The sun-god Surya was born as the son of Kashyapa and Aditi. Surya was born as an egg (anda). For days on end, nothing happened to the egg and the shell did not crack. Vishvakarma was the architect of the gods. When Vishvakarma found that nothing was happening to the egg, he cracked it open with a blow. Kashyapa was initially alarmed at this act, since he thought that the son who was inside the egg might have died. But he soon discovered that this was not the case and remarked, ―the egg (anda) has not died.‖ From this expression, the son who was born came to be known as Martanda.
As Martanda, Surya named Samjna, Vishvakarma‘s daughter. They had two sons and a daughter. The sons were named Yama and Vaivasvate (this was the one who became a Manu). The daughter was Yami (or Yamuna).
But so strong was the sun‘s radiance that Samjna could not bear to look at her husband. She eventually thought of a way out. She created an image who looked exactly like her. This image was called Chhaya (shadow).
―I am going off to my father‘s house,‖ replied Samjna. ―Please stay here and pretend to be Samjna. Take care of my children and under no circumstances divulge that you are Chhaya.
―I will do what you ask,‖ said Chhaya. ―But there is a condition. I will not relate the truth as long as I am not cursed or as long as no one grasps me by the hair. But the moment this condition is violated, I will reveal all.‖
Samjna agreed to this condition and left for Vishvakarma‘s house. Initially, Vishvakarma was quite happy to see his daughter. But days passed and Samjna did not return to her husband. Vishvakarma now realized that something must be amiss and cajoled his daughter to return to Surya. A thousand years passed. Finally, Samjna could bear her father‘s scolding no longer. Pretending to return to her husband. She left Vishvakarma‘s house. She adopted the form of a mare and started to live in the region known as Uttarakuru.
Meanwhile, Surya had not realized that Samjna had been substituted by Chhaya. Surya and Chhaya had parented two sons and a daughter. The sons were Savarni (who would become a Manu) and Shani (the planet Saturn) and the daughter was Tapati.
Chhaya was clearly more attentive towards her own children than towards Samjna‘s.
Vaivasta did not mind this at all. He was the eldest and more collected and balanced. But Yama resented this apparent partiality. In a fit of childish petulance, he kicked Chhaya.
―You have dared to kick your father‘s wife,‖ said Chhaya. ―I therefore curse you that your feet will decay. It is with these feet that you have kicked me.‖
Yama went and complained to Surya. He had sinned, but he was no more than a child. Might his transgression not have been overlooked on that account? Besides, whatever the provocation, does a mother ever curse her own son?
Surya did what he could to mitigate the effects of the curse. But he also realized that something must be wrong. When he demanded to know the truth from Chhaya, she revealed nothing. But when Surya threatened to curse her and grasped her by the hair, the conditions Chhaya had made were violated. Chhaya now revealed the truth.
Surya went to Vishvakarma‘s house to find out where Samjna was. Hearing what had happened, Vishvakarma offered to slice off some of the sun‘s energy. It was, after all, the sun‘s great radiance that had motivated Samjna to do what she had done. Surya was not averse to this idea and his appearance was greatly improved by Vishvakarma‘s action.
Surya got to know that Samjna was in the Uttarakuru kingdom in the form of a mare. He adopted the form of a horse and joined his wife there. As horses, Surya and Samjna had two sons named Nasatya and Dasra. Since the word ashva means horse and since these two had been born when their parents had adopted the forms of horse, they were also known as the two Ashvinis. They became the physicians of the gods. After the Ashvinis were born, Surya and Samjna returned to their original forms.
Therafter, nine sons were born to Vaivasvata. They were Ikshvaku, Nahusha, Dhrishta, Sharyati, Narishyanta, Pramshu, Nabhaga, Karusha, and Prishadhra.
The kings of the solar line (surya vamsha) were descended from Ikshvaku. In this line was born Rama of the Ramayana fame.