Video
illustrated SCRIPT
To measure the length of a lunar orbit, one needs to see any prominent stars that the moon is passing by in her orbit.
This sort of procedure is implied in Indian astronomy, and other traditions, where the sky was first divided into 28 or 27 equal parts, called lunar mansions (nakshatras).
The lunar mansion on a given day was identified by the name of the lunar mansion seen behind it. In B.C. 2350, the system started with the spring equinox which stood on the seven sisters or Krittikas.
This was in the Zodiac constellation Taurus the Bull, but the mansion, one of 28, was called Ashwini – the Horse’s Head. In this way Indians still speak of “the moon in Ashwini”.
The division into 28 probably came from the observation of Saturn’s loops which resembled the twenty seven and a third days for a lunar orbit, though the first is a frequency of synods while the latter is a frequency of earth rotations, both relative to the ecliptic / Signs / Mansions.