I suspect that the stars and sun and moon were believed to be either deities or to have sacred properties, to the ancients. Formal observations of the stellar bodies were just beginning to occur and, no doubt, there were magical ceremonies to invoke the 'power in the planets' in mundane life. (Wishing for luck). If the festivities, sacrifices, ceremonies took place when the sun rose on the longest day, for instance, and the sun rose behind a small hill on the horizon, for instance, then that alignment would have been associated in the mind of the community with good luck, good harvest, whatever. If the idea of using the alignments in magic rituals is possible then you'd expect monuments to incorporate as many alignments as possible. It's a similar concept to a church being consecrated for a particular saint and then windows, and other bits, dedicated to other saints.