In theory an equinox should be easy to define. It’s the point halfway between the solstices. However, in what way do we mean halfway? Halfway in space or halfway in time? Or something completely different?
You would think that marking the equinox would be easy. You simply mark on the horizon where the solstices lie and then point your megalithic row at the midpoint. Which works perfectly if you have a flat horizon, but in many places the horizon is not perfectly flat. A higher horizon has the effect of moving the sunrise points to the south (or if you’re in the southern hemisphere to the north).
The effect of a higher horizon on azimuth positions
You will also find that while the sun is rising half-way between one extreme and the other, the year is divided into unequal parts. This is because the Earth’s orbit isn’t circular, it’s elliptical. The Earth moves faster round the Sun in January when it’s closer to the Sun and slower in July when it’s further away. If you want the equinoxes to help divide the year into four equal parts then you’ll need to mark the equinox on a different day.
The name Equinox comes from the Latin meaning equal night. If you want the Equinoxes to be the days that the length of the night is equal to the length of the day then you have to remember it gets quite light before sunrise. Another definition would be when the Sun rises and sets on the exact opposite side of the sky, or when Sun rises due east and sets due west, but both of the events rely on there being a flat horizon.
The usual definition used by astronomers is that the Equinoxes are the days when the Sun crosses the Celestial Equator. This is the midpoint between the two extremes the Sun can occupy in the sky, which is not necessarily the same as the difference between the two extremes on the horizon. This definition is derived from Greek astronomy, which is fine if you’re talking about Greeks, but more problematic if you’re talking about peoples who didn’t use Greek-based astronomy. There are many claimed Equinoctially significant sites across the world, but the sheer number of definitions can make it harder to be certain that an alignment is deliberate.
The most striking equinoctial event is the descent of the serpent at Chichen Itza. The pyramid of Quetzalcoatl is aligned to so that the rising Sun casts shadows and light across one side of the pyramid down the steps. The Sun also lights the head of the serpent, so it looks like a serpent of light has descended the pyramid.
Photo under a CC licence by Fanny
See also http://www.locogringo.com/past_spotlights/apr2002.html for a view including the head.