At the fall equinox, the Sun rises due east and sets due west. It continues on it's journey southward until, at the winter solstice, the Sun rises are far to the south as it ever does, and sets as far to the southwest. Many, if not most, prehistoric cultures tracked these rising and settings points with great detail. If they had jagged mountains along the horizon, the exact points could be readily remembered. Without a suitably interesting horizon, standing stones could be arranged to line up with the various rising and setting points.
Or, tree poles could replace the standing stones. Or, rock cairns could be used. How does this work? The dioramas simulate the rising and setting points of the Sun, and its tracks across the sky at summer solstice longest track , winter solstice shortest track , and the spring and fall equinoxes medium track. A bead placed on one of the tracks simulates the Sun rising along the eastern horizon, travelong along the sky, and setting on the western horizon. Imagine a tiny version of yourself standing in the middle of the wooden disk.
And imagine that the outside rim of the disk represents your horizon. Have you ever wondered why the direction of sunset changes throughout the year? We usually speak of the sun setting in the west, but technically it only sets due west at the spring and autumn equinoxes. For the rest of the year, the direction of sunset pivots about this westerly point, moving northerly in winter, and towards the south in summer. In the northern hemisphere, the sunset tends more northerly in summer and more southerly in winter.
But it must have taken years for ancient astronomers to notice the particulars of all these patterns, let alone explain them astronomically. One of the earliest records of such an observation may lie in the alignment of the Wurdi Youang stone circle in Victoria, on land traditionally owned by the Wathaurong people. But historians do know that ancient Greek-speaking people deduced an extraordinary amount of astronomical information from the way the sunset and sunrise directions change throughout the year.
Most of the early Greek astronomical manuscripts have been lost, but their content was developed and codified by Claudius Ptolemy, about CE, in a book known as Almagest. Its first printed appearance, in Venice in , was based on a mediaeval Latin translation of the Arabic adaptation of the original Greek — quite a multicultural achievement! The sunset direction reaches its northerly and southerly extremes at the solstices, while the noon altitudes are also at their extremes highest and lowest at the solstices.
The midpoints in both cases occur at the equinoxes. Thanks largely to Hipparchus of Nicaea — who worked about BCE, and to whom Ptolemy was indebted — astronomers had already figured out the geometrical configuration of the equinoxes. You can get the idea by visualising a third, more obvious plane — an extension of the one we appear to be standing on, bounded by the horizon with the sky sitting on top like a hemispherical dome.
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