What is an Equinox?

Celtic Calendar logoIn 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).

[FLASH]http://archaeoastronomy.co.uk/wp-content/images/horizons.swf, 500, 375[/FLASH]
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.

The serpent descending the pyramid at Chichen Itza
Photo under a CC licence by Fanny

See also http://​www​.loco​gringo​.com/​p​a​s​t​_​s​p​o​t​l​i​g​h​t​s​/​a​p​r​2​0​0​2​.​h​tml for a view includ­ing the head.

Part One of A Celtic Calendar was — The ‘Celtic Calendar’ and the Solstices — from May 2
Part Three — The Midquarter Days — fol­lows on May 16
Part Four — The Coligny Calendar- on May 23


When he's not tired, ill or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.

3 Responses

  1. tyshay says:

    This web­site needs to put more things about the equi­nox on here because i couldn’t find any­thing about the equinox!!!

  2. Alun says:

    It cer­tainly needs a re-write, not least because the anim­a­tions have broken since mov­ing to WordPress​.com. I’ll give it some thought in the New Year.

  1. June 20, 2006

    What is an Equinox?

    […] There seem to be two con­nec­ted issues here. One is “Is a Native American site worth spend­ing $600K to pre­serve?” I don’t know. I haven’t seen the site. If it’s a couple of rocks then maybe not, but if it’s a hitherto unre­cog­nised major site then the people of Sharon may have got them­selves a bar­gain. Slightly more press­ing is whether or not the site is what it’s claimed to be. I get wary when equi­noc­tial alig­ments are claimed for sites. If the people are fully informed, and choose to spend the money any­way, then it’s their money. But if this is a fantasy then it’s a fantasy that leaves $600K + interest for future gen­er­a­tions to pick up. […]