A sundial has a shadow casting device called a gnomon, attached to a dial plate
which has markings. The actual shadow used to read the time is created by a
part of the gnomon called the"style". The markings almost always have hours,
and may or may not be legal clock mean time, often it is local sun time. Local
sun time (local apparent time or L.A.T.) must have the longitude factored in,
typically somewhere between -30 to +30 minutes. Also, the clock and the sun are
not in synch, the difference is called the equation of time (EOT) and varies
predictably throughout the year, the limits being about plus or minus 16 minutes.
Other markings may appear on a sundial, the date (declination lines or curves) is
common. Sometimes the hours until the next sunset (Italian lines) is another. Yet
another is the Islamic prayer times. The analemma, and so on.
The markings have hour lines or hour points, and sometimes
calendar information and the like, collectively called dial
furniture. Calendar lines may show specific dates, or the
solstice and the equinoxes. The dial furniture is marked on the
A nodus is sometimes on the gnomon and it highlights the
calendar information. If there is no nodus then the tip of the
gnomon's sloped edge (style) is the de-facto nodus.
The gnomon is often a triangle, and is fixed to the
dial plate by the sub-style.
The tip at the base of the gnomon, at the tiny end,
is called dial center, from whence hour lines are
The angled bit which actually casts the shadow for
the hours is called the style.
The (in this example vertical) third leg of the
gnomon has no name as such.
The angle between the sub-style and the style is
the latitude for horizontal dials that are flat. It is
called the style height. The linear distance from
the nodus directly to the dial plate is the "gnomon
linear height" or GLH.
|EOT for 2013 EOT for 2014
EOT for 2015
EOT for 2016 EOT for 2017 EOT for 2018
EOT for 2019 EOT for 2020
Simple Shadows talks about building a sundial and
explains how it works. Free.
This web site is financed by sales of Illustrating
Time's Shadow, and online download
see paypal page here for details.
|the author has a rather interesting
family, so do check it out! He is the
great grandson of Harry Gordon
Selfridge and custodian of his personal
effects (currently on loan to a library in
Wisconsin: 2016), worked with Lindy
Woodhead on "Shopping, Seduction,
and Mr Selfridge", and on "Secrets of
Selfridge" (PBS), and various news
paper and magazine articles.
Also Wiasemsky (Russia), and Craig (Last
Chance Mine, NM).
One of hundreds of dials made by the author,
shows one Italian hour line (3 hours before sunset),
the ASR prayer curve, the declination (calendar)
curves, the longitude corrected hours, and an
equation of time graph. This is a horizontal dial
specific to a latitude and longitude.