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Sundials

A sundial is a simple way of telling the time using the motion of the Sun across the sky. In its simplest form it is just a stick stuck vertically into the ground but there are more complex and attractive forms like the one shown in the photograph.

Please click on the WORD link to download a sheet showing the sundial parts.


 

To make a simple horizontal sundial you need a gnomon (a pointer with an angle equal to your latitude) and a horizontal scale. You will also need a compass to set your gnomon and scale so that it lines up with the north-south line where you are.

The horizontal, scale should look like the one drawn below. The lines show the hours. This one is correct for a person at 51o N, you will have to use the table below to draw your own if you live at another latitude.



The gnomon goes along the centre line of your dial face and the thickness of this line should be the same as the width of the gnomon. The angle from the noon line (12:00) is called the hour angle (H). Since the Sun moves through 15o each hour the hour angle is 15 x the number of hours from noon.

You can calculate the angle (A) from the noon line for various hours using your latitude (L). The table below shows the angle A for different hours at various latitudes
Hours from noon/Latitude 50o 51o 52o 53o 55o
0 0.0 0.0 0.0 0.0 0.0
1 11.6 11.8 11.9 12.1 12.4
2 23.8 24.2 24.5 24.7 25.3
3 37.4 37.8 38.2 38.6 39.3
4 53.0 53.4 53.8 54.1 54.8
5 70.7 71.0 71.2 71.4 71.9
6 90 90 90 90 90

You can use these to make a sundial that should be accurate for 51o N. Don't forget to change the angle of your gnomon to that of your latitude.

A = tan-1[sin L x tanH]

You must always line up the sundial so that the line marked NORTH points north. The square piece of card must be on a flat surface.

There are two scales, one on either side of the paper. One is for summer time and the other Greenwich Mean Time. This allows for the changing of the clocks (one hour forward in Spring and one hour back in Autumn).

The sundial in the photo shows that it was nearly 8.30 in the morning when the photograph was taken.

It is important that the triangular piece of card (the gnomon) is stuck exactly along the north-south line with the vertical edge to the north.
 
 
 
 
© Keith Gibbs 2020