Mârtânda is etymologically derived from mârta meaning "dead or undeveloped," (being connected with mrita, the past participle of mri to die) and ânda, an egg or a bird; and it denotes a dead sun, or a sun that has sunk below the horizon.
In the tenth chapter of Rigveda, it is mentioned that -
Aditi first had only seven sons but later gave birth to an eighth son named M?rtanda. Although many hymns in the Rigveda mention him along the other Adityas as a form of Surya, but as evident from the verse above, Aditi shunned him.
The Taittirîya Aranyaka reads -
tat parâ Mârtândam â abharat (she set aside Mârtânda for birth and death).
The Aranyaka then proceeds to give the names of the eight sons, as Mitra, Varuna, Dhât?i, Aryaman, Amsha, Bhaga, Indra and Vivasvat. But no further explanation is added, nor is it told which of these eight sons represented Mârtânda.
In the post-Vedic period, when the number of Adityas increased to twelve, another name Vivasvat was added to the canon. Vivasvat and Martanda are often used interchangeably.