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

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"SUNNY DAY U"


Make a Polar Sundial - Graphical Method

Polar Sundial: is a sundial in which the dial plate is set along the East-West direction and inclines so that it is parallel with the polar axis. The standard polar pointing gnomon is thus also parallel to the dial plate.


Like the equatorial sundial the polar sundial is a universal sundial and can be used at any latitude. To work properly the dial plate must be tilted at an angle equal to the latitude and the style pointed to the celestial pole.

Figure 1 shows the relationship between a polar sundial and the equatorial sundial that is used to create it.  Notice that the two sundials have a common style or shadow casting edge that points to the celestial pole. The style is parallel to the face of the polar sundial and it is perpendicular to the face of the equatorial sundial. For this to be, the equatorial sundial is perpendicular to the face of the polar sundial.

This dial is designed for the Northern Hemisphere and the style must point to the North Celestial Pole (True North) or Polaris. To design a sundial for the Southern Hemisphere you need to reverse the numbering of the hours. Instead of the hour numbers going from 6 a.m. to 6 p.m. from left to right they will go from 6 a.m. to 6 p.m. from right to left. Also the style must point to the South Celestial Pole (True South).

This sundial will tell solar time but not clock time. You must learn about longitude correction and the Equation of Time to correct your sundial reading so that it will give you clock time.

 

Figure 1: Polar Sundial

Figure 1: Polar Sundial (CAD)

It would be difficult to project the hour lines of the equatorial sundial shown in Figure 1 on to a perpendicular plane to create the polar sundial. A graphical method is available to do this on a flat surface, your piece of paper. Follow these instructions and in no time you will have your very own polar sundial and it will be designed for where you live.

What do you need to know? All you need to know is the latitude of where you live. The latitude is not required to design the dial but to position it properly. Now let's make a polar sundial.

Step 1

Step 1

  • Draw a horizontal line.

  • Draw a vertical line that passes through the horizontal line at E. This is the centre of your equatorial sundial.

  • Select a point A on the vertical line. The line EA is the radius of your equatorial sundial. This is also the height of the style from the face of the polar sundial.

  • Place your compass point at E and draw a circle starting at A.  You have now drawn the face of your equatorial sundial.

Step 2

Step 2

  • Starting at EA draw lines spaced at 15 in both directions until they are horizontal. The hour lines on an equatorial sundial are spaced at 15 intervals. You now have the hour lines for the full hours between 6 a.m. and 6 p.m.

  • If you want smaller time intervals add more lines. Remember that 15=60 minutes so 7.5=30 minutes, 1=4 minutes. You can figure out the rest.

Step 3

Step 3

  • Extend as many lines as you wish to the horizontal line. This horizontal line is one side of the dial face of your polar sundial.

Step 4

Step 4

  • Draw a line parallel to the horizontal line passing through A. This line is the other edge of the dial face and determines its width. Draw a vertical line from point A to point B. Draw vertical lines from all the other points. You have now transferred the hour lines from the equatorial sundial to the polar sundial.

Step 5

Step 5

  • Remove all the extra lines from your drawing or trace the hour lines on to a clean piece of paper.

  • Glue the paper on to some cardboard.

  • You can make your dial face any shape you want and decorate it too. Cut your sundial out with scissors.

  • The gnomon should be made out of cardboard. It is a rectangle with a base of length AB and a height H, where H is the radius of the equatorial dial. Notice that distance H is the same as the distance from 9 a.m. to noon or noon to 3 p.m. The gnomon is attached to your sundial so point A on the gnomon is at point A on the sundial's face and point B is on point B. Try to make it stand as straight as possible.

A polar sundial could tell the time between 6 a.m. and 6 p.m. but it would be of infinite length. The period of time that your polar sundial will indicate depends upon how long you want to make it.

Congratulations! Your sundial is now ready to use. All you need to do is find a sunny spot and find the direction of true north, that is the celestial pole. Make sure that your sundial is tilted so the gnomon is pointing to the celestial pole. To find true north please go to "Finding True North" at The Sundial Primer.

Happy Dialling!