The word hatha derives from the two seed or bija mantras "ham" meaning sun and "tham" meaning moon. Like male/female, hard/soft or exhale/inhale, the sun and moon are opposite qualities, one defining the other as an interdependent aspect of a whole, bound in exquisite tension until the moment they reunite.

On the macrocosmic level, the celestial sun and moon divide time into day and night and only meet during the twilight known as sandhya (gap), or the mystic hour of meditation. On the microcosmic level, the human biological system is programmed to the cycles of the sun and moon. This affects the body/mind complex, pulling the sympathetic and parasympathetic nervous systems, and the conscious and subconscious awareness from one pole to the other, tying us to the duality of rigmarole.

Advancement of yoga may be measured by equanimity in any given situation. In hatha yoga, this state of balance is achieved by resolving the vacillation of energy between the inner sun and moon. These two opposing forces, prana shakti and manas shakti, universally known as the force of life and the force of consciousness, flow like an electric current in the human body through the psychic or astral circuitry of 72,000 or more channels called nadis. Of the three main nadis that run along the spinal cord from the base to the crown, two correspond to the sun and moon: pingala, the solar, right channel and ida the lunar, left channel. The third and central channel is sushumna the ?Royal Road? of spiritual awakening, which when connected, acts as the cable for the supra voltage force of Kundalini shakti (primal prana). The solar, lunar and central channels intersect at the seven chakras, vortices of energy that act as electrical transformers to regulate the prana throughout the nadi network.

When the right nostril is more open and freely flowing, the left hemisphere of the brain is engaged and the body is restless (waking state). When the left nostril is dominant, the right side of the brain is engaged and the mind is overactive (sleeping state). The two streams, pingala and ida, merge in the river of sushumna through the various practices that include pranayama (cultivating life force through breath), kriyas (pranic/energetic techniques), asana like surya and chandra namasakar (the sun and moon salutations) and meditation. If the pingala is the positive charge and ida the negative, then sushumna is the earth. When all three flow simultaneously and the current of Kundalini ultimately flows to the highest center in the crown, that human being attains the fruit of yoga and exists in the perfectly balanced state of enlightenment.

Hatha yoga prepares the practitioner for the tremendous responsibilities of functioning fully switched on. While the Buddhist, Jain and Patanjali raja yoga systems stress the yama (moral codes) and niyama (self-restraints) as the preliminary underpinning of spiritual work, hatha yoga and her sister science of wellbeing, ayurveda prepare the way by first purifying the subtle and gross systems of the body complex whereby a virtuous attitude of mind is a natural outcome of their lifestyle.

Paramahamsa Satyananda Saraswati wrote, "Hatha Yoga is practiced in order to initiate a process in this physical body, whereby the pranic currents and the mental forces which interact with each other in the scheme of life and existence may be transformed. Unless the physical molecules are transformed, it is no use discussing compassion or unity." The hatha yogi undertakes their spiritual journey from within the body, clearing the multitude of internal passages for the unobstructed rise of vital life force (Shakti) as it seeks union with consciousness (Shiva). By stabilizing the body as a touchstone, meditation develops safely into pratyahara (withdrawal of senses), dharana (pointed concentration), dhyana (deep meditation) and samadhi (ecstatic merging with the object of meditation).

In the experiential tradition of tantra, the human being is an instrument of union between heaven and earth. Thousands of years ago, the yoga of sun and moon was developed to attune the human with the divine from the inside out.