Capricorn: Dec.22-Jan.29 The Sundial Primer
created by Carl Sabanski
Capricorn: Dec.22-Jan.29

The Sundial Primer Index

Finding True North/South

North: the intersection of the local meridian with the horizon, in the direction of the north celestial pole.

South: one of the cardinal points of the compass, in the direction opposite north, in the direction of the south celestial pole.

Meridian: the great circle (or, more usually, half of a great circle) passing through the North and South poles. The same as a line of longitude. The term is sometimes used to mean the meridian line (local) passing through the observer's location, or its representation on the dial face.

Celestial Pole: the points on the celestial sphere where it meets the Earth's axis. The stars appear to rotate around these poles.

Magnetic Variation / Magnetic Declination / Magnetic Deviation: is the angle between the true North pole and the magnetic North pole.

Polaris (or Pole Star): actually Ursae Minoris, it is the star which appears quite close to the North Celestial Pole and is frequently used for finding north by navigators. It currently appears to rotate daily around a circle of radius 1. The size of this circle varies over the centuries with the precession of the equinoxes.

To accurately tell time, the gnomon of a horizontal sundial must be parallel to the earth's axis and therefore must point directly to the celestial pole. To accomplish this in the Northern Hemisphere one must determine the position of the true North line at the place where the sundial is to be located.  In the Southern Hemisphere one must determine the position of true South. There are a number of methods that can be used to do this, some more accurate than others. Method 1 applies only to the Northern Hemisphere. Although some of the other methods may reference locating true North they apply equally to locating true South.

1. Position the sundial so that the gnomon points at the Pole Star Polaris.

This method may take some time to perfect as it must be done on a clear night and you really can't check the accuracy of the alignment until the next sunny day. Also because Polaris appears to rotate daily around a circle of radius 1, you must account for this offset. Might be a little tricky!

Star Trails

Star Trails!

2. Use a compass.

This can work but will be inaccurate because the compass points at magnetic north and you must correct for magnetic deviation. Magnetic deviation varies around the world and even changes over time. It can even be affected  by local forces such as the presence of steel and magnetic rocks. This is not a recommended method.

If you are interested in learning more about magnetic deviation, visit the Natural Resources Canada web site on "geomagnitism". The site even has a "Magnetic Declination Calendar".

3. Mark the shadow of a vertical object at local apparent noon.

Using a vertical rod or a plumb line, mark the position of the shadow at precisely local apparent noon, the time when the sun souths. To do this you must have an accurately set watch and you must know the time when local apparent noon occurs at your location and on the day when carry out this procedure.

The photos below show a variation of this technique. A slot was cut in a thin board that was mounted on a tall tripod. An optical plumb was used to ensure that the apparatus was vertical above a marked point on the ground. A narrow beam of light was cast on the ground and at precisely local apparent noon both edges of the light were marked on a piece of paper. The centre of the beam was determined. The line from the beam's centre point to the point beneath the tripod was the true North line. This method was used to confirm a line that was previously surveyed and the two matched precisely.

Casting a light beam. Finding True North.

   A method of finding true North.

To easily determine local apparent time as well as other dialling information it is good to get a copy of "The Dialist's Companion". This program allows you to set any location, date and time. The image below shows that the program has been set for June 20, 2004 and the "Dial Time" or local apparent time is 12:00 pm. Clock time for local apparent noon can be determined by entering the date of interest and reading the time for "Solar Noon" on the display. This is the time when you will mark the position of the shadow. Don't forget to allow for DST in the summer.

Figure 1: Detemining local apparent noon using "The Dialist's Companion".

Figure 1: Determining local apparent noon using "The Dialist's Companion".

4. The "Method of Equal Altitudes".

This method requires a sunny day and plenty of time. Prepare a board by drawing a number of concentric circles. Position a nail vertically at the centre point of the circles. Place the board at the location where the sundial will be located ensuring that it is level. Mark the point on the circles just as the shadow of the nail head passes through, both in the morning and the afternoon. As the sun passes through the same circle in the morning and afternoon it will be at the same altitude.  Draw a line joining the two points on the same circle and then find its midpoint. A line from the nail through the midpoint will define the true North line. By marking a number of circles, the accuracy of this method can be increased by averaging the midpoints if they do not all fall on one true North line.

Figure 2: Method of Equal Altitudes

Figure 2: Finding True North - "Method of Equal Altitudes" (CAD)