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This may sound stupid. but could it all b down 2 farming perhaps?

Sut mae fella, i think youve hit the nail on the head there, either directly or indirectly many of these alignments have everything to do with farming, solar and lunar cycles seem to have been critical. Marking the solstices seems to have taken on a ritual significance for many cultures.
it may be that in zones near the equator where more harvests were possible per year, and the difference between summer and winter were less marked, that these alignments may have been in some places less ritually important, and other factors like cardinal and stellar alignments had more cultural significance. even here farming played a crucial role, linked strongly with mythologies and priesthoods. It is probable that in many cases the actual building work was carried out by farmers who had free time while their crops grew, or were not needed to tend the land. Like in egypt for example, it is believed that the monumental works like the pyramids etc, were carried out by farmers who inherited a second trade from their fathers, and in times when the nile was flooded participated in national building projects.
from what ive read hunter gatherer cultures found lunar cycles more important, and in cultures that farmed solar cycles became dominant in ritual. Sometimes it was the rising and setting of stars that was used as markers, but in most cases (dont want to say all because there's usually an exception) it is the planting and/or harvest times that were being regulated.
.....i think.