Tuesday, June 21, 2011

Great time of year to erect a menhir

Now is a great time to begin tracking the sun's path where you live, if you haven't done so already. You can erect a gnomon or a menhir in a part of your property where the shadows from other objects (such as trees, hedges, fences, or your house) will not interfere with its shadow during the day.

A gnomon is the general term for the central "fin" or obelisk in a sundial, designed to cast a shadow which has a definite point that can be tracked across the ground or the sundial surface. The word is derived from the Greek word "to know" and is related to the word "gnosis."

A menhir is a more specific term for the unhewn standing stones found throughout the world (and especially in western Europe and the British Isles), some of them of great size. The word is derived from French by way of Breton, a language spoken in Brittany and related to Brythonic (a Celtic language). It is composed of two words, men meaning stone and hir meaning long.

The fact that most menhirs found around the world are unhewn or undressed stone is another clue in the discussion found in the post "Who were the ancient Celts and Druids?" in which allegations of a connection between the ancient Hebrews and Phoenicians and the ancient Celts are examined. The Hebrew Scriptures stipulate in Exodus 20:25 "And if thou shalt make me an altar of stone, thou shalt not build it of hewn stone: for if thou lift up thy tool upon it, thou hast polluted it." We also find in Genesis 28:18 "And Jacob rose up early in the morning, and took the stone that he had put for his pillows, and set it up for a pillar, and poured oil upon the top of it."

In any event, if you set up a gnomon or a menhir on your property (or in some open space near your home), you can observe the changing shadows cast by the sun throughout the year. This phenomenon is caused by the tilt of the earth's axis, which causes the path taken by the sun (the "ecliptic") to change its angle as the earth goes around the sun (for this reason the tilt of the earth's axis is also known as "the obliquity of the ecliptic").

This tilt of the earth's axis causes the sun to rise and set in the northern hemisphere from a point furthest north at the summer solstice (where earth is now), which makes sense if you think of a globe with an axial tilt and its north pole pointed most directly towards the sun. During the opposite solstice, when the earth has moved around its path such that its north pole is pointed most directly away from the sun, the sun's rising and setting will be furthest south in the northern hemisphere.

This phenomenon is depicted in the rough sketch above. The earth's surface (with a menhir or gnomon) is drawn as a rectangle, with the east edge and west edge labeled. The unlabeled north edge is of course the one to the left and the unlabeled south edge the one to the right. Along the east edge, where the sun rises each morning, the rising points of the summer solstice, winter solstice, and both equinoxes are marked with the letters SS (for summer solstice), E (for the point at which the sun rises on both equinoxes), and WS (for winter solstice).

Note that the ecliptic path traveled by the sun looks like a "tilted rainbow" or arch in this diagram. The arch is more tilted at winter solstice (that is, its apex is closer to the horizon) than at summer solstice (when its apex is further from the horizon). This also makes sense if you imagine again the globe with an axial tilt going around the sun.

The differences in the tilt of the sun's path will create a different shadow length throughout the year. If you think about it for a moment, you will realize that the sun traveling along the more upright arc traveled by the sun at summer solstice will cast a shadow that is much closer to the gnomon or menhir than will the sun when it is traveling along the more tilted arc of winter solstice, which will cast a much longer shadow.

The path of the shadow will actually make a tight arc around the gnomon or menhir on summer solstice, which will gradually uncurl into a straight line at the equinoxes, and then begin to curve outward as it begins to approach winter solstice. The three lines approximating shadow paths made on the solstices and the equinox are sketched into the drawing above*.

This pattern was discussed previously in the post entitled "What are cross-quarter days?" and again in greater detail in the post on "The Solar Double Spiral," which included a link to an excellent site by artist Charles Ross illustrating this principle (to see the animation of the shadow field, follow the link and then click the link for "Solar Pyramid and Shadow Field" and then click the link for "Shadow Field"). That post also explains how this curling and uncurling shadow pattern is related to the ancient double spiral that represented the sun's path from one solstice to the other throughout the year.

So, if you don't have a sundial or menhir, today is a great day to set one up and begin tracking the shadows through the year. It takes only a small bit of commitment to set up a good one (certainly much less commitment than it takes to get a solar double spiral tattooed across the bridge of your nose).

* Note that this pattern is that which is found in between the tropics and the arctic or antarctic circles in either hemisphere. North and south of the arctic and antarctic circles, respectively, the sun will not rise at all during the winter, and will not set during the summer. From the equator to the lines of the tropics north and south of the equator, the sun will actually cast a shadow on either side of the gnomon or menhir depending upon the time of year. This is an important piece of information to know if you are ever stranded on a desert island like Robinson Crusoe or Tom Hanks: observation of the shadow of a gnomon or menhir can help you determine north and south, and whether you are within the two tropics or outside of them.