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
à
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)
à eclipses repeat every 18yrs +
11.3 days