The ‘Celtic Calendar’ and the Solstices

If you visit www.archaeoastronomy.com between now and May 5 and look at their very cool graphic of the Earth’s orbit, you’ll notice that the festival of Beltane is on its way. Beltane is the origin of what are now the May Day celebrations and part of what is often referred to as the ‘Celtic Calendar’. This is the idea that the year can be divided into eight parts. Between them the solstices and the equinoxes divide the year into four parts and four additional mid-quarter days placed between a solstice and an equinox divide the year into eight. Controversially many people hold that the calendar isn’t merely Celtic, but has roots deep in the Stone Age which can be seen in the alignments of stone rows in the British Isles. The evidence is ambiguous but intriguing. Some of the events as clearly observable and some rely on abstract geometry.

Additionally some people claim that these days are shared by the world. This is absolutely true. So are days like July 14th, but July 14th is really only significant to the French. Were the days of this eight-fold calendar significant across the world? Certainly the easiest event to see is the solstice and there is evidence that this is something noted by cultures across the planet.

Solstice is derived from the Latin word Solstitium (sun stands still). It refers to the movement of the sun’s rising point on the horizon. Contrary to what you may have learned in school the Sun does not always rise due east. The tilt of the Earth means that the Sun’s apparent position is not constant. In the winter the observer is tilted away from the Sun, so it lies low in the sky. In the summer the observer is on the side of the planet angled towards the Sun, and it appears higher in the sky (see animation below).

This affects the where the Sun rises over the horizon. In the spring the Sun does rise to the east, but as spring becomes summer the point where the sun rises moves north. At the time of the summer solstice, usually June 21st, the Sun reaches its most extreme northerly position. After this is starts moving south along the horizon. In the autumn once again it rises due east. As autumn turns into winter the sun continues to rise further and further south, until the winter solstice, usually December 21st, when it lies low in the sky. This is the most southerly extreme. After this date the Sun’s rising point moves back towards the north until it’s Spring and the cycle starts again. The animation below shows this in action.

If you watch the animation again you may notice something else about this rising point. Not only does it move. It swings across the horizon like a pendulum. In spring and autumn it moves rapidly across the horizon. The position changes by about half a degree a day. This might not look a lot, but it’s about the same size as the Sun’s disc viewed from Earth. If you hold your hand out at arm’s length then your little fingernail is about half a degree across.

As the Sun approaches the solstices its movement on the horizon slows down. Around the solstice its position only changes by about 1/60th of a degree a day. This means that at the solstice the Sun looks like it’s rising over the same point as it did yesterday or will do tomorrow. If you have a notch in the horizon you might have a means for telling which day is the solstice but at somewhere like Stonehenge with fairly flat horizons? Not a chance.

This doesn’t mean the solstice is unimportant. The place on the horizon of the solstice sunrise could be extremely special because it marks a physical limit beyond which the sun may not go. The most famous place with a solsticial interest is Stonehenge, but there is also evidence of the solstices being marked on every continent on the planet – though not always in the same way.

Part Two of A Celtic Calendar – What is an Equinox? – follows on May 9
Part Three – The Midquarter Days – on May 16
Part Four – The Coligny Calendar- on May 23
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