Message text written by "John Hall" >One thing that confuses me about the Oughtred dial is the height of the gnomon at the business end. Is this height the same as the radius of the Oughtred projection ? I have Fred Sawyers article on the dial from the Compendium 4/1-97 but I am still finding it confusing. <
Hi John, The height of the vertical gnomon on the Oughtred dial isn't relevant. None of the readings that you would normally take with the traditional form of this dial depends on the length of the shadow - and therefore you don't need to know where the shadow of a specific-length gnomon ends. So, for example if you take a reading, you are primarily measuring the sun's azimuth - since it's the direction of the shadow that matters for this dial. You then notice where the line for the current date crosses the shadow. At that point, you read from altitude lines (or the alidade, as mentioned in the article) what the sun's altitude is. Similarly, you note where that point falls among the hour curves, and you can tell what time it is. The sun's altitude is always 1) an output item or 2) an input item from another knowledge source independent of the dial (e.g. an observation with a quadrant) - so you never need to know exactly how tall the gnomon is or how long its shadow is. Hope that helps. Fred Sawyer
