Purana - the story of Samjna and Surya

Surya or Vivasvan was born to the sage Kashyapa and his wife Dakshayani. His other name was Martanda. He was married to the daughter of Viswakarma, Samjna and they had two sons and a daughter. The two sons were Vivasvat Manu and Yama and the daughter was Yamuna. Yama and Yamuna were born as twins. Samjna could not withstand the tremendous power of the sun and therefore she created a look alike of hers, called Chhaya, and went to live at his father's place. She entrusted her sons and daughters to Chhaya imploring her not to reveal her identity. Chhaya agreed. However she said that if nobody insulted her by dragging her by hair, she would not reveal anything. Samjna thereafter went to the place of Viswakarma. But Viswakarma, when he heard that Samjna had left her husband, became very angry and rebuked her harshly. He asked her to go back to her husband's place and take care of her children. Samjna, who was in no mood to relent therefore left her father's place, took the form of a mare and went to roam freely in the Uttara Kuru area. Now Chhaya was managing well but in the meanwhile she had her own sons. The first one was called Savarni Manu as Chhaya was also called Savarna, the second one called Shanaischara. When she had her own sons, Chhaya began to neglect the sons and daughters of Samjna. Vivasvat Manu was a good natured child, so he did not protest. But Yama was ill tempered and he could not accept this behavior. In a ft of anger once he showed his feet to Chhaya who promptly cursed him that he foot would fall down. Yama was afraid and he went to his father, Vivasvan crying, saying that Chhaya could not eb his mother since she cursed him. Vivassvan after listening to Yama pacified him saying that only a portion of his foot would come out being affected by worms, so he had no fear. When Sun asked Chhaya the reason for her negligence of her sons, Chhaya could not give a good answer. Vivasvan through his Yogic power came to know of everything and he took Chhaya's hair and was about to curse her. Then Chhaya revealed everything about Samjna. Sun hurriedly went to Viswakarma's house seeking Samjna. But Viswakarma told him that Samjna left his house and went as a mare to Uttara Kuru. Vivasvan therefore took the form of a horse and went in search of Samjna. When he got her finally they spent a delightful time as horse and a mare in Uttara Kuru and two sons were born to them who were known as Aswini Kumaras - Nasatta and Dasra. The Aswini Kumaras became physicians of the gods.

Then when Vivasvan came back with Samjna Viwakarma devised an idea. He put Sun on one of his giant wheels to get him rid of some of his lustre and power. The Sun then became more bearable and tolerable to Samjna and they began to live happily together in their previous forms. The excess power given up by the Sun was used in making a trident for Shiva and a Chakra or Discuss for Vishnu. 

Yama later became the Lord of all the pitris and the dead. Yamuna became the famous river on earth. Shani or Shanaischara became a planet Shani. Savarni Manu would become a Manu of the later ages and Vaivasvat Many became the Manu or the king of the present Manvantara.