after 30 SOLAR years, add one LUNAR month of each type, respectively, to bring the SUN & MOON back into alignment
so,
30 SOLAR = 10,957 days.
SIDEREAL = 400 months is 10,928 days. add another month of 27.32 days
SYNODIC = 370 months is 10,926 days. add another month of 29.53 days
APOLLO visited the HYPERBOREANS every 19 years :)
a "myth" linked to the very scientific observation of the astronomical eclipse cycles
and only TWICE will you have had to intercalate over 60 years, as opposed to the hebrew (7 in 19), or gregorian (1 in 4)
SYNODIC
29.53 x 12 = 354
29.53 x 12.369 = 365.2
SIDEREAL
27.32 x 13 = 355
27.32 x 13.369 = 365.2
13.369 / 12.369 = 1.080