You have probably forgotten by now that Kashyapa and Aditi had a son named Vivasvana. This was the sun god, also known as Surya or Martanda.
Surya was married to Samjna, Vishvakarma’s daughter. They had two sons. The fist son was Vaivasvata Manu and the second son was Yama or Shradhadeva, the god of death. Yama had a twin sister named Yamuna. The sun’s energy was so strong that Samjna could not bear to look at her husband. Through her powers, she created an image from her own body that looked exactly like her. This image was called Chhaya (shadow).
Samjna told Chhaya, “I cannot bear the energy of my husband, I am going off to my father’s house. Stay here, pretend to be Samjna and look after my children. Under no circumstances tell anyone, certainly not my husband, that your are not Samjna.”
“I will do as you have asked me to,” replied Chhaya. “But the moment someone curses me or pulls me by the hair, I shall be forced to reveal the truth.”
Samjna went to her father Vishvakarma kept asking her to return to her husband. But this Samjna refused to do. Instead, she went to the land known as Uttara Kuru and started to live there as a mare.
Meanwhile, Surya, who had not realized that Samjna had been replaced by Chhaya, had two sons through Chhaya. They were named Savarni Manu and Shani (Saturn). As soon as her own children were born, Chhaya no longer displayed as much of love for Samjna’s children as she used to do. Vaivasvata Manu was a quiet sort of person and he ignored the implied neglect. But Yama was not the tolerant. Besides, he was also younger. He raised his leg to kick Chhaya. At this, Chhaya cursed Yama that his legs would fall off.
Yama went and complained to Surya. ” I have not really kicked her,” he said. “I only threatened to. And does a mother ever curse her children?”
“I can’t undo the curse, ” replied Surya. “At best, I can reduce its severity. Your legs will not actually fall off. Some of the flesh from your legs will fall off onto the earth and create worms. Thereby, you will be freed of your curse.”
But nevertheless, Surya felt that there was some truth in Yama’s asking whether a mother would ever curse her children. He taxed Chhaya with the truth, but Chhaya would not reveal anything Surya then grasped her by the hair and threatened to curse her. Since her conditions were now violated, Chhaya blurted out the truth.
In an extremely angry mood, Surya dashed off to Vishvakarma’s house. Vishvakarma tried to cool him down. “it is all because of your exercises energy that this has happened, exclaimed Vishvakarma. “If you permit, I will shave off some of the extra energy. Then Samjna will be able to look at you.”
Surya agreed to this proposition. With the shaved off energy, Vishvakarma manufactured Vishnu’s chakra (a weapon like a bladed discus).
Surya found out that Samjna was in Uttara Kuru in the form of a mare. He joined her there in the form of a horse. As horse, they had two sons named Nasatya and Dasra. Since ashva means horse, the sons were also known as the two Ashvinish and became the physicians of the gods.
Surya and Samjna then gave up their equine forms and lived happily ever after.