
"The word Yoga is derived from the Sanskrit language and means 'to unite' or 'to harmonize.' In other words, Yoga means working toward a level where the activities of the mind function together harmoniously creating balance in our lives.
In the compound word Hatha, 'Ha' means sun and 'tha' means moon. This refers to the balance of male and female, day and night, positive and negative, yin and yang, hot and cold, or any opposites, yet balancing pairs.
When this concept is applied, it could refer to bringing two sides of the body into a state of balance, through the development of equal strength and flexibility on both sides. It could also refer to bringing the two hemisphers of the brain into balanced functioning, so that the logical, mathematical side and the creative, intuitive side are encouraged to function harmoniously and equally efficiently.
In Yoga, the flow of the breath through th right nostril is called the sun breath; the flow through the left nostril is called the moon breath. Breath regulation (pranayama) in some form is central to all hatha Yoga practice."
From the book The Yoga Handbook by Noa Belling