John Carmichael wrote: > **** Does this mean that there is no upper limit for the size of a > sundial? *****
Seems obvious to me. The limitation in most configurations is the fuzziness of the shadow, which also implies that size doesn't improve precision. > If this is true, then one second time line markings could be placed on the > dial face, couldn't they? I haven't done the math, but if the one second > lines at high noon ,when they are closest, were spaced at an easy to read > distance of about a 1/2 inch apart on a giant horizontal > sundial, then the > height of the style and the diameter of the face could be determined. It > would be a large sundial indeed! The Earth rotates at 360 degrees/day = 0.073 mrad/sec. Divide this into 1 cm/sec and you get the scale of the sundial, 140 m. Monumental, but doable. > It has long been my dream to design and construct such a sundial, > maybe not > with one second markings, but with 30, 20, or 10 second time lines. (What > are the time divisions on the large sundial in Japur India, does anyone > know?) I'd like to use the same basic design that I use for my horizontal > string sundials (see website). The sundial face could be located > in a park > and people could walk on it. The cable style would reach way up > to a pulley > attached to a building roof edge or southern wall. A very heavy > counterweight suspended from the cable would apply tension, > making the cable > straight. The diameter of available stranded metal cable may be > the limiting > size factor here because if the sundial were too large and the cable too > narrow then the shadow would completely disappear (like telephone lines do > on the ground). The diameter of the sun is 8.7 mrad, so the style would have to be at least 140 m X 8.7 mrad = 1.2 m thick to provide an umbra. Consider using a thinner cable with a 1.2 m ball attached, so that the date can also be read with great accuracy. You should be able to determine the exact day of the solstice with that precision, and the equinox within 15 minutes! I think you will never be able to locate the position of the shadow this accurately, however, without imaging optics. The most convenient lens to use for dialing purposes is that in your eye. You can get a very accurate reading if you look for the place or time where the image of the sun disappears behind an appropriately sized object. The biggest drawback of this approach, particularly in a public park, are the hundreds of people who will be blinded by looking too long into the sun. One solution that would be appropriate for a park would be a shallow, arc-shaped pond, preferably with the bottom black except for the dial markings. The visitors would walk along the pond until the reflection of the sun is blocked by the reflection of the style. This also solves another problem of a configuration where the style is viewed directly, namely the error due to different eye levels. This would be a sundial the builders of Stonehenge could be proud of! --Art Carlson
