Equatorial Equatorial Sundial

Human Sundial

Original Image

At the equator, the line of stones would run directly East / West and you would be unable to adjust for the person’s height by having them step along the path. So making this kind of sundial at the equator has a unique set of problems. However, you could have them stand in the middle of a wheel, arms extended N / S, and make an equatorial sundial… A Human Equatorial Equatorial Sundial ;-)

I was contemplating the steps needed to make a Henge. Something I do from time to time as I’d like to have a Henge of my own someday… and (sung to the tune of “Everybody ought to have a slave” from A Funny Thing Happened On The Way To The Forum ):

“Everybody ought to have a Henge. Everybody ought to have a work-ing dial!”…

I’d started listing all the uses, how to get them from a Henge, and working out the most likely steps and stages along the way. One of the uses is as a calendar. Another is as a time standard (we’ve looked at that briefly in earlier postings). That got me thinking about alignments and clocks. Was there a ‘clock’ function at Stonehenge? I’d proposed using a star transit timing as the first step of making a length standard and as part of laying out a Henge; but might there be a more formal ‘clock’?

At which time I remembered the thousand and one ways to make a sundial and realized that just about any vertical pole could have been a sundial (and horizontal poles too…).

Oh, and I’ve been thinking about making a Henge at the North Pole and at The Equator and what would that do to various aspects. Basically, how much does a Henge change as you move from Equator to Pole. That got me thinking about a sundial at the Equator.

OK your typical sundial has a center line that points at the pole star. This aligns it with the axis of rotation of the earth and you get a nice shadow line for marking time. (There’s a lot more than that that goes into sundials and some are truly magnificent with built in allowance for all sorts of things, some seasonal and some longer… but that’s out of this scope.) But at the equator, that line would be perpendicular to the earths surface. If placed on the top of a pole, it would make a T shape (or look like a crucifix if placed part way up the pole) with the top aligned North South.

At that point, you would just mark the ground with 5 divisions each side on an east west line and have your sun dial (each being 15 degrees of arc about the top of the T. You don’t need a 6 am or 6 pm line as they are an infinite distance away floating over the horizon ;-)

As the seasons change, the sun is straight overhead during the equinox and slightly north or south during the solstices. So you could mark lines (about 3.9 degrees each N / S or 23.4 / 6 ~= 3.9 ) and put month names next to them. As the months progress, the shadow would move north or south and align with a month marker. For each month, you mark the arc the shadow traces on the ground and then mark the hours on it. Now you have a sundial that tells you both month and time. (If you wanted to get really fancy and were precise enough you could even mark weeks or days, but that precision is hard to come by in outdoor constructions).

This device is basically an “Equatorial” sundial, but built at the equator (thus the Equatorial Equatorial posting name ;-) as that puts the ‘axis’ parallel with the earth axis as the cross beam of the T or crucifix. For an in depth look at equatorial sundials, this page has directions on how to make one. They often take the form of a large wheel with a pole through the axle pointed at the pole star and markings on the wheel for time. Like this one:

Equatorial Sundial in Beijing

Original Image

At the equator, the ‘axel rod’ would be parallel to the earth surface and pointing north south. You could use a wheel for the markings, or just have a pole hold up the rod and mark the ground as described above.

But at that point, you don’t really need the cross beam and can simply leave it out. The end of the vertical pole alone being enough to mark month and hour… At that point, you have become a different specific kind of sundial, the vertical pole analemmatic sundial rather like the one here that I usually think of as the antithesis of the equatorial sundial. Odd, that at the equator one transitions into the other…

Not surprisingly, there is a North American Sundial Society for folks really into this kind of thing and all the ways to tell time by sundials. (There are also various ways to do the same at night, but that’s for another posting…)

http://sundials.org/

and a wiki on sundials:

http://en.wikipedia.org/wiki/Sundial

So at this point I’ve come to realize that in my quest for a Henge at the equator, the very first step is to put in a flag pole or crucifix and some paving stones or selected flowers arround it. “Half past the petunias on the granite month line” has a certain charm to it ;-)

Then I could properly schedule all the work on the Henge construction project and know when it was time to break for lunch. It will also give you the equinox and solstice days (as maximum and minimum N/S excursions of the shadow) for your initial star sightings for Henge alignment.

With proper placement, this pole can also serve as either the first ‘central sighting pole’ or the ‘solstice sighting pole’ of the first step of Henge layout. Either at the exact center of your future Henge, or on the perimeter of the outer ring at the end of the Solstice Line from the center toward the horizon on the day of the Summer Solstice.

So where my ‘first step’ had been “get two poles and some rope”, instead it’s become “get two poles and some rope, and some stones. Make one pole into a sundial at the center of the future Henge.”

Odd that the first step of creating a Henge ends up at time keeping, project management and the metrology to do it. Or maybe not so odd. It would be a big project.

In an earlier posting on the English Foot we saw one of the ‘next first steps’ is to mark a 15 degree angle on a circle. I also find it interesting that the Equatorial Sundial uses 15 degree arcs for each hour. In the case of the sundial, we align the plane of the circle to the plain of the ecliptic and use it to mark hours. In the case of the Henge, we align the plane of the circle to the earth, then mark the transit of a star from pole to pole and use that to make a time standard, and from it a length standard (the Rod and the Megalithic Yard).

This, now, makes me wonder if perhaps the first step ought to be making an equatorial sundial and using it to make a standard of time, and from that a standard of length. Basically, the length standard can be made more precise with a larger “star dial”, but does not actually need one and could be made with a sundial instead. Each is timing the rotation of the earth. Each is then calibrating a pendulum from that to make the length standard.

So we could use a transit of the sun, moon, or stars, as we like it, and as we need more or less precision. I’m going to have to think a bit about exactly when is the best time of the year to use a sundial to make a Rod unit of length. Or perhaps look at the rod and figure out what time the ancients selected. Basically, their length standard ought to tell us when and what time standard was used to create it within the known margin of error. Did they know the date with the most precise hour? And how much does it vary from place to place and season to season? How much did they know about The Equation Of Time? And is that reflected in their unit of length? I note that the equation of time has a ‘near zero’ at June 13th, and that’s rather close to the Summer Solstice in the northern hemisphere…

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