Note: All this information is based on inference, and off-hand comments.
Astrology is predicated on the belief that celestial objects (such as stars, moons, etc.) can have a profound effect on Aravorn. Those who study such movements, and occurrences may influence matters on a more earthly scale.
For example, the presence of a shooting star during the birth of a child is a prediction that such a child will have a profound impact on the world. (source: start of Spirited Heart)