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Today it is snowing in Washington DC and it brings to mind a winter some 43 years ago when the Washington Monument was turned into a sundial.  Over the years many have proposed turning this spiring monument into a sundial.  For example at the first North American Sundial Society (NASS) Conference in 1995 Robert Terwilliger drew a map of the shadow's excursion from Independence Avenue to E Street NW, repeated here at left. In June 2011 in the NASS Quarterly Journal The Compendium Robert Kellogg suggested that "...the precise shadow lines drawn in [Terwilliger's map] are precise geometrical constructs.  The problem is the sun is not a point source, but a disk about 1/2 degree in diameter.  The result is the sun casts a penumbral (partial) shadow when it is partially obscured, resulting in a range of light to dark sunlight that blurs the edges of shadow, making a gradient from light to dark.

To get a feeling for the impact of the penumbral shadow from the Washington Monument, we start with a brief summary of the Monument:   A competition was held in 1836 and won by architect Robert Mills who designed a tall obelisk.  Excavation began in early 1848 and the cornerstone was laid as part of the 4th of July ceremony by the Freemasons.  Lack of funds created a hiatus in construction in 1858, and finally the capstone to create a pyramidal point to the obelisk was laid December 6th, 1884. The obelisk as we see it today is 169.29 meters tall, 16.8 meters wide at the base and 10.5 meters wide at the start of the capstone pyramid.  The pyramid itself is 17m tall, with a small aluminum capstone at the tip and by the time the sun is sufficiently blocked by the top of the oblesk for us to see the shadow, the sun is well below the tip.