Rhythmic minima and maxima of tides caused by three gravitational phenomena: perigee of the moon (i.e. when it is nearest to the earth; every 411 days), the perigee of the earth (when it is nearest to the sun), and the alignment
of the earth, moon and sun that causes a lunar
eclipse. These three events
are synchronous approximately once every 1.8 kya
; it has been suggested that this synchronicity could cause powerful tides that may cause deep
cooling in the Atlantic, and trigger
global cooling, including the Little Ice Age
500 years ago.