In reality, the period between eclipses is 173 days and about eight hours, just a little less than the 177 days of six lunar months. The Maya may have been aware of the lesser number. The Dresden Codex contains a table which deals with the timing of lunar eclipses. It shows a column containing the number 177, occasionally interspersed with the number 148. The 148 equals the number of days in five lunar months. This seems to have been an effort to adjust the record for the six-month intervals between eclipses.
The rule for prediction may have been that, when a lunar eclipse occurs, the next one would occur on one of three days: (1) 177 days hence; (2) 325 days hence (177+148); (3) 354 days hence (177+177).
This technique had to fit the Sacred Round of 260 days. This was achieved through the overall length of the table, just under thirty-three years. Forty-six rounds equal 11,958 days. This is very close to 405 lunar months, which equal 11,960 days.