The Race of Varuna : Birth of Asvin Gods1 - Verse 71-74

O suppressor of enemies ! At the outset, I shall change your personal form into an excellent one”. The form of Vivasvan, then, was sideways, upwards and below (i.e. the rays were diffused everywhere). Hence Divaspati (the Sun-god) was ashamed of that form. So the deity of great penance praised the wheel of Tvastr. Tvastj- was then permitted to change the form. With the concurrence of the Sun-god (Martanda Vivasvan) , Tvastr mounted him on the wheel and whetted his brilliance. Then the brilliance had pleasant refulgence due to reduction in dazzle.