# The Saros — the same eclipse returns to the same node (~18 yr 11 d)
model.geocentric
reset
show.bodies
show.moon
show.moonNodes
show.eclipticBelt
show.constellations
show.sunPointer
jump.eclipse
overlay.show {date} · Saros {eclipse.saros} · {moon.node}
overlay.show One eclipse. Now step forward by 18 years and 11 days, again and again, and watch what stays fixed
wait.next Step through the series

repeat 8
  wait 2
  jump.saros
  overlay.clear
  overlay.show {date} · Saros {eclipse.saros} · {moon.node}
end
overlay.clear
overlay.show Same Saros number, same node, eclipse after eclipse
wait.next Why does that work?

overlay.clear
overlay.show 223 synodic months and 242 nodal months come out very nearly equal — 18 years, 11 days
overlay.show After that interval Sun, Moon and node are all back in the same arrangement, so the eclipse repeats
overlay.show A cycle like this lets eclipses be predicted from records alone, before any theory of why
wait.next Done

overlay.clear