As the Markandeya Purana tells it
Markandeya Purana, cantos 77 to 78 and 106 to 108 · 3 min read
Samjna was the daughter of Vishvakarma, the craftsman of the gods, and her father gave her in marriage to the Sun.
In those days the Sun was a ball of fire with nothing held back, and he scorched the three worlds. Whenever he turned his face to Samjna she shut her eyes.
The Sun took it badly. "Because you close your eyes when you see me," he said, "the son you bear will be Yama, the one who closes the eyes of men."
She trembled, and her glance darted about in fear. "And because your eyes are restless," he said, "your daughter will be a restless river."
So after Manu, her firstborn, Samjna bore Yama, and then the river Yamuna. And still she could not bear the heat.
One day she looked at her own shadow and spoke to it.
"Stay here in my place," she said. "Be kind to my two boys and my girl, as kind as you are to the Sun. And tell no one that I have gone. If you are asked, say: I am Samjna."
"I will keep your secret," said the Shadow, "until I am seized by the hair, or until a curse is about to fall on me. As far as that I will go."
Samjna went home to her father. Vishvakarma was glad to see her, but before long he told her gently that a wife's place was her husband's house.
She left, but she did not go home. She went north, to the land of the Northern Kurus, and there she took the shape of a mare and lived by fasting and prayer.
The Sun noticed nothing. He took the Shadow for his wife, and she bore him two sons and a daughter. One son was Shani, the planet Saturn.
The Shadow did not treat all the children alike. Her own three got the caresses. Manu put up with it. Yama did not. One day in a temper he lifted his foot as if to kick her. He held it back, but she had seen.
"You raise your foot against your father's wife?" she said, her lip shaking. "Then let that foot fall to the ground!"
Yama ran to his father. "I was wrong to lift my foot, Father, and I ask pardon. But no mother curses her own son, however bad he is. She is not a mother to us as she is to the little ones."
The Sun could not undo a mother's curse, only soften it. "Worms shall carry a little flesh from your foot down to the earth," he said. "So her word comes true, and you keep your foot."
Then he turned to the Shadow. "Where has Samjna gone?"
"I am Samjna, your wife, the daughter of Vishvakarma," she said. He asked again and again, until he made ready to curse her. That was the limit she had set, and she told him the truth.
The Sun went straight to his father-in-law in a rage.
Vishvakarma received him with honour and calmed him. "Your form is more than anyone can bear," he said. "That is why she left. She is in the forest even now, fasting. If you allow it, I will trim your blaze."
"Do it," said the Sun.
So Vishvakarma set the Sun on his lathe and turned him. The worlds lurched as the wheel spun, and the gods and sages stood by singing his praise. Fifteen parts of his splendour were pared away and one part in sixteen was kept. From the shavings Vishvakarma made the discus of Vishnu, the trident of Shiva, the spear of the war god and the staff of Yama.
Then the Sun, milder now and beautiful, went north in the shape of a horse. The mare turned to face the strange stallion. Their noses touched, and from that meeting the twin Ashvins were born, the physicians of the gods. Then he showed her his true form, and she found that she could look at it. She took her own shape again and came home with him.
As the Rigveda and its old commentary tell it
Rigveda 10.17 and the Brihaddevata · 2 min read
The oldest telling is two verses long.
They stand at the head of a funeral hymn in the tenth book of the Rigveda, and they read like a riddle that everyone was once expected to know.
Tvashtri, the maker god, is holding a wedding for his daughter, the first verse says, and the whole world comes together for it. Then, at once: the mother of Yama, the wife of great Vivasvat, vanished while she was being led to her new home.
The second verse says what was done next. "They" hid the immortal woman from mortals. They made one who looked like her and gave that one to Vivasvat. Then comes the bride's name, the only time the Rigveda speaks it. She is Saranyu. She brought the two Ashvins into the world, the verse says, and she left behind two pairs of twins.
That is all. The hymn turns to the god Pushan and the roads of the dead and says no more about her. It does not say why she went, who "they" were, or what Vivasvat did when he found out.
Some centuries later a book called the Brihaddevata, a guide to the gods of each hymn, set the tale out for students.
Tvashtri, it says, had twins of his own, the girl Saranyu and a three-headed son. He gave Saranyu to Vivasvat, and she bore him twins in turn, Yama and Yami.
Then, while her husband was away, Saranyu herself made a woman who looked exactly like her. She left the two children in that woman's care, turned herself into a mare and went away.
Vivasvat suspected nothing. With the woman he took for his wife he had a son, Manu, who became a royal sage.
When at last Vivasvat learned that Saranyu had gone off in the shape of a mare, he changed himself into a horse of the same kind and went after her. She knew him at once, even as a horse, and came to him. From their meeting two young gods were born, Nasatya and Dasra, whom people praise together as the Ashvins, the horsemen.
There the Brihaddevata stops. Nobody is cursed, nobody is put on a lathe, and Saranyu is not brought home.