They determined correctly within a small fraction the length of the synodic revolution of the moon.
Why, because twenty-seven days is about the synodic period of solar rotation.
The correlation with the synodic rotation of the Sun was practically perfect.
Lunar Year, a period of 12 synodic lunar months, being about 354.5 days.
Relating to the conjunction of celestial bodies, especially to the interval between two successive conjunctions of a planet or the Moon with the Sun as viewed from Earth. For example, the new moon comes at the conjunction of the Moon with the Sun; the interval between successive new moons (the time it takes for the Moon to orbit the Earth and return to conjunction with the Sun) is the Moon's synodic period, also called a lunar month. Synodic time differs from sidereal time, which is measured in relation to the stars and is generally more appropriate to astronomical observation.