HelioTrak Sundial Compass Calibration

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 map 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 inaccuracy.

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 page