moon orbits earth. it's viewable somewhere in the world every 28 days. if the place where it's viewed were by the sea, then the tidal patterns of water - high tide and low tide - could be correlated to the moon being seen or otherwise. If the moon is is visible, it presumably gives one gravitational pull on the water. If the moon is not visible, then it presumably gives a less gravitational pull. Maybe "spring" suggests a high high tide (as in growth ?) and "neap" suggests a low high tide ???
