|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.
|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.
|Also see here:
|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 in
Wisconsin: 2016/2017), worked with
Lindy Woodhead on "Shopping,
Seduction, and Mr Selfridge" over a
period of 2 years, and on "Secrets of
Selfridge" (PBS), and various news
paper and magazine articles.