Make a Horizontal Sundial - Graphical Method

Horizontal Sundial: the common or garden sundial with a horizontal dial plate and polar-pointing gnomon.

The sundial you will be making is one that you can find in most garden supply shops. The difference is that this one will actually work. Give it a try!

Figure 1 shows the relationship between a horizontal 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 at an angle equal to the latitude relative to the face of the horizontal sundial and it is perpendicular to the face of the equatorial sundial. For this to be, the equatorial sundial is tilted at an angle equal to the co-latitude, or 90°-latitude, relative to the face of the horizontal 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. in the clockwise direction they will go from 6 a.m. to 6 p.m. in the counterclockwise direction. 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: Horizontal Sundial (CAD)

It would be difficult to project the hour lines of the equatorial sundial shown in Figure 1 on to a horizontal plane to create the horizontal 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 horizontal 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. Now let's make a horizontal sundial.

Step 1

• Draw a horizontal line.

• Draw a vertical line that passes through the horizontal line at O.

• Select a point C on the vertical line. This point will be the centre of your horizontal sundial and eventually all the hour lines will radiate from it.

• Draw the line CD at an angle equal to the latitude (LAT) from OC.

• Draw the line OB from O and perpendicular to line CD. The line OB is the radius of the equatorial sundial you will be drawing next.

Step 2

• Place your compass point at O and draw an arc from B until it crosses the vertical line at E. This is the centre of your equatorial sundial.

• Place your compass point at E and draw a circle starting at O. Notice that OE=OB=R. You have now drawn the face of your equatorial sundial.

Step 3

• Starting at OE 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 4

• Extend all the lines to the horizontal line.

Step 5

• From C, the centre of the horizontal sundial, draw a line to each of the points on the horizontal line. You have now transferred the hour lines form the equatorial sundial to the horizontal sundial.

• As the hour lines approach 6 a.m. and 6 p.m. they move further away and if you want to mark off smaller time intervals between 6 and 7 a.m. and 5 and 6 p.m. you will probably run out of paper space......table space...........room space.  Do you know how you can get these hour lines? Think about it.

Step 6

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

• The gnomon should be made out of cardboard. The angle at C must be equal to your latitude. The gnomon is attached to your sundial so that point C on the gnomon is at point C on the sundial's face and the base of the gnomon is on the noon line. Try to make it stand as straight as possible. Try different sized gnomons to see what happens to the shadow.

Step 7

• A horizontal sundial can tell the time before 6 a.m. and after 6 p.m.. How much earlier and later depends upon where you live.

• To add these extra hour lines all you have to do is extend the hour lines you have already drawn. For example, 5 a.m. is the extension of 5 p.m. and 7 p.m. is the extension of 7 a.m..

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 level and that the gnomon is pointing to the celestial pole. To find true north please go to "Finding True North" at The Sundial Primer.

Happy Dialling!

There are a couple of other methods that can be used to draw a sundial. The following pages will show you these methods and how you can use them to draw a horizontal sundial. They can also be used to draw a vertical sundial and if you are interested you can easily figure that out. Just start with the vertical sundial page and remember that the hours a vertical south sundial can show are limited.