HelioTrak Sundial Compass
Some compasses are consistently off by a few
degrees or more. To check the compass of the HelioTrak
sundial, point the north-south line to geographic north, then
check the direction pointed by the compass.
If the compass is working perfectly, it points to a direction
exactly matching the magnetic declination for your location,
which you can find on a special
or by looking it up for your city. This is normal and
correct operation of your compass.
If your compass is "off", it will point some number of degrees
in a different direction. To calibrate your sundial to correct
for this error, mark the compass direction on the flat plate
when you know that the north-south line of the plate is aligned
to true geographic north.
Here are three possible ways to make sure your device is aligned
to true geographic north.
- Look on a map and find a local street that runs exactly
north-south. Go to that street and align your device to the
street curb or sidewalk.
- At night, find the North Star. Take a plumb line (a string
with a weight tied to the end) and hold the string up to the
star. Note the location where the string visually meets a
distant object such as a building or tall tree. Point your
device at the distant object.
- On a sunny day, use the HelioTrak sundial "in reverse."
Find the date in the figure-8 analemma on the top of the
solar cage. Follow either right (AM) or left (PM) to reach
the current clock time. Adjust the time for Daylight Savings
Time, your east-west (longitude) location in your time zone,
and the analemma deviation (see Sundial
Accuracy in the main HelioTrak page). Mark the
intersection of the date and time on the solar cage using a
piece of transparent tape and a marker pen.
While keeping the plate level, turn the device parts until
the marked spot casts a shadow on the center of the plate.
This orients the plate to geographic north and tilts
the solar cage for your latitude.
With the device pointing true geographical north using any of
the methods above, mark the direction that the compass is
pointing. Now your device compass is calibrated, taking into
account both your local magnetic declination and any compass
A free downloadable declination marking tool
can help you
draw a straight line through the compass. Print out the tool at
100% actual size and glue it to an index card or cereal box. Cut
out the tool, then use it like a ruler to draw the declination
line at the proper angle. Use the 5-degree tick marks on the
sundial as guides for accuracy.
If you go on a long trip (many hundreds of miles or km),
recalibrate your compass for the new location, as magnetic
declination varies from place to place.
Back to HelioTrak Sundial main