The above photo shows the Sun just behind the apex of the Washington Monument on the Spring Equinox (c) P.Field 1998
A nodus is a specific point along the Gnomon of a sundial. This specific point is usually used to show the declination of the sun on a sundial, or the zodiac lines. However the nodus can be used as the style of the dial.
Consider using an existing monolith (or obelisk) as the gnomon of a sundial. I believe that the Washington Monument (an obelisk) would make an excellent pointer for a sundial.
The diagram below shows how the Washington Monument could be utilized as the nodus of a large horizontal sundial.
The dial pictured above is a horizontal dial which is painted on my patio. The nodus (labled) lies along a vertical support column. Let us imagine that the pictured brick column is the vertical Washington monument. As seen a dial can be constructed which will show the time.
The picture below the dial is a close-up of the Equation of Time graph.
Let us calculate the distances from the base of the Monument to the maximum and minimum (and midpoint). If we use the Noon line, which would lie due North of the base of the Monument we find that on the Winter Solstice (the shortest "day" of the year) that the shadow would be 1060 feet, 1 3/4" from the base. On the longest "day" of the year (the Summer solstice) the shadow would be 153 feet, 10" inches and on the equinox(s) the length of the shadow would be 442 feet.
The Monument is about 687 feet from the dial center.
The map below shows the layout of the Washington Monument dial to scale. The declination lines are for the Fourth of July and February 22 and the Spring/Fall Equinox.
(c) 1998 P. Field I/O Asso.
A shadow has two parts, an Umbra in which the Sun is totally obscured and a Penumbra where the obscuration is only partial. We found that the Washington Monument has a penumbra that is rather large, thus detracting from the accuracy.