The Moon has been important to many prehistoric societies, and in particular the extremes of moonrise and moonset. The rising and setting of the moon follows a complex pattern which repeats ever 18.6 years. The Station Stones are aligned in the direction of the northernmost moonset at the 'major standstill' and also the southernmost moonrise at the major standstill.

At the latitude of Stonehenge, this axis crosses the midwinter sunset/midsummer sunrise axis at right angles. This relationship is only possible at around the latitude of Stonehenge  it may well have been significant in the siting of the circle. 
For the archaeoastronomists, the Aubrey Holes served as fixed reference points along a circle, and their number was essential to astronomical calculations. The cycle of the moon, for example, which takes 27.3 days, can be tracked by moving a marker by two holes each day to complete a circuit in 28 days.
A much longer calculation is to move the marker by three holes per year to complete a full circuit in 18.67 years. In this way, it is argued, it would be possible to keep track of the nodes, points where the paths of the sun and the moon apparently intersect to produce an eclipse. Because the moon slews around in its path, the two nodes move along the path of the sun, a complete circuit of which takes 18.61 years. By means of the markers in the Aubrey Holes and keeping track of the directions of the sun and the moon, the astronomer at Stonehenge could calculate nodal points ahead of time and thus predict both solar and lunar eclipses.
Plan of Stonehenge with the Aubrey
holes, the Heel Stone, and the Station
Stones 91, 92, 93, 94 marked