Is the Enneagram similar to astrology?

Not really. Although there is likely some overlap in the types of people who are interested in both personality models and astrology, and (broadly) they’re both tools to examine and explain people, the two are quite different. Astrology uses the movements and positions of stars and planets to explain human affairs and terrestrial events. The Enneagram is a psychology-based personality system that clusters people into “types” that share common motivations and behaviors. Its closest comparisons are Carl Jung’s theory of psychological types and the Myers Briggs Type Indicator (MBTI).