Chaaya or Chaya means shadow or shade. Chaaya is also personified as the goddess of shadow, the consort of Surya, the Hindu sun god. She is the shadow-image or reflection of Saranyu (Sanjna), the first wife of Surya. Chaaya was born from the shadow of Sanjna and replaced Sanjna in her house, after the latter temporarily left unable to bear Surya's fierce splendour.
Chaaya is usually described as the mother of Shani, the god of the planet Saturn and a feared graha; goddess Tapti, the personification of river Tapti; and a son Savarni Manu, who is destined to be the next and eighth Manu (progenitor of mankind) - the ruler of the next Manvantara period.
In the Rigveda (c. 1200-1000 BCE), which is the earliest narrative about the Chhaya-prototype. After the birth of twins to Vivasvan (Surya), his consort Saranyu - the daughter of Vishwakarma - abandons him and flees in form of a mare. The woman the divine Saranyu places in her place is called Savarna ("same-kind"): similar to Saranyu, but mortal. Savarna has no children by Surya. A later (500 BCE) addition to the Rigveda by Yaska in his Nikuta says that Manu (the progenitor of mankind, called Savarni Manu in later Puranic text) was born to Savarna. While the original text says "they" (interpreted as the gods) substituted Savarna for Saranyu, Yaksa's Nikuta tells that Saranyu created Savarna and substituted her. Brhad devata calls the prototype of Chaaya as Sadrisha ("look-alike"), a woman who looked like Saranyu. Sadrsha begot Manu by Surya, who became a royal sage.
By the time of the Harivamsa (c. 5th century CE), an appendix of the epic Mahabharata; Saranyu is called Sanjna and her double is reduced to her shadow or reflection Chhaya. It narrates that Sanjna, after giving birth to three children of the Sun, abandons him, and leaves Chhaya to take care of her children. Surya mistakes Chaaya for Sanjna and begets Manu by her. As Manu looked just like his father, he was called Savarni Manu. When Chaaya, also called parthvi ("earthly") Sanjna, became partial to her own son and ignored those of Sanjna, Yama threatens her, and Chaaya casts a curse on Yama. Upon discovery of this, Surya threatens Chaaya, and she discloses the story of her creation; whereafter Surya finds Sanjna and brings her back. The text also states that Shani was a brother of Savarni Manu, though his birth is not explicitly stated. According to Wendy Doniger, the substitution of the dark shadow was apt by Saranyu for the Surya darkened by her.