| The parts of a sun dial |
A sundial has a shadow casting device called a gnomon, attached to a dial plate which has markings. The markings almost always have hours, and may or may not be legal 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 vaies 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. And so on.
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 vaies 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. 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 dial plate.
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.
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 dial plate.
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 of the gnomon at the tiny end is called dial
center, from whence hour lines are marked.
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 is irellevant.
The angle between the sub-style and the style is
the latitude for horizontal dials that are flat. It is
called the style height.
dial plate by the sub-style.
The tip of the gnomon at the tiny end is called dial
center, from whence hour lines are marked.
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 is irellevant.
The angle between the sub-style and the style is
the latitude for horizontal dials that are flat. It is
called the style height.
| EOT for 2007 EOT for 2008 (year to year, the EOTs are very close) Simple Shadows talks about building a sundial and explains how it works. Free. This web site is financed by sales of Illustrating Shadows and also Illustrating More Shadows, and the big CD, see paypal page here for details. |