An important American polar dial that indicates standard time. It is located on the campus of the National Institute of Standards and Technology at Gaithersburg Maryland.
The images can be viewed by clicking the hypertext links, or the thumbnails below.
The dial has three faces. The upper and lower faces utilize half-analemmas as hour lines to indicate standard time corrected for the longitude. Each is used for half the year. The time shown in the photograph of the dial face is about 10:30 AM on a date when the sun's declination was a few degrees north of the equator.
The sunburst at the top of the dial surrounds the aperture through which the sun falls on the analemma, which is located behind the dial. The third face is in the center, and indicates local solar time.The dial was designed by R. Newton Mayall, the author of Sundials, How to Know, Use, and Make Them. I was pleased to be able to recreate Mayall's inspired design and produce a drawing of the dial face using DesignCAD 2D and its internal programming language. The drawing shows more detail than the photograph.A photograph of the side of the dial shows the shape of the gnomons, and the position and shape of the analemma receiving surface. (Seen at A in the small image.) Since the analemma is projected on a semi-cylindrical surface, it is somewhat more compressed north to south than we are used to seeing. Mayall has included the Time Zone offset on the analemma as well as the declination and the meridian altitude. Curious that he did not include an indication of the equation of time, which I suppose you could extrapolate from the dial itself.
The illustrations used on this page were originally published in Compendium 1-4 of the North American Sundial Society, November 1994. Drawings by Bob Terwilliger.