Using a Shadowplot to find the North-South line in the Southern hemisphere

After one day of shadow measurements you are ready to draw the North-South line. (If you have not yet made your shadow plot, click here to learn how to do so.) On your completed shadow plot draw a smooth curve through all of the marks that you have made, without moving the paper. The more often you have taken your measurements, the easier it will be to draw this curve accurately.

Once you have drawn a smooth curve through the markings, you want to find the shortest distance between the dowel base and this curve. To do this place a meter stick so that one end is at the dowel base and the curve crosses the meter stick at some other point. Pivot the meter stick about the end at the dowel base until the find the location on the curve that is the shortest distance from the dowel base.

Draw a line from the dowel base to this point as in the image below.

This line you have just drawn is called a North-South line. It is the line along which the Sun will cast a shadow at local noon. (Your local noon may not be exactly when the clock says noon depending on where you are in your timezone.)

This North-South line points exactly North and South. A line drawn perpendicular to this line will point East and West. You will need to know the exact direction of South to use your horizontal sundial.

Now that we know the direction of south, we need to know our latitude and then we are ready to make the sundials.

Shadowplot sundials Latitude