Their priests were called Druids. It was the Druids who established the Celtic Zodiac and invented the complex method of Celtic Tree Astrology.
The Celtic comprehension of the world was represented by the kind of a tree: deep roots in the earth; branches reaching toward the skies; eternal cycle of death and renewal. The Druids called the Sun as emblematic 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 attributes and subtle nuances of every sort of tree became magnified to them. They thought these qualities were the spirits, or essences of being, that the Sun gave into the trees as a sacred gift.
The Moon made a string of steady changes which were repeated approximately every 28 days, making a total of 13 cycles for each complete cycle of the seasons. Therefore, the Celtic calendar is composed of 13 months annually.
Since the Druids were such skilled observers of nature, their faith was based on the idea of supernatural and natural energies. Trees dominated their surroundings, so they thought these energies, or spirits, lived in the trees, thereby making the trees themselves sacred. With time, the mysterious qualities of a specific tree were correlated with each lunar month.
The lunar calendar and also the soul of the tree delegated to every lunar cycle is what forms the cornerstone of Celtic Tree Astrology. Other variables are then figure within this framework: a Celtic God, a Guardian creature, a letter from the sacred alphabet, and at times a gemstone or other special slice of nature. Lastly, the Druids began to find that human traits also fit into this frame. They found that the patterns in an individual’s life moved in accordance with the movements of the Moon and that these patters diverse depending on the season where a person was born. Herein lies the arrival of Celtic Tree Astrology: a predictable system that systematically gives reason to human behaviour and places the human soul in an area of significance within the world.