Surya Namaskar A,B & C is progressively harder to master & a lot more strenuous. Each salutation adds an extra movement.

"Surya namaskar c" is the traditional surya namaskar. Surya namaskar a & b are modern versions from Ashtanga Vinyasa Yoga. 1) "Most of the aasanas in the procedure themselves have documented in old literature. "Sashtang dandavat" which is the central aasana of the surya namaskar was followed from time immemorial in India as a form of showing respect and complete surrender to God. "Bhujangasana" was described as one of the 32 important aasanas in "Gheranda Samhita" (dated around 1802 A.D.) which describes the yoga prevalent in north-east India. The "Adhomukh Swanasan" was described in the old wrestling text of "Mallapurana" (dated before 1750). "Sarpasana" (Bhujangasana), "Gajasana" (Adhomukh Swannasan), "Uttanasana" and series of postures done in tandem, similar to surya namaskar are all described in Sritattvanidhi which was written by the order of Krishnaraja Wodeyar III (1799-1868) to capture the Hindu knowledge of his time. The use of surya namaskar for physical exercise is also not modern. Bhagavat Simhaji on Page 61 in the book "A Short History of Aryan medical science" published in 1896 says "There are various kinds of physical exercise indoors and outdoors. But some of the Hindoos set aside a portion of their daily worship for making salutations to the Sun by prostrations. This method of adoration affords them so much muscular activity that it takes to some extent the place of physical exercise". Historically it is widely believed in the state of Maharashtra that Shivaji Maharaj, Sage Samarth Ramdas and the Marathas have performed surya namaskar as a physical exercise to develop able bodies. This is not surprising since 'vyayama' (Physicial exercise in Sanskrit) traditionally has been influenced by spirituality. Many physical practices have ingrained spiritual values in them. In addition spiritual training is considered as a part of physical training from ancient times in India." Source and further information: http://en.wikipedia.org/wiki/Surya_Namaskar_Origins 2) "Ashtanga Vinyasa Yoga is a system of yoga that has its origins in an ancient manuscript known as the Yoga Korunta, compiled by the sage Vamana Rishi. Its current form was developed at the Mysore Palace in Mysore India. and is commonly attributed to Sri K. Pattabhi Jois by way of his Satguru, Sri Tirumalai Krishnamacharya." "Although many practitioners assert that this yoga was devised by Jois from reading the Yoga Korunta, no one (aside from Krishnamacharya and Jois) has ever seen this text and Jois himself has occasionally dismissed the story as untrue. A far more likely explanation for Ashtanga's creation is that Jois was asked to devise a yoga sequence for children and adolescents, whom he had been asked to teach by his guru. Noticing that their attention spans were short, particularly for poses held for any length of time, and that introspection was not one of their strengths, Jois began to formulate a style of yoga that would cater to the youths' natural vigor and flexibility, while minimizing aspects they found tedious. And so he devised a new form of surya namaskara with athletic jumps and challenging push ups, and a series of poses -- none of which would be held for more than five breaths with the exception of shoulder and headstand -- that were visually exciting, and physically demanding. The poses were sequenced to be performed without interruption, and the sequences were designed with young, flexible bodies in mind" Source and further information: http://en.wikipedia.org/wiki/Ashtanga_Vinyasa_Yoga Further information: http://en.wikipedia.org/wiki/Sri_K._Pattabhi_Jois 3) "Surya Namaskar B Surya Namaskar Sun Salutation (Ashtanga B) Sun Salutation Surya Namaskar C Surya Namaskar Sun Salutation (Sivananda) Sun Salutation Surya Namaskar A Surya Namaskar Sun Salutation (Ashtanga A) Sun Salutation" Source and further information: http://home.comcast.net/~brucche/asanas/