Computus week before the Gregorian Easter in 1724, 1744, 1778, 1798, etc. The Easter cycle groups days into lunar months, which are either 29 or 30 days long. There is an exception. The month ending in March normally has thirty days, but if 29 February of a leap year falls within it, it contains 31. As these groups are based on the lunar cycle, over the long term the average month in the lunar calendar is a very good approximation of the synodic month, which is days long. There are 12 synodic months in a lunar year, totaling either 354 or
Lunar month days. The Moon completes one orbit around Earth every 27.3 days (a sidereal month), but due to Earth's orbital motion around the Sun, the Moon does not yet finish a synodic cycle until it has reached the point in its orbit where the Sun is in the same relative position. This table lists the average lengths of five types of astronomical lunar month. These are not constant, so a first-order (linear) approximation of the secular change is provided: Valid for the epoch J2000.0 (1 January 2000 12:00 TT): "Note:" In this table, time is expressed in Ephemeris Time (more precisely