Celtic Astronomy (a little more archaeoastronomy)

Until the late 1800s, little was known about the ancient Celts' knowledge of or interest in astronomy because archaeologists just couldn't find any artifacts to indicate it.  For centuries, most of the knowledge held by the Celtic peoples was concentrated in a small group of wise people they called Draoi, or Druids, and they passed their knowledge from generation to generation orally.  In 1897, archaeologists found a calendar that dated back to the Gauls in modern-day France.  This tablet, called the Coligny Calendar, was the first evidence of ancient Celtic interest in astronomy.

Taken from the Coligny Calendar page on Wikipedia

Measuring about 1.5 x 1 m, the calendar is a lunisolar calendar thought by anthropologists to be a rebellion against the implementation of the Julian calendar in the 2$^{_nd}$ century AD.  The Gauls measured time starting with night, so night came before day and dark seasons(what we call winter) came before light seasons (summer).  

The ancient Gauls had no astronomical tools to observe the sky.  They made their calculations based on years--even generations--of naked-eye observations.  And (as far as we know) they didn't write any of it down!  Druids used the modern statistical method of Stochastic Estimation to make their predictions, and though their work wasn't as accurate or as far-reaching as the famous Mayan Calendar, it was accurate enough.  

They were able to discern that the phases of the moon were periodic, and even figured out the geocentric relationship between the Earth and the moon.  Their work in predicting eclipses was particularly accurate, discerning an 18-year-long cyclical pattern, which they called the Saros Cylcle.  

By pinpointing different spots in the sky and keeping track of how long it took the moon to reach those spots time after time, the Druids developed an intricate system of cycles.  When combined, the different cycles allowed the Druids to accurately track time, including corrections similar to our leap year to make up for the difference between solar and sidereal times.  

Based on the evidence provided by the Coligny Calendar that the ancient Celts were interested in studying the moon, archaeoastronomers are convinced that they also would have studied the stars.  Now it's just a matter of finding the artifacts to prove it.