, then Tamasa, then Raivata, then Chakshusha. But all the six Manus have passed away with the passing ages. Vaivaswata, the seventh Manu presides over the sun and is the Manu of the present age.

Swayambhuva: The era of Swayambhuva, had taken place, in the beginning of a Kalpa together with the celestials, saints and other personages.

Swarochisha: During the time of Swarochisha, two classes of celestials had flourished namely the Parvatas and the Tushits. The king of the celestials was the powerful Vipaschit. The seven Rishis were Urja, Stambha, Prana, Dattoli, Rishabha, Nischara, Arvarivat and the sons of the Manu were Chaitra Kimpurusha and others.

Auttama: In the third Manwantara of Auttami, Susanti was the king of the celestials, who were severally denominated as the Sudhamas, Satyas, Sivas, Pradersanas and Vasavertis and each of these orders consisted of 12 deities. The seven sons of Vasishtha were the seven celestial saints and Aja, Parasu, Divya and others were the sons of the Manu.

Tamasa: In the reign of Tamasa, the fourth Manu, the Surupas, Haris, Satyas and Sudhis were the orders of the celestials, Sivi was their king who was named Satakratu by his performance of hundred sacrifices; the seven Rishis were Jyotirdhama, Prithu, Kavya, Chaitra, Agni, Vanaka and Pivara. The sons of Tamasa were the powerful Kings Nara, Khyati, Santahaya, Janujangha and others.

Raivata: The fifth Manwantara was ruled by Raivata Manu; Indra was their king and the celestials were Amitabhas, Abhutrajasas, Vaikunthas and Sumedhasas each consisting of 14 divinities. The seven Rishis were Hiranyaroma, Vedasri, Urddhabahu, Vedabahu, Sudhaman, Parjanya and Mahamuni. The sons of Raivata were Balabandhu, Susambhavya, Satyaka and other brave kings.

These four Manus, Swarochishas, Auttama, Tamasa and Raivata were born in the race of Pryavrata who had propitiated Lord Vishnu by his devotions and had obtained in consequence thereof these rulers of Manwantaras as his son.

Chakshusha: In the sixth Manwantara, Chakshusa was the Manu and Manojava had become the king of the celestials during that time. They were grouped as Adyas, Prastutas, Bhavyas, Prithugas, and the high-minded Lekhas each consisting of 8 divinities; the seven Rishis were namely Sumedhas, Virajas, Havishmat, Uttama, Madhu, Abhinaman and Sahishnu. The sons of Chakshusha were mighty Uru, Puru, Satadyumna and other kings of the earth.

Vaivaswata: Purana narrates that the Manu, who reigns a period is the wisest and the most illustrious lord. The celestials are the Adityas, Vasus and Rudras. Their king is Purandara. Vasistha, Kashyapa, Atri, Jamadagni, Gautama, Vishwamitra and Bharadwaja are the seven Rishis. And the nine pious sons of Vaivaswata Manu are the kings Ikshwaku, Nabhaga, Dhrista, Sanyati, Narishyanta, Nabhanidishta, Karusha, Prishadhra and the well-known Vasumat.

Vishnu Purana states that in the next Manwantara Vishnu was born of Vikunthi, as Vaikuntha along with the celestials called Vaikunthas. In the present period Vishnu was again born as Varnana, the son of Kasyapa by Aditi. With three paces he had conquered the worlds and having released them from all disturbances he had given them to Purandara.

Divinity of Manwantaras in Different Ages
It is believed that the incomparable energy of Vishnu and the quality of goodness and the quality of preserving all created things rules over all the Manus during their Manwantaras in the shape of divinity. From a part of that divinity Yajna was born in the Swayambhuva Manwantara the well-begotten child of Akuti. And at the arrival of the Manwantara of Swarochisha the irrepressible Yajna was born as Ajita along with Tushitas, the sons of Tushita. And at the advent of the Manwantara of Auttama, Tushitas were born as the excellent Satyas. In the Manwantara of Tamasa, Satya became Hari along, with the Haris, the children of Hari. And in the Raivata Manwantara of Sambhuti the excellent Hari was born as Manasa along with the celestials called Abhutarajasas.

By these seven persons, in the various Manwantaras, the created beings have been preserved. He is called Vishnu because his energy pervades the whole world from the root.