STEPHEN HOLEHAN SUNDIAL AND SIGN MAKER
Workshop/mobile 07947 214802
Wall dials, finding the angle of declination. Why can’t I use a compass?
If you take a good look at this cottage in the village where I live you will see that the front wall reclines forward. ( the door is vertical!)
In order to calculate the hour lines of the dial an accurate measurement of the deviation, forward or backwards, from vertical is required. If this is the case with your house I will send detailed information to allow you to take the required measurements.
Wall dials, My house is leaning (forward / backwards), can I still have a wall dial?
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The diagram above shows the relative differences between the hour lines of vertical wall dials as they range between due (solar) south to due (solar) west, the converse applies for due (solar) east. A magnetic compass will point to magnetic north, a position which lies to the side of the true axis of the earth. This position in fact moves over time and is shown on maps as the “angle of magnetic deviation” Imagine a knitting needle pushed through a ball, now spin the needle with your fingers- this is the true axis, the one we need. This is found by measuring the suns angle relative to the globe when the sun is at its highest point ( Zenith) on any given day. Finding this line or meridian is not too difficult to do and I will send a detailed information pack to you and provide telephone support, you will, however, have to provide a sunny day!
In order that the dial I make for you is accurate I will need to have the angle of declination for your chosen wall, this is how many degrees to the East or West that your wall points or declines to .
Angle of sun when at its zenith
Wall
Angle of declination
Wall declines West
West
East
Magnetic North South
WALL PROCLINES,
LEANS FORWARD
WALL RECLINES