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.
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EOT for 2015
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