An eclipse in a semester series of solar eclipses repeats approximately every 177 days and 4 several hours (a semester) at alternating nodes of the Moon's orbit. Littmann, Espenak, Willcox: Totality: Eclipses of the Sun. Jubier, Xavier Espenak, Fred. fifteen (4): 431-443. PMC 3898681. PMID 24459410.