It takes many occultation observations of many stars by many observers to accurately determine the orbit and true motion of the moon. From observations like these astronomers found out that the moon kept a different time than the earth. Earthly time is based on the rotation of the earth. But, it turned out that the earth is slowing down. So time based on that changing rate cannot be used for time based position calculations of the sun, moon and planets.
Atomic clocks are also immune to the earth's slowing rotation.
Now, atomic time has become the basis for earthly time. So to keep our clocks a tuned to the sun via the earth's rotation, an occasional leap second is added at approximate 18 month intervals. The actual rate of slowing is irregular and can only be accurately determined by observation. When a star is occulted by the airless moon it simply winks out, since a star's actual diameter is so tiny. The moon occults several dozen bright stars a month, and since the moon's orbit processes or wobbles, it cuts a wide swath of sky over time.