"Cosmic Orbit (zhou tian)" ( 周天 zhoutian ) is a term of Inner Alchemy ( 內丹 neidan ). In view of traditional philology, the word "zhou" has multiple meanings, and an important meaning of it is "moving in cycles". This meaning originates from astronomy. Through observing astronomical phenomena, ancient people learnt many laws of the motion of celestial bodies, such as that of the celestial way, the terrestrial way, the middle way and the ecliptic. The "celestial way" refers to the way in which the sun, the moon and stars in heaven move; the "terrestrial way" refers to the law of terrestrial generation and transformation in accordance with the heavenly law; the "middle way" refers to the way in which the Yang breath of heaven and the Yin breath of earth communicate; the "ecliptic" refers to the envisaged track of the sun’s annual movement in the celestial sphere. When we observe the sun on the earth, we find that the sun does not move along a straight line, but between the Tropic of Capricorn and the Tropic of Cancer, i.e., eight degrees to the either side of the ecliptic in the celestial sphere. This zone is the "inhabiting belt" of the sun, the moon, the five stars and the twenty-eight constellations, which is generally called "zodiac". The track of the ecliptic should be understood not only spatially, but also temporally. Getting inspirations from the movement of celestial bodies and summarizing their practice of Inner Alchemy, Daoists have discovered the "Cosmic Orbit" laws of the movement of the Vital Breath in the human body. According to the trigram principles in the Book of Changes ( 周易 zhouyi ), the numeral representing the Cosmic Orbit ( 周天火候 zhoutian huohou ) is actually the numeral representing the intercourse of Yin and Yang. For the Kun trigram, taking six as the base and multiplying it by six, we get thirty-six, the perfected Yin number; and for the Qian trigram, taking nine as the base and multiplying it by six, we get fifty-four, the perfected Yang number. Six plus nine makes fifteen, the "number of the one-breath" in the context of the Fire Phases of the Cosmic Orbit in Inner Alchemy. According to the principle of "following the laws of heaven and earth", Daoist Inner Alchemists believe that the human body also has the numbers of heaven and earth, Qian and Kun. The circulation of one-breath forms the Microcosmic Orbit ( 小周天 xiaozhoutian ), and if the practitioner Increases Fire ( 進火 jinhuo ) at the proper time, and the breath moves with the passage of time to achieve 360 degrees, then the perfected number of Qian-Kun is reached and the Macrocosmic Orbit is formed. The "Fire Phases of the Cosmic Orbit" is a symbolic description; its central idea is the regularity of the movement of the Vital Breath in human body. Cultivation of Inner Alchemy must follow the laws of the movement of heaven and earth instead of acting blindly.