john hoy wrote: > I've gotten a fair amount of email lately from people who want to make an > analemmatic dial but are not comfortable with technical or theoretical
> explanations -- they just want a sundial. > > I've tried to prepare some simple, hopefully not confusing, instructions, > which I would like to invite any of you who are interested to review. I > will welcome any criticism or corrections. > > It's at > http://www.cyberspace.org/~jh/dial/analemmatic.html > > Thanks, > > John Hoy As a project for the children at a club I go to, I created an analemmatic sundial with the minimum of measurement and calculation. For anyone who prefers a geometric approach I offer the following, which needs only a protractor and a piece of string and even includes correction for your longitude. I established the north-south and east-west axes of the proposed dial as described by John, checked by a noon sight. We fixed the size by choosing the desired height, rather than the width. Then we used the protractor (set to the latitude) to find the foci, where we fixed two pins. We tied the ends of the string to the pins, adjusting its length so it would just reach to the 12 o'clock point. Then we used the string to draw the ellipse in pencil. To fix the hour marks, we put a pin at the centre, looping both ends of the same piece of string over it. We made a circle of points, every 15 degrees, starting one degree left of north, as we are one degree west of Greenwich. A second set, starting one degree right of north, gave us points due north or south of the first. This made it easy, using the string, to find the points on the ellipse due north or south of the first circle of points. These are the hour marks, corrected for longitude. To draw the date scale is just as easy. >From one of the foci, use the protractor to find the points on the N-S axis making an angle of 23.5 degrees north or south of the east-west line. Draw a circle in the centre of the dial passing through these points, and also draw arcs through the same points, centred on the foci, to make a lens shape within this circle. Divide the circle into 365, and mark dates on it, starting with December 22nd at the south. To draw the actual date scale, for any date go due east/west from the circle to the lens, then radially from the lens towards the opposite focus, but stop at the north-south axis. This point is where the user should stand on the chosen date. So there you have it, an accurate sundial without resort to tables or calculators. By the way, there is even a simple correction you can apply to adjust for the different lengths of the seasons due to the eccentricity of the earth's orbit, but that's perhaps going a bit far. I trust the above is neither too brief to be useful, nor too long to be boring. It is not intended as a correction or criticism of John's method - just an alternative for those who might prefer it. My one criticism of John's method is that it does not adjust for one's longitude relative to the local time zone. I would comment, too, that some makers now prefer to mark such sundials with daylight saving time. You could do both, using different colours, perhaps. Chris Lusby Taylor 51.4N 1.3W Please note: My email address has recently changed: from [EMAIL PROTECTED] to [EMAIL PROTECTED]
