The equinoxes occur when the sun sets due west, and the days and nights are (virtually) of equal length everywhere on Earth. Which means that the ecliptic and equator must be in different planes, and must intersect at the equinoxes. So, in its yearly journey along the ecliptic, there are only two days when the sun crosses the equator. At the equator, however, the days and nights are always 12 hours long, so the spring and autumn equinoxes must happen when the sun is “on” the equator.
The equinoxes and the directions of sunset show why. Still, these astronomers had figured out that these two paths lay in two different, intersecting planes.
It must have seemed strange to the ancients that the sun had two different paths in its journey about the Earth. The second plane is that of the “ecliptic”, the sun’s apparent yearly path around the Earth. Today, following pioneers such as Nicolas Copernicus, we can imagine this more easily, because we know it’s not the sun that is moving but the Earth, which is rotating on its axis from west to east – that is, in the plane of the equator – producing the illusion that the sun rises and sets each day. This is the plane in which the sun appears to make its daily journey about Earth, from sunrise to sunset and on through the night until sunrise again. The first plane is that of the “celestial equator”, which is parallel to the plane of the Earth’s equator. 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. To do this, they used the concept of two imaginary planes cutting through a huge imaginary “celestial sphere” centred on the Earth. Thanks largely to Hipparchus of Nicaea – who worked about 150BCE, and to whom Ptolemy was indebted – astronomers had already figured out the geometrical configuration of the equinoxes. The midpoints in both cases occur at the equinoxes. 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. Its first printed appearance, in Venice in 1515, was based on a mediaeval Latin translation of the Arabic adaptation of the original Greek – quite a multicultural achievement!Īmong many other things Ptolemy was interested in was the fact that the symmetry in the arc of sunset directions is reflected in the symmetry between the sun’s midday altitude at the summer and winter solstices. It’s one of those ancient Greek classics that owe their survival to mediaeval Arabic scholars. Most of the early Greek astronomical manuscripts have been lost, but their content was developed and codified by Claudius Ptolemy, about 150CE, in a book known as Almagest. 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. We’ll never know just why, or even if, the builders of Wurdi Youang, Stonehenge, and other stone arrangements marked out the directions of sunrise or sunset at solstices and equinoxes. The circle’s age is unknown, but it could be as old as 11,000 years, and researchers – including former Monash academic Duane Hamacher – think it's likely that the circle includes deliberate markers of the direction of sunset at the solstices and equinoxes.
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. The Wurdi Youang stone circle near Little River, Victoria. But it must have taken years for ancient astronomers to notice the particulars of all these patterns, let alone explain them astronomically. If you’re an early riser, you’ll have noticed that the same symmetries apply to the easterly directions of sunrise. Technically the sun only sets due west at the spring and autumn equinoxes. (In the northern hemisphere, the sunset tends more northerly in summer and more southerly in winter.) 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. 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.
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