Complex sentence: So Kidinnu developed from Nabu-rimanni's System A. A more refined system (now called System B) was used by the Chaldean astronomers to describe more clearly the motions of the Sun and planets.
Split into simpler sentences: So Kidinnu developed from Nabu-rimanni's System. A the more refined system (now called System B) used by the Chaldean astronomers to describe more clearly the motions of the Sun and planets.