Alidade
Each vane (also called a pinnule or pinule) has a hole, slot or other indicator through which one can view a distant object.The figure on the left displays drawings that attempt to show the general forms of various alidades that can be found on many antique instruments.Real alidades of these types could be much more decorative, revealing the maker's artistic talents as well as his technical skills.In the diagram, the openings are exaggerated in size to show the shape; they would be smaller in a real alidade, perhaps 2 mm or so in width.He did have access to the telescopic sights that were being used by astronomers in other countries, however, he chose to use naked-eye observations for his positional instruments.Hevelius' design[2] featured a pivot point with a vertical cylinder and a vane at the observer's end.In a sextant or octant the alidade is the turnable arm carrying a mirror and an index to a graduated circle in a vertical plane.Two vertical sight apertures are arranged opposite each other and can be rotated along the graduated arc of the horizontal table.
Adelaideceiling projectortriangulateplane tablepolar measurementcircular graduationsastrolabesArabicgraphometermariner's astrolabesJohannes HeveliusTycho Braheparallaxtheodolitetelescopesextantoctantfire towersforest firetopographic maplightningOsborne Fire FinderGunsightPelorus (instrument)public domainChambers, EphraimCyclopædia, or an Universal Dictionary of Arts and Sciences