Therefore, the synodic month takes 2.2 days longer than the sidereal month. Thus, about 13.37 sidereal months, but about 12.37 synodic months, occur in a Gregorian year. The calendar recognises two types of months&colon; synodic month and sidereal month. The calendar recognizes two types of months&colon; synodic month and sidereal month. The Babylonians expressed all periods in synodic months, probably because they used a lunisolar calendar. The synodic month is used to calculate eclipse cycles. The period between two new moons is a synodic month. This period of time corresponds to 152 synodic periods of Venus. In all, the difference in period between synodic and draconic month is nearly days. Also any calendar synchronized only to the synodic period of Venus would not be solar. However, the synodic and draconic months are incommensurate&colon; their ratio is not an integer number. Nineteen tropical years are as long as 235 synodic months (Metonic cycle). These respective events repeat themselves every eight years pursuant to the planet's synodic cycle. Page 2a of the Madrid codex is an almanac of the synodic cycle of Mars. It includes 111.5 synodic months, or 111 synodic months plus one fortnight.