The name Hatha Yoga goes back to the truth on which this system is founded. Our body is enlivened by positive and negative currents, and when these currents are in complete equilibrium, we enjoy perfect health. The positive current is designated by the letter “HA” which is equivalent in meaning to “SUN”. The negative current is called “THA” meaning “MOON”. The word YOGA has a double meaning. On the one hand is it equivalent to “joining” while the second meaning is “yoke”. Thus “HATHA YOGA” signifies the perfect knowledge of the two energies, the positive sun and negative moon energies, their joining in perfect harmony and complete equilibrium, and the ability to control their energies absolutely, that is, to bend them under the yoke of our “SELF”.
This wisdom is from the book YOGA and HEALTH by Selvarajan Yesudian and Elisabeth Haich, First published in 1953.