Reiche theorized that the builders of the lines used them as a sun calendar and an observatory for astronomical cycles.

The great Egyptian river has followed clear rhythmic variations that approximate certain astronomical cycles.

The main astronomical cycles used to construct the calendar were the synodic month, equinox year, and sideral day.

Generating Economic Cycles traces the rainfall cycle back to an astronomical cycle.

The cause of glaciation may be related to several simultaneously occurring factors, such as astronomical cycles, atmospheric composition, plate tectonics, and ocean currents.

A problem with the theory is that the astronomical cycles have been in existence for billions of years, but glaciation is a rare occurrence.

The original purpose of such a calendar, with no obvious relation to any astronomical or geophysical cycle, is not securely known, but there are several theories.

A drop in heat received from the sun below some critical threshold, caused by those astronomical cycles, triggers a switch from warm to coldish.

But when the sun's heat finally builds up again to another threshold value, thanks once more to the three astronomical cycles, then the climate switches back to warm.

To reconcile the calendar year with an astronomical cycle (which could not possibly be reckoned in a whole number of days), certain years contain extra days.