The Pleiades.

3600 years ago the Pleiades disappeared in the dusk 12 days before the onset of Spring. If ideally, the first crescent moon appears by the Pleiades on the last day of their visibility, then the full moon and the start of Spring and with it, coincide.

From Wolfhard Schlosser's theory.

A Mesopotamian text named from the opening words, the MUL.APIN  originally composed in 1000 BC from older texts, and used for at least 500 years after that gives us this rule:

If the first crescent of the new moon appears by the Pleiades at the spring equinox (21st March) then this is a normal year.If, however, during this month the Moon does not appear in the Pleiades until the third day, this is a sign that a leap month must be added.

The Nebra sky disc is dated to 1600 BC.

This diagram shows where the  Pleiades were in relation to the sun at sun rise and sun set in the year 1610 BC.

After the spring equinox the Pleiades can be seen in the west, in the evening sky as the sun sets. The Pleiades become day time stars after the spring equinox and invisible, until the autumn equinox.

After the autumn equinox, the Pleiades become night time stars, rising in the east at sun set.

A month always begins with the sighting of a new moon at sun-set.

At the spring equinox, the angular distance between the Pleiades and the sun is getting smaller each day and the sun will soon drown out the light from the Pleiades making them invisible.

The moon measures months, and there are more lunar months in the year than solar months. The Pleiades act as a marker for the equinox, and the phase of the moon mark out the months.