'nomos' in greek means Law, and 'astro' means Star so literally you have astronomy = Star Law. on the other hand, 'logy' means Study, so astrology means Star Study. Astrology was considered during the 17th century and before, to be the practical application and art of astronomy which provided the laws. It wasn't until quite recently that astrology became identified with fortune telling and such things, exclusively, and the scientific study was relegated to astronomy. Properly speaking, Copernicus, Kepler, Tycho Brahe and Galileo were astrologers.