Eclipses occur when: •  moon is on the line of nodes and •  phase is new (solar) or full (lunar) . torque of sun on moon’s orbit causes, the orbit to precess  line of nodes shifts •  points to sun twice per 346.6 days (eclipse yr)                                        (19 days less than year) moon’s synodic cycle and eclipse yr are commensurate (19 eclipse yrs  223 synodic months) = 18 yrs + 11.3 days (Saros cycle)  eclipses repeat every 18yrs + 11.3 days