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When my sundial customers ask me if they can use
their GPS devices to determine True North for setting their sundials, I tell
them no. I just don't trust GPS for that. I tell them to use this
method instead:
FINDING TRUE NORTH Using The Noon Shadow
Method 1. Finding True
North Before
sundial installation, you need to determine True North (not magnetic
North). Do NOT use a magnetic
Compass. To find True North you will need the
following: A piece of clothes line rope or
think string about 8 ft. long. A plumbob or heavy
weight A free standing ladder 5 or 6 feet
tall A broom handle or stiff
board Duct tape or
clamps A flat board like plywood (about 3
x 3) An accurate time source (cell phone,
clock or watch) The exact time of High Noon for your
location on the date you perform the measurement. A sunny day Finding True North using this method
is easy and extremely accurate. You
will use sun to give you true North by looking at the shadow cast from a
vertical rope at HIGH NOON (local apparent solar noon). At High Noon, the sun is exactly due
south and will cast a shadow on the ground directly towards the north.
1. Determine the time of High
Noon: High noon does not occur at the same watch time every
day and is different at different locations! This website will tell you the exact time
of High Noon (Apparent Solar Noon) at your location and date: http://www.solar-noon.com/ If you have questions, contact John
Carmichael. 2. Setup plumbob: First attach a broomhandle or stiff
board horizontally to the top of the ladder with ductape or clamps. Tie the end of the string or rope to the
end of the broomhandle, then tie the pumbbob to the other end so that it hangs
just above ground level. Place the ladder off to the side so that it is not in
the way of the construction zone and so that it wont shade the plywood
board. If its a windy day, you can
keep the plumbob from swinging if you place it in a glass of water to dampen the
movement. 3. Setup plywood projection
screen: Place a leveled horizontal plywood board on the
ground to the north of the plumbob. So that the strings shadow is visible on the
board. The shadow cast onto the
board by the rope will point due North at High Noon (Solar
Noon). 4. Mark the North/South
line: Looking at
your accurate clock, at the moment of Solar Noon, mark on the board with a
pencil the position of the strings shadow. This line is a true North/South
line. 5. Rebar Placement: Tie a string between two pieces of
rebar and position the string directly above the North/South line on the plywood
board. Pound rebar into ground so
that you will know where the North/South line is after you remove the
board. When positioning the sundial
to true North, use this string. You can measure off a line thats parallel to
the rebar string.
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