A little-known branch of astrology, yet deeply rooted in religious mysticism is Celtic Tree Astrology. The Celts are a people who lived in the British Isles around 1,000 B.C. Their priests were known as Druids. It was the Druids who established the Celtic Zodiac and devised the complex system of Celtic Tree Astrology.

The Celtic understanding of the universe was symbolized by the form of a tree: deep roots in the ground; branches reaching toward the heavens; everlasting cycle of death and renewal. The Druids referred to the Sun as symbolic of a Supreme Being, recognizing that the Sun was the source of life for all beings. They were nature-watchers, and in their keen observations, unique characteristics and subtle nuances of each kind of tree became magnified for them. They believed these qualities were the spirits, or essences of being, that the Sun gave to the trees as a sacred gift.

The Druids noticed that the cycle of life demonstrated by the trees was in direct correlation with the season, and the seasons were marked by the Sun and the Moon above. The Moon made a series of steady changes that were repeated approximately every 28 days, making a total of 13 cycles for each complete cycle of the seasons. Thus, the Celtic calendar is made up of 13 months per year.

Because the Druids were such skilled observers of nature, their religion was based on the concept of natural and supernatural energies. Trees dominated their environment, so they believed these energies, or spirits, lived in the trees, thereby making the trees themselves sacred. Over time, the mystical qualities of a particular tree were associated with each lunar month.

The lunar calendar and the spirit of the tree assigned to each lunar cycle is what forms the basis of Celtic Tree Astrology. Other factors are then figure into this framework: a Celtic God, a Guardian animal, a letter from the sacred alphabet, and sometimes a gemstone or other special piece of nature. Finally, the Druids started to see that human characteristics also fit into this framework. They noticed that the patterns in a person's life moved in accordance to the motions of the Moon and that these patters varied depending on the season in which a person was born. Herein lies the birth of Celtic Tree Astrology: a predictable system that methodically gives reason to human behavior and places the human spirit in a place of significance within the universe.