Sun Salutation are not a part of the tradition of Hatha Yoga. Originally from the enlightened sages of the much older Vedic age. The corresponding Sanskit mantras are already mentioned in the Rig Veda, one of the oldest known scriptures of India, composed around 1700-1100 BC.
The sun as the center of solar system symbolizes Ultimate Consciousness. Surya Namaskara is performed as a smooth flow of movements,or one may choose to hold the body for 1-2 seconds in each posture.
Specific breathing pattern are synchronized with the sequence of movements between the 12 postures.