Sundial Theory



Sundials have been used over the ages to provide accurate timekeeping.  
Of course, today, most people use wristwaches and clocks to make sure 
that important times and events are not missed!  Does this mean there 
is no place for a sundial in today's world?  Heavens No!!!  Sundials 
are gentle reminders that day after day, and year after year, the earth 
follows the same path around the sun, and does so in a precise amount 
of time.  This regular movement is the basis for sundial construction. 
Some simple mathematics, to take into account the angle of the sun in 
the sky, and an adjustment to "standard time", are added 
ingredients used to make an accurate timepiece, suitable as an ornament 
for your garden, a craft hobby, a youth project, or just something easy 
and fun to do!  The following paragraphs will take the mystery out of
how sundials work.

The basic componets of a horizontal sundial are the BASE and the GNOMON.
  
The Gnomon:

The gnomon is the part that casts a shadow on the base of the horizontal 
sundial, and indicates the time of day.  The angle of the gnomon to the 
base is critical if you want the sundial to be accurate.  The angle must 
equal the latitude of the location where the sundial will be used.  For 
example, in Anchorage, Alaska, the latitude is 61 degrees North.  The 
angle between the base and the gnomon would therefore be 61 degrees.  At 
other locations, the angle would be different, and would equal the lattiude 
of the location the sundial (your town, for example!).  

The Horizontal Sundial Base:

The BASE of the horizontal 
sundial is basically a square, with lines drawn on it to mark the hours of 
the day.  See the illustration above.  The two basic lines are the noon 
line, and the horizontal line along the bottom edge of the sundial face.  
The lines that mark the hours of the day are also related to trigonometric
angles!  The formula for figuring out the slope (or angle) of the lines is:

TANGENT of "D" = (TANGENT of "t") X (SINE of "PHI"),
 
where "D" is the angle which the hour line makes with the noon line, 
"t" is the time measured from noon in degrees and minutes of arc, and 
"phi" is the lattitude of the place where the dial is to be used.

The sun doesn't respect our man-made conventions of Standard Time Zones.  
But a sundial's calculations can be adjusted to compensate for the 
difference between the "real"  sun time and man-made standard 
time zones.

Sundial time is not as accurate, day to day as a "clock", in that 
the orbit of the earth around the sun is not a perfect circle, and the speed 
of the orbit is not constant throughout the orbit.  The average days of the 
year when sundial and clock time most agree are near April 16, June 14, 
September 2, and December 26.  So setting up the sundial orientation on or 
close to those days will ensure the most accuracy.

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