Observations on the rising or setting sun of the solstices
Some early calendrical monuments suggest that the equinox was fixed by observations on the rising or setting sun of the solstices (December 21st and June 21st), when the sun rises and sets at its most extreme positions towards the south and north repectively. In the figure, A and B are two poles placed in alignment with the setting sun of the winter solstice. The distance between A and C in line with the setting sun of the summer solstice is the same as the distance between A and B. Midway on its journey between the two extremes the sun rises and sets due east and west, and the lengths of day and night are equal. Hence these two days (March 21st and September 23rd) are called the equinoxes. In ancient ritual they were days of great importance. The east and west points on the horizon can be obtained by bisecting the angle BAC.
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