The meridian was a very understandable derivative from what was probably the most ancient astronomical instrument of all , the gnomon , used for measuring the different lengths of the sun 's shadow .
Looking south , the observer measured the shadow at noon ; looking north during the night , he measured the times at which the various circumpolar made their upper and lower transits across the meridian .
Their astronomical system was thus closely associated with the concept of the meridian ( the great circle of the celestial sphere passing through the pole star and the observer 's zenith ) , and they determined systematically the culminations and lower transits ( meridian passages ) of these circumpolar stars .
